# data[].attributes.attachment_link.description
Easily create surveys and polls to collect customer feedback, measure employee satisfaction, and organize team events.

# data[].attributes.attachment_link.description
Easily create surveys and polls to collect customer feedback, measure employee satisfaction, and organize team events.

# data[].attributes.body
Talks about the different exercises to hit horizontal extension, vertical extension and external rotation.

# data[].attributes.attachment_link.description
Welcome back to Instagram. Sign in to check out what your friends, family & interests have been capturing & sharing around the world.

# data[].attributes.body
Quads quiz

# data[].attributes.attachment_link.description
Easily create surveys and polls to collect customer feedback, measure employee satisfaction, and organize team events.

# data[].attributes.body
Please do not complete until you have done the classroom session.

# data[].attributes.attachment_link.description
Easily create surveys and polls to collect customer feedback, measure employee satisfaction, and organize team events.

# data[].attributes.attachment_link.readable_text
Academic Department of Military Rehabilitation (ADMR): avoiding the pitfalls of ' the Walker Dip '

Academic Department of Military Rehabilitation, Defence Medical Rehabilitation Centre, Loughborough, UK

Correspondence to Russell J Coppack, Academic Department of Military Rehabilitation, Defence Medical Rehabilitation Centre, Loughborough LE12 5QW, UK; russ.coppack100{at}mod.gov.uk

Abstract

A key research theme identified during the 2021 Strategic Delivery Plan (SDP) for Defence Medical Services (DMS) Research was preventing and treating musculoskeletal injury (MSKI). MSKI is a significant burden to military populations globally, reducing both operational strength and force readiness. It is therefore essential that research is conducted to gain a greater understanding of the epidemiology, aetiology and risk factors associated with MSKI to develop targeted prevention strategies and rehabilitation interventions. The Academic Department of Military Rehabilitation (ADMR) and the recently established MSKI research-theme working group must use a combination of balance and flexibility when coordinating research priorities to ensure they align and reflect both higher level DMS and UK Defence Rehabilitation practitioner-driven priorities. This article describes the response ADMR have taken to meet and align with the requirements of the 2021 SDP.

REHABILITATION MEDICINE

Musculoskeletal disorders

OCCUPATIONAL & INDUSTRIAL MEDICINE

SPORTS MEDICINE

Statistics from Altmetric.com

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center's RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.

Read the full text or download the PDF:

# data[].attributes.attachment_link.title
Academic Department of Military Rehabilitation (ADMR): avoiding the pitfalls of 'the Walker Dip'

# data[].attributes.attachment_link.description
A key research theme identified during the 2021 Strategic Delivery Plan (SDP) for Defence Medical Services (DMS) Research was preventing and treating musculoskeletal injury (MSKI). MSKI is a significant burden to military populations globally, reducing both operational strength and force readiness.

# data[].attributes.headline
Strength trg in military rehabilitation 

# data[].attributes.subject
Strength trg in military rehabilitation 

# data[].attributes.attachment_link.readable_text
Abstract

The use of strength and conditioning (S&C) in musculoskeletal rehabilitation has gained wide acceptance among the rehabilitation community. However, there is an absence of evidence demonstrating how to best integrate the principles of S&C into rehabilitation practice. This article discusses four broad themes: (1) an overview of the UK Defence Rehabilitation care pathway, (2) the historical and current approaches to physical training to support operational readiness of the British Armed Forces, (3) the current and future challenges of integrating S&C into Defence Rehabilitation practice and (4) research priorities relating to the use of S&C in Defence Rehabilitation. We detail the importance of strength/power-based physical attributes within our military population. We recommend that consideration be given to the benefits of an alternative education/coaching-based model to be used during the current 3-week residential care pathway, which aims to ensure effective implementation of therapeutic S&C over a longer period of care.

http://creativecommons.org/licenses/by-nc/4.0/

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

Background

The most common reason for medical non-deployable status among military personnel is musculoskeletal (MSK) injury. 1 Soldiers injured during basic training, field exercise and sport may be unable to deploy on operations, while soldiers injured during deployment may not be fit to return to active duty. 2 Subsequently, there is a large economic and operational cost to UK Defence associated with MSK injury. Functional status during rehabilitation is most closely associated with muscle strength. 3 Therefore, maximising the potential for adaptations during strength training is a crucial factor in the progression of any MSK rehabilitation programme. However, there is an absence of evidence demonstrating how to best integrate the principles of strength and conditioning (S&C) into rehabilitation practice. 4 A significant challenge lies in designing optimal rehabilitation programmes that facilitate both neurological and muscular adaptations while concurrently accommodating biological healing, recovery and patient safety. 5 Historically, it has been widely accepted that to elicit significant gains in muscle hypertrophy and strength requires loads equivalent to a minimum of 70% of an individual's one repetition maximum for a given movement. 6 For individuals undergoing MSK rehabilitation, heavy-load resistance training can be contraindicated or they are limited by their symptomatic impairment, including pain and immobility, to attain the recommended heavier loads. 7 Therefore, patients with MSK injuries are often advised to reduce their training load, potentially limiting the desired muscular response to treatment, presenting a challenge to effective service delivery. Despite these known challenges, the use of S&C in MSK rehabilitation has gained wide acceptance among the rehabilitation community. Therefore, the purpose of this clinical commentary is to discuss issues surrounding the challenges of integrating S&C principles into UK Defence Rehabilitation practice.

Overview of UK Defence Rehabilitation service delivery

UK Defence Rehabilitation services are provided through a tiered network within Defence Medical Services. This consists of unit-level primary care rehabilitation facilities (PCRFs), 14 regional rehabilitation units (RRUs) and the Defence Medical Rehabilitation Centre (DMRC) at Stanford Hall (formerly DMRC Headley Court). PCRFs are unit/station-based outpatient departments providing exercise rehabilitation therapy for acute MSK injury. Military personnel with injuries that are unsuitable or do not respond at this level of care are referred to RRUs to allow rapid access to imaging services and residential rehabilitation. This intermediate level of care nests between the PCRF and DMRC. DMRC delivers consultant-led residential rehabilitation for more complex and chronic disorders and injuries, including complex trauma, neurological injury or illness, and chronic or specialist MSK injury/pain. DMRC provides an interdisciplinary approach to rehabilitation with services, including occupational therapy, social work, mental health, prosthesis/orthosis provision, podiatry, speech and language, dietetics and pain management support. Each admission at the RRUs and DMRC is traditionally 3 weeks in duration (readmissions may be provided for more complex or chronic conditions). The overall aim of treatment at each level of rehabilitation care is to restore optimal function and to regain full occupational employability to maintain force readiness. While there is some evidence supporting the efficacy of this 3-week model of residential treatment, 8-10 to our knowledge, there is only one descriptive study specifically investigating the integration of strength training into UK Defence Rehabilitation practice. 11 The primary opportunity for clinical staff at RRUs/DMRC to provide treatment and facilitate favourable medium-term to long-term rehabilitative outcomes is during this 3-week residential admission. Delivery of the exercise component of the multidisciplinary team rehabilitation care pathway and overall responsibility for integrating the principles of S&C into rehabilitation rests with the exercise rehabilitation instructor (ERI) and the physiotherapist. It is acknowledged that there is a particular challenge integrating S&C practice into the residential RRU/DMRC settings. This complex challenge is therefore reflected as a focus for this clinical commentary.

Historical and current approaches to physical training to support operational readiness of the British Armed Forces

Military training can be physically and mentally demanding. Through necessity, it is vital that soldiers are prepared to function under the uncomfortable conditions they may be exposed to during combat. The UK military's historical approach to physical training involved high-volume running, group circuits involving callisthenic-based exercise and marching with load. 12 It was traditionally advocated that high levels of aerobic capacity and muscular endurance were the components of fitness most closely associated with operational readiness. 12 13 However, the training culture of volume over intensity and aerobic conditioning over strength/power-based physical attributes has since been challenged. Many military tasks require an underlying level of muscular strength to perform. 13 14 For example, handling heavy machinery, casualty extraction and performing functional tasks while carrying heavy load (ie, a Bergen). 14 The ability to produce high quantities of force (maximum strength) provides the foundations of a soldier's ability to create explosive movements necessary during close-quarter combat, jumping/landing, multidirectional speed and agility, sprinting and throwing. 12 Therefore, improving or maintaining strength and power is essential to optimise the number of military personnel fit for operational duty. To meet this task, rehabilitation practitioners must individually tailor their programmes to meet the needs of the individual and the occupational standards expected by the British Armed Forces. Furthermore, strength training is consistently shown to be a key component of MSK injury prevention strategies. 15 ERIs and physiotherapists across UK Defence Rehabilitation should become familiar with the updated physical employment standard (PES) and, in particular, the 6-monthly soldier conditioning review (SCR) that measures a soldier's (army-specific) ability to perform various components of fitness (see online supplemental file). Rehabilitation outcomes following traditional MSK injury should therefore be assessed against these new physical standards and gym-based assessment criteria. The updated PES provides a strong rationale for prioritising the development of these strength and power-based physical attributes towards the end stage of rehabilitation and reinforces the importance of why ERIs and physiotherapists must understand the principles of S&C and how to integrate them into MSK rehabilitation. Strategies to improve strength and power during rehabilitation using athletic populations returning to high performance have been described elsewhere. 16

A summary of commonly used programme variables and monitoring methods is presented in Table 1. These guidelines are well established and widely accepted, but describe recommendations resulting from research using healthy adult populations. 17-19 It is the challenge of incorporating these principles into UK Defence Rehabilitation practice that provides the focus for this commentary.

Current and future challenges of integrating therapeutic S&C into UK Defence Rehabilitation practice

Despite an abundance of information on the implementation of S&C principles with healthy adults, investigations regarding the application of these principles in MSK rehabilitation programmes (or therapeutic S&C) are lacking. 4 Clear clinical reasoning relating to effective progression of therapeutic strength training is essential to achieve positive clinical outcomes. 20 In the absence of evidence to inform decisions regarding the integration of S&C training in rehabilitation, it is recommended that rehabilitating a patient at the site of injury be considered similar to an untrained individual when designing their training programme. 5 While useful as a means of progressing single exercises, this approach is limited in its ability to apply progressions within a multidimensional rehabilitation programme at different stages of a patient's recovery. 20 Consequently, it is difficult to justify the use of formulas and principles for the physiological adaptation to S&C established with healthy adults as a basis for clinical reasoning in the rehabilitation setting.

Figure 1 presents a model that describes the challenges of applying 'mainstream' S&C principles into a complex multimodal rehabilitation programme. In this context, the term mainstream applies to the accepted knowledge and principles derived from research using healthy adults. In this model, the horizontal axis represents a continuum of physical function ranging from low to full functional capacity. The application of mainstream strength training principles (eg, intensity, duration, frequency and exercise order) is maximally exploited when approaching the later stages of functional recovery but is less applicable when physical function is significantly impaired, particularly in the presence of pain. This is represented in the model by the blue triangle and proportional vertical arrows. The green triangle and arrows represent the integration of therapeutic/restorative strength training principles and knowledge employed in the clinical rehabilitation setting. This knowledge is principally applied with patients at the early stage of functional recovery with less reliance at the 'higher functioning' end of the continuum. Therefore, while acknowledging a degree of overlap during functional recovery ( Figure 1, shaded rectangle), this model visually shows an inverse relationship between the integration of mainstream versus therapeutic resistance training knowledge, principles and techniques at different stages of the recovery process. However, in the absence of clear guidelines on the integration of therapeutic strength training, decisions on when to increase load and add an external resistance are disproportionately driven by mainstream strength training principles, with little or no rationale for such decision making in a clinical rehabilitation programme. We would argue this approach promotes eminence rather than evidence-based practice. The examination of what constitutes an optimal therapeutic strength training programme is a research priority for UK Defence Rehabilitation.

While the use of S&C within MSK rehabilitation has acceptance across UK Defence Rehabilitation, there remains much debate surrounding how best to integrate accepted principles into the current treatment care pathways. Tables 2 and 3 identify some of the common challenges and proposed solutions from a clinical delivery, education, training and research perspective.

Figure 2 provides a template for exercise programme design considerations and how different components of therapeutic S&C should be prioritised and progressed from point of injury to discharge from rehabilitation care, while taking into consideration operational 'group' and using the updated PES (using the Army PES as an example). During the restorative recovery ( figure 2, phase I), the priority is promotion of pain-free movement. Once the therapist is satisfied the patient has completed the early restorative phase, they will perform a thorough needs analysis and identify which occupational group the patient most closely aligns with: group 1: medically discharged (return to civilian life and community reintegration); group 2: sedentary role (eg, desk-based role, low physical demand); group 3: low operational tempo (eg, this will be a unit who has just returned from ops and is just required to meet normal single-service physical testing criteria relevant); group 4: medium operational tempo (eg, units that are about to enter the readiness training cycle for future deployment); group 5: high operational tempo (eg, units that are undertaking training for deployment or units that have ongoing operational commitments, eg, special operations forces). The length of the blue arrows reflects the level of function, training load and physical capacity requirements to meet operational readiness. Operational readiness is determined using the updated PES. Groups 2 and 3 reflect non-ground close combat (GCC) (army non-GCC, not yet complete, to be published in 2021/2022); groups 4 and 5 represent army GCC (see online supplemental file 1). Preparing the injured serviceperson for operational readiness ( figure 2, phase II) will require a semistructured, progressive periodised training programme.

One of the most important challenges raised in Table 2 is the lack of objective performance-based outcome measures that can be directly used to inform current and future exercise prescription and programme design across all tiers of UK Defence Rehabilitation. Using examples of measurements currently used within different components of the PES whole career testing continuum (during entry selection, end of basic training and in-service); figure 3 plots these measures against a force-velocity curve. This approach to testing can provide therapists with a simple yet more diagnostic representation of their patient's force-generating capacity, which can be quantified and monitored over time against the updated employment standards expected by the British Armed Forces.

Research priorities

UK Defence Rehabilitation must respond to and incorporate an ever-evolving evidence base to meet the needs of its injured personnel while remaining sufficiently flexible to meet the dynamic/changing priorities of the Defence Medical Services and UK Military. Research priorities relating to the integration of S&C into MSK rehabilitation include optimal loading to aid safe progression that simultaneously accommodates tissue healing and recovery while achieving strength/hypertrophy adaptations. A review of how exercise therapy is currently prescribed and managed across Defence Rehabilitation (particularly during the structured 3-week residential care model delivered at RRUs/DMRC) is warranted. In addition, determination of whether rehabilitation outcomes are optimised using alternative methods of delivery (an education-based model) should also be considered.

Summary

In this clinical commentary, we detail the importance of strength and power-based physical attributes within our UK military population and reinforce the reasons why medium-term to long-term MSK rehabilitation training programmes could potentially be designed to reflect the updated PES. We provide some areas for consideration that may address these commonly cited challenges. We discuss the potential benefit of developing objective performance-based outcome measures that can be used across all tiers of Defence Rehabilitation that more closely reflect the updated PES/SCR. We would also recommend that consideration be given to the benefits of an alternative education/coaching-based model that aims to ensure effective implementation of therapeutic S&C over a longer period of care (eg, 3 months). Determining optimal loading to aid safe progression of strength/hypertrophy adaptations while simultaneously accommodating tissue healing and recovery is poorly understood across the global MSK rehabilitation community and has now become a research priority for UK Defence Rehabilitation. This article was written immediately prior to the COVID-19 outbreak in the UK. Therefore, it is too early to speculate on what impact the ongoing contingency measures may have on long-term service delivery in Defence Rehabilitation.

Acknowledgments

The author's thank all staff and patients across UK Defence Rehabilitation for their support and contribution to this article. The authors also thank Vanessa Walters (Academic Department of Military Rehabilitation) for her technical input and comments on the final draft.

# data[].attributes.attachment_link.title
Integration of strength training into UK Defence Rehabilitation practice: current trends and future challenges

# data[].attributes.attachment_link.description
The use of strength and conditioning (S&C) in musculoskeletal rehabilitation has gained wide acceptance among the rehabilitation community. However, there is an absence of evidence demonstrating how to best integrate the principles of S&C into rehabilitation practice.

# data[].attributes.headline
Journal: Return to Play After Soleus Muscle Injuries

# data[].attributes.subject
Journal: Return to Play After Soleus Muscle Injuries

# data[].attributes.body
Location of muscle

Recovery time for soleus muscle injuries

# data[].attributes.attachment_link.readable_text
Introduction

The rectus femoris is the anterior thigh compartment's most superficial and nearly vertically oriented muscle. This bipennate structure is a component of the quadriceps muscle complex, one of the knee's most important dynamic stabilizers.[1] The rectus femoris is also known as the "kicking muscle" for its involvement in activities involving forceful knee extension. As in other musculoskeletal structures, many rectus femoris injuries arise from sports activities, trauma, and occupations requiring repetitive movement of this muscle.

Structure and Function

Structure of the Rectus Femoris

The rectus femoris is fusiform in shape with superficial bipenniform fibers and deep, vertically oriented fibers (see Image. Right Quadriceps Femoris, Anterior View). This muscle has two origins. The "direct head" originates from the anterior inferior iliac spine, while the superior acetabular ridge is the origin of the "indirect head." The muscle heads fuse distally at an acute angle to form a common tendon.[1][2]

Inferiorly, the rectus femoris inserts conjointly with the other quadriceps muscles on the patellar base via the quadriceps tendon. The connection between the quadriceps tendon, patella, and patellar ligament allows the quadriceps muscle complex to have another insertion point on the tibial tuberosity.

Thus, the rectus femoris can be easily distinguished from the other quadriceps muscles by its nearly vertical orientation, superficial location, and crossing of 2 lower limb joints, namely, the thigh and knee.[1]

Function of the Rectus Femoris

The rectus femoris flexes the hip along with the sartorius and iliopsoas and extends the knee as part of the quadriceps femoris. This superficial muscle antagonizes the hamstrings at the knee and hip.[1][3]

Active and passive insufficiency are functional limitations that multi-joint muscles can acquire. In the case of the rectus femoris, active insufficiency occurs when full extension of the knee limits its ability to flex the hip and vice-versa. The muscle is highly contracted in such a state. Thus, the rectus femoris becomes a weak knee extensor in the sitting position when the hip is flexed.

On the other hand, passive insufficiency of the rectus femoris occurs when full knee flexion limits its capacity to extend the hip and vice-versa. In this case, the muscle is at its longest.[1]

Embryology

The lower limb bud appears during the 5th week of development. The bud originates laterally from the L2 to S2 spinal segments and contains all the primary germ layers, the ectoderm, endoderm, and mesoderm. The muscle tissue of the rectus femoris arises from the myotomic portions of somites, which arise from the mesoderm. Meanwhile, the skeletal elements supporting the lower limb come from the lateral plate somatic mesoderm. During embryogenesis, the lower limb rotates 90 degrees medially about the longitudinal axis, bringing the knee to the anterior aspect of the fetus.[3]

Blood Supply and Lymphatics

The femoral artery supplies blood to the quadriceps muscle via the lateral femoral circumflex branch. The muscle's venous drainage is via the femoral vein and its branches.

Lymphatic vessels of the anterior thigh drain into the superficial inguinal nodes. From there, the vessels ascend to the external iliac and lateral aortic lymph nodes.[15]

Nerves

Cortical signals from the precentral gyrus apex descend via the corticospinal tract. About 90% of the fibers in this nerve tract decussate at the level of the lower medulla at a region called the "pyramidal tract decussation." These fibers continue to descend on the contralateral side as the lateral corticospinal tract until they reach the lower motor neurons below the upper thoracic spinal cord. The remaining 10% of the fibers that do not decussate continue as the ventral corticospinal tract, but they do not innervate structures below the upper thoracic spinal cord.

To reach the quadriceps femoris muscle, the lateral corticospinal tract's fibers synapse with the lower motor neurons at the L4 level. From there, the neuronal signal propagates toward the L4 anterior rami and the lumbar plexus.

The femoral nerve forms from the L4 anterior rami and has contributions from L2 and L3 fibers. The femoral nerve innervates all 4 components of the quadriceps femoris, including the rectus femoris.[1]

Physiologic Variants

Accessory muscles can arise from the rectus femoris. These variations include a muscle slip from the acetabulum that inserts directly into the vastus lateralis. The rectus femoris muscle's proximal origin may also have only 1 instead of 2 heads, with the lone tendon arising from the lower anterior iliac spine and the acetabular origin missing.[1][4]

Surgical Considerations

Acute rectus femoris muscle injuries present with a tearing sensation and abrupt-onset pain. By comparison, subacute injuries manifest gradually, usually with mild-to-moderate pain aggravated by running at the onset. Ultrasonography and MRI aid in the diagnosis and monitoring of injuries to this muscle.[5]

Proximal rectus femoris tears are uncommon but are associated with soccer and other sports activities requiring frequent kicking and sprinting. The direct head is more often involved than the indirect head. Total avulsions involve both. Acute single-head proximal rectus femoris tears may be managed conservatively, though they are prone to recurrence. Surgery is appropriate for total avulsion and recurrent single-head tears. Both conditions have a good prognosis, with patients being able to return to preinjury functional levels after surgery.[16]

A distal rectus femoris tear is more commonly associated with quadriceps tendon ruptures, which affect the quadriceps muscle as a whole. Isolated tears are uncommon. Quadriceps tendon tears are disabling and usually require surgical management.

Avulsed tendon repairs involve sewing the tendon into sutures anchored to the bony origin. The torn tendon may be sewn to the tendon stump if it has sufficient tissue for surgical sewing. Tendon allografts may be used for reconstruction if the tendon edges are unsuitable for repair or cannot be adequately mobilized.[6]

Clinical Significance

The rectus femoris muscle is the most frequently injured quadriceps muscle due to its superficial location. Contusion and rupture of this muscle are common in sports.[5][7][8][9] Trauma and intense exercise can tear the rectus muscle at the proximal or distal insertions.[10] A partial tear is usually treated conservatively with activity modification and positional restriction.[11] Physical therapy may also help rehabilitate torn muscles.

Acute rectus femoris strain typically presents with swelling and moderate to severe pain in the groin or anterior aspect of the hip. Knee extension is also impaired.[12] MRI detects rectus femoris strains.

Myositis ossificans involving the rectus femoris may arise from both traumatic and non-traumatic causes.[13] Most patients are managed conservatively with excellent outcomes. Typically, athletes progress to light activity in 2 or 3 months, full activity in 6 months, and return to their preinjury performance level in 1 year. Persistent cases may require surgical excision.[14]

References

1.

Bordoni B, Varacallo M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 8, 2023. Anatomy, Bony Pelvis and Lower Limb: Thigh Quadriceps Muscle. [ PubMed: 30020706 ]

2.

3.

Mahabadi N, Lew V, Kang M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Apr 11, 2023. Anatomy, Abdomen and Pelvis: Femoral Sheath. [ PubMed: 29494010 ]

4.

Biondi NL, Varacallo M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Aug 8, 2023. Anatomy, Bony Pelvis and Lower Limb: Vastus Lateralis Muscle. [ PubMed: 30335342 ]

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

Disclosure: Christopher Murdock declares no relevant financial relationships with ineligible companies.

Disclosure: Andrew Mudreac declares no relevant financial relationships with ineligible companies.

Disclosure: Kofi Agyeman declares no relevant financial relationships with ineligible companies.

# data[].attributes.attachment_link.title
Anatomy, Abdomen and Pelvis, Rectus Femoris Muscle

# data[].attributes.attachment_link.description
The rectus femoris is the anterior thigh compartment's most superficial and nearly vertically oriented muscle. This bipennate structure is a component of the quadriceps muscle complex, one of the knee's most important dynamic stabilizers.[1] The rectus femoris is also known as the "kicking muscle" for its involvement in activities involving forceful knee extension.

# data[].attributes.body
Taken from the following journal: https://www.sciencedirect.com/science/article/pii/S0031940617300226#tbl0015

# data[].attributes.body
Highlights:

Depth into origins and insertions of the gastrocnemius.

Issues that may arise.

Treatment modalities.

# data[].attributes.attachment_link.readable_text
Introduction

The gastrocnemius muscle is a complex muscle that is fundamentally involved in walking and posture. It affects the entire lower limb and the movement of the hip and the lumbar area. It is a muscular district called to work during daily and sports activities and maintain orthostatism. This article reviews the anatomical and functional information of the gastrocnemius muscle and its embryological derivation. This review will also illustrate the vascular and lymphatic network and the innervating nerve branch. The text describes the concept of fascial continuum, which explains the influence of a muscular district compared to those bordering and distant.

It highlights the anatomical variants such as gastrocnemius tertius or quadriceps gastrocnemius. The text describes some pathological conditions and surgical approaches related to gastrocnemius and the muscle's instrumental and palpatory clinical evaluation. The paper concludes by describing conservative approaches in cases of calf dysfunction, such as physiotherapy and osteopathy.

The important reflection from the text is that a contractile district relates to the body system, and it is limiting to consider a muscle only as an anatomical segment.

Structure and Function

The muscle of the triceps surae is part of the muscles of the posterior area of the leg. It is comprised of three heads, two of which are superficial; the medial head and the lateral head, which form the gastrocnemius muscle, and a deeper muscle, the soleus muscle, and in a small percentage, the plantaris muscle.[1] The triceps surae determines the plantar flexion of the foot and its supination, and with the muscle heads that originate from the femur, it helps to flex the knee. It intervenes decisively in the posture of the foot (the sagittal plane) and walking, running, and jumping.[1] While walking, the quadriceps muscle and the gastrocnemius activate, particularly during the first phase of the foot support and the swing phase; in these movements, there is a complete extension of the knee. The gastrocnemius intervenes to stop the tibia translation forward while the quadriceps extend the knee.[2] The medial head of the gastrocnemius muscle intervenes with greater power in plantar flexion than the lateral head.[1] In a walk without running, it is the soleus muscle that intervenes most in the plantar flexion of the foot.[1]

Anatomy

The medial head of the gastrocnemius originates from the epicondyle and the posterior surface of the medial condyle of the femur. We can identify a lateral origin and a medial origin. The latter involves a flattened, strong, thicker tendon, which contacts the upper area of the medial condyle, below the tendon's insertion of the muscle adductor magnus, and the medial supracondylar crest. The lateral origin has a short tendon in admixture with muscle fibers contacting the knee joint capsule and on the popliteal area of the medial femoral condyle. It knows this insertion zone as the medial supracondylar tubercle.[1] The medial head is generally thicker and wider than the lateral head. At the medial condyle of the femur/origin of the medial head, there is a serous bursa, which allows the proximal muscular body of the lateral head of the muscle to slide.[1] This bursa comes in contact with the bursa of the semimembranosus muscle, creating a functional link of continuity. The bursa involving the medial head may be the site of Baker's cyst.

The lateral head of the gastrocnemius originates from the lateral surfaces of the epicondyle, in a posterior and lateral fossa, proximal to the popliteal muscle tendon, attaching to the lateral supracondylar crest.[1] Tendon and muscle fibers originate from the joint capsule of the knee and, in a lower percentage compared to the medial area, from a lateral supracondylar tubercle. In 10% to 30% of the population, there is a small sesamoid bone (Fabella), which is imbibed in the origin of the lateral head of the gastrocnemius. The fabella is covered by hyaline cartilage and is articulated with the lateral femoral condyle. It rarely connects with the medial head. It could play a role in stabilizing the knee. There are bursae similar to those found with the medial head (not as complex as the previous one and not always) at the level of the lateral head.[1]

The two heads are directed downward, delimiting the popliteal fossa, and at the level of the middle third of the leg, they unite to constitute a large aponeurosis that completely covers the anterior area of the muscular bodies going downward. The aponeurosis continues distally as a calcaneal tendon. The gastrocnemius muscle is a bi-articular muscle and morphologically defined as pennate.

The soleus muscle has a fibrous origin from the head and the posterior face of the fibula, from the soleus muscle line of the posterior face of the tibial bone, and the fibrous arch between the fibula head and the soleus muscle line-a wide, flattened belly forms from these origins, which continues in a large aponeurosis directed downward. At the level of the lower third of the leg, it joins the deep fascia of the aponeurosis of the gastrocnemius muscle, constituting the calcaneal ligament. The latter reaches the foot and inserts into the tuberosity of the heel; a mucous bursa separates the calcaneal bone and the deep face of the tendon; a second, more superficial bursa lies between the skin and the tendon (subcutaneous calcaneal bursa). The soleus muscle is mono-articular and is covered by the twins of the gastrocnemius.[1] See Figure 1.

The plantaris muscle, absent in about 10% of the population, inserts with a tendon lateral to the insertion of the calcaneal tendon (rarely joining); in cases of rupture of the calcaneal tendon, the plantaris muscle-tendon remains intact.

Embryology

At the eighth week of gestation, the skeletal muscles are formed. The gastrocnemius muscle derives from the paraxial mesoderm, the latter in the post-otic region from the somites. The somites differ in dermomyotome, sclerotomies, and myotomes.[3] The precursors of the muscles derive from the lips of the lateral dermatomyotome. The last stage of muscle development is identified by the presence of transverse striations, a typical sign of the maturation of the muscles.[3] At this stage, there are the myotubes (precursor cells of muscle fibers).

Blood Supply and Lymphatics

The sural arteries (or twin arteries) are collateral arteries of the popliteal artery (which derive from the femoral artery). The sural arteries originate at the level of the popliteal fossa and nourish the twin muscles and the soleus. The fibular circumflex artery, derived from the collateral branches of the posterior tibial artery (which derives from the popliteal artery), nourishes some deep muscles, including the soleus.[4] The popliteal vein drains the venous blood from the gastrocnemius muscle. The popliteal vein is formed by the junction of the saphenous vein, posterior tibial veins, and anterior tibial veins on the lower edge of the popliteal muscle and the medial side of the popliteal artery. Ascending, it passes through the opening of the adductor to become the femoral vein.[5][6] The perforating veins that influence blood flow during calf contraction are called Cockett veins.[6]

The lymphatic vessels of the lower limb are divided into superficial and deep. The superficial vessels originate from the integuments, run subcutaneously, and are grouped into medial, lateral, and superficial gluteal collectors, which then flow into the superficial inguinal lymphatic grouping. The deep vessels originate from the bones, muscles, and joints to become satellites of the deep vessels, ending in the deep inguinal lymph nodes. The lumbar lymph nodes left, and right are the efferent vessels of the lateral aortic group of lumboaortic lymph nodes. They drain the sub-umbilical region of the abdominal wall, the pelvic wall, the perineal wall, the lower limb, and the vascularized territory from the splanchnic branches of the aorta.[7]

Nerves

The tibial nerve is the reference nerve structure for the gastrocnemius muscle.

Before dividing at the level of the upper corner of the popliteal fossa, in a tibial branch and a peroneal branch, these two nerves constitute the main trunk of the sciatic nerve; they are separated inside the sciatic by a fascial/adipose tissue or septum, known as the septum of Compton-Cruveilhier.[8]

The tibial nerve arises from the trunk of the sciatic nerve above the popliteal fossa and descends into the popliteal fossa between the two heads of the gastrocnemius, anterior to the soleus muscle; about 15 centimeters above the ankle, it travels medially to the calcaneal tendon and superficially.[9] In the calf, it innervates several contractile districts of the superficial and deep posterior compartment: the plantar, the gastrocnemius; the soleus; the tibialis posterior; the long flexor of the fingers; the long flexor of the hallux.[9]

Muscles

Myofascial Continuum

When talking about skeletal muscle, one should think not of a single district but a myofascial continuum. The gastrocnemius muscle tension is transmitted not only to the foot but to the knee, hip, and lumbar area, thanks to its fascial connections: the latter is the anatomical links with other tendons (insertion and origin) and the fascia covering muscles. A shortened gastrocnemius muscle could cause dysfunctions in the physiological movements of the hip, decreasing its anteversion.[10] The fascial system plays a fundamental role in transmitting the force produced by the contraction of the contractile component of the muscle.[11]

When the muscle contracts, the first tissue that receives the tension produced is the connective tissue of the muscle stroma. The tension travels along the axis of the fibers through the epimysium, up to the tendon (if the muscle has a fusiform morphology), with a longitudinal pattern. If the muscle is pennate, like the gastrocnemius, the force produced follows the interfilament inside the Z line of the sarcomeres, up to the sarcolemma, the extracellular matrix, and the epimysium (and hence the tendon); the vector has a transverse trajectory, and only later, follows the longitudinal structures of the epimysium.[12]

In the first case, with the fusiform muscles, the speed of arrival of the expressed force is rapid but less effective, while in the second case, with the pennate muscles, the arrival of force to the tendons is slower but with values of tension expressed more wide. For example, the biceps brachii is faster but weaker than the deltoid muscle, just as the gastrocnemius muscle is stronger than the soleus muscle, which is faster but less powerful.

The calcaneal tendon is the widest and longest tendon in the human body.[1] The tendon connects with the plantar fascia of the foot (from the calcaneus to the metatarsal heads), which is considered fundamental for the stability of the different tarsal joints. Dysfunction of the gastrocnemius muscle negatively affects the tension of the plantar fascia, altering the physiological support of the foot.[13]

Muscular Phenotypes

Another important difference between the gastrocnemius muscle and the soleus muscle is that the first muscle has a majority of anaerobic or white fibers, while the latter is rich in aerobic or red fibers.[14] In a movement that requires the expression of great power quickly, the gastrocnemius will be called to intervene. If walking is the main component of physical effort, it will be the soleus muscle that plays the main role.

Physiologic Variants

Nature and anatomy always reserve extraordinary adaptations and differences. The gastrocnemius does not escape the rule. One can find the gastrocnemius tertius, an accessory muscle district that is an integral part of the muscle, as well as an extra soleus muscle. Rarely, these accessory muscles can be found simultaneously and bilaterally, without effects on joint function and innervation by the tibial nerve. The gastrocnemius tertius could cause problems in the popliteal fossa if it should come into contact with the popliteal artery and create an arterial entrapment.[15]

There are anatomical variants in the literature. Specifically, it is possible to find a quadriceps gastrocnemius muscle. Practitioners do not know if this variant causes dysfunctions and pathologies; the tibial nerve innervates this variant.[16]

The gastrocnemius may present the absence of a head (in particular the lateral one) or be absent or present anomalies not in the number of heads but with respect to the area of origin.[16] A case of compression of the popliteal artery and popliteal vein is reported in the literature due to an anomaly of the origin of the lateral head of the gastrocnemius. This alteration led to the formation of a venous and arterial thrombus, resulting in secondary pulmonary hypertension.[17]

Surgical Considerations

One of the dangers during gastrocnemius muscle surgery is the accidental injury of the sural nerve. Furthermore, the patient may be unsatisfied with a poor cosmetic result. It is possible to divide the anatomical area for a surgical procedure into five levels, with the fifth level starting proximally and the last level ending distally.[1] The fifth level includes the origin of the two muscle heads of the popliteal area; the fourth level involves the muscular portion of the two heads (before arriving at the aponeurosis); the third level begins where the contractile fibers of the muscle merge with the aponeurosis until the fusion of the fascial area of the gastrocnemius and the soleus. The second level includes the aponeurosis of the gastrocnemius and the soleus, up to the distal part of the calcaneal tendon, before its insertion; the fifth and last level comprises the portion of the calcaneal tendon that involves the heel and its final insertion.[1]

Reconstruction

The medial head of the gastrocnemius can be used to detect an innervated flap; the latter is used for the recovery of several pathological pictures, such as the foot drop, muscular defects of the upper limbs, Volkmann contractures, and for the recovery of the movement of the tongue. In cases of post-surgical infection (replacement with knee joint prosthesis), a gastrocnemius muscle flap can be used to improve the clinical results of cicatrization. A medial head flap is used to improve the cicatrization of transtibial amputation intervention.[18] Using a part of the gastrocnemius muscle has few negative collaterals and fast use.[19]

Baker Cyst

This popliteal cyst comes from the doctor who first described it in 1877, surgeon William Morrant Baker. These cysts are formed in the presence of an already compromised knee joint (osteoarthritis, meniscal rupture), which creates effusions from the articulation toward the posterior area of the knee, toward the bursa of the medial head of the gastrocnemius. Often, there is a joint material in the cyst coming from inside the capsule; this transit of fluids is always unidirectional. Generally, they are asymptomatic and discovered with the magnetic resonance in a knee with known functional problems. Surgical excision is used only in limited cases, with a posterior approach.[20]

Injury of Muscle Fibers or Tearing

Muscle tearing or muscle distraction can be common for those who play sports. Depending on the severity of the injury, one can distinguish three types of calf muscle tears. The most serious is the third degree, which involves at least three-quarters of muscle mass. In first-degree injuries, when the lesions are modest and do not exceed 5% of muscle mass, there is no significant reduction in muscle function. The subject usually experiences a sensation of continuous cramping. In the second-degree lesion, the manifestations are those of pain; pain is felt during movement. Generally, the indications for the first two levels are conservative. In the third degree, muscular tear, hematomas, edema, and swelling may appear, the pain becomes very strong, and the subject is in a disabling condition because he can not move the leg. This lesion is felt on palpation as a depression, which testifies to the extent of the breakup.[1]

Achilles Tendon Disorders

Achilles tendon (or "calcaneal" tendon) disorders are relatively common injuries. Disorders can occur in adolescents and adults, and etiologies include traumatic and nontraumatic mechanisms. The spectrum of pathology ranges from insertional tendinitis, intra-substance tears and/or tendinopathy, and partial versus complete ruptures.[21]

Acute ruptures of the Achilles tendon may be misdiagnosed in up to 25% of cases. The most common misdiagnosis is that of an ankle sprain. Missed or delayed diagnosis can be problematic for patients, and a chronically dysfunctional tendon is harder to repair surgically.[22] Acute and sudden pain in the calcaneal tendon, the functional impotence, and the palpatory sensation of the interruption of the tendon are the basis of the diagnostic suspicion confirmed by the ultrasound investigation. This injury is the consequence of chronic tendinitis, often unrecognized or undervalued. Other causes may be related to systemic diseases (diabetes, lupus erythematosus) or chronic administration of steroid drugs.[23] Collagen fibers change their phenotype; with increased collagen type III, they are less elastic and less able to handle mechanical stresses.

The injury mainly involves jumpers, runners, soccer players, and tennis players, realized as a consequence of a sharp muscle contraction. Treatment is surgical. Many types of tendon suture can be performed. The calcaneal tenorrhaphy is now performed with a technique that uses tiny incisions to avoid the scarring disorders related to very long incisions and the benefit of reduced recovery times. Sometimes, a reinforcing plastic is made using the tendon of the gracilis muscle plantar or the peroneus longus muscle tendon.[22]

Gastrocnemius Shortening

Several diseases cause a shortening of the gastrocnemius muscle: lesions of the nervous system, central or peripheral, hereditary diseases such as myopathy and dystrophies, or acquired disease like diabetes. All age groups can be involved, from the children to the elderly. The most evident functional disturbance of this shortening is the equine foot, where the tip of the foot points toward the bottom, and the gait is characterized by the toe of the foot to the ground. First of all, it is essential to distinguish the cause of equinism, evaluate its characteristics in walking, and establish an appropriate treatment. The treatment uses conservative methods (physiotherapy, stretching, tutors, botulinum toxin) and surgical methods, such as stretching the calcaneal tendon.

The Silfverskiold test makes it possible to discriminate whether the equinism is caused by a retraction of the soleus muscle (extended knee) or the gastrocnemius muscle (flexed knee). If the ankle can be dorsiflexed with the knee bent at 90 degrees but can not be dorsiflexed with the extended knee, it is probable that the gastrocnemius muscle is shortened, but not the soleus; it can be done with a patient sitting or lying on the back.

The most recent surgical procedures use endoscopy to reduce collateral damage by creating a small cut at leg level 4; the release of the calcaneal tendon could produce an evaluation error with an excessive elongation of muscle.[24][25]

Clinical Significance

The clinician should first observe how the patient moves, how he compensates, and whether there are any differences in muscle shape between the two calves. The clinician will palpate the tone of the muscles, both on the fleshy level and on the fascia (aponeurosis and tendon); the movement is first activated by the patient so as not to condition the person or the perception of pain. Passively evaluate the knee and ankle after active movements. Finally, the anamnesis will be fundamental.

Correct Evaluation of the Equine Foot

To perform the Silfverskiold test correctly, the force applied by the operator must be about 2 kilos, with the hand placed under the sole or under the head of the second metatarsal. If the patient has a non-rigid flat foot, the heel is in a position of valgism; in this case, pure dorsiflexion does not occur, with a movement that expresses itself on an oblique plane, thanks to the joints of the hindfoot. To obtain a clean movement and to improve the execution of the test, the hindfoot must be placed in a neutral or slightly variant position. Before performing this test, it is possible to carry out the Taloche test or the Taloche sign. If the gastrocnemius muscle is contracted, the patient will not be able to maintain the orthostatism if it is resting on an inclined plane (a tablet).[26]

Tibial Nerve Trapping

A tibial nerve tract can be palpated both in the popliteal cavity and slightly below; in these areas, it is superficial and very simple to hear manually. The nerve becomes the posterior tibial nerve when it reaches the ankle area and travels in the tarsal tunnel, deeper than the flexor retinaculum and the posterior tibial artery and vein.[9] Another point of contact with the tibial nerve is behind the medial malleolus, just below and medially the calcaneal tendon; with the thumb, look for the malleolus and slowly bring a finger toward the tendon with a horizontal pattern. It is often a painful area of which the patient is unaware.

The tibial nerve rarely undergoes an entrapment to the popliteal cavity, while it more frequently has an entrapment to the tarsal tunnel, as previously mentioned. At this level, there are different causes such as a tumor, a previous intervention, trauma, systemic diseases such as diabetes or neurological diseases, and deformity of the same foot. It is possible to carry out a stress test with the Tinel test to evoke the painful or paresthesia symptoms.[9]

Calcaneal Tendon Rupture Evaluation

The Thompson squeeze test or sign of Simmonds-Thompson is a semiotics maneuver of the objective examination of the lower limb, which, if positive, indicates a complete rupture of the calcaneal tendon. The examiner compresses the patient's calf between the fingers, lying in a prone position with the feet over the edge of the bed. Under normal conditions, the squeezing of the muscular masses causes the plantar flexion of the foot, while when there is a full-thickness tear of the posterior calcaneal tendon structures, it evokes intense pain with no movement of the foot.[27]

Other Issues

Physiotherapy for Tears: First and Second Degree

With an anatomical lesion, the treatment in the first 24 to 48 hours consists of the RICE protocol ( Rest, Ice, Compression, E levation). This treatment avoids the aggravation of the lesion and inflammation. This protocol includes rest, ice 3 to 4 times a day for 20 minutes, bandaging, and elevation of the limb.

Generally, muscle relaxants and analgesics are taken, the latter being preferable to anti-inflammatories (NSAIDs) because post-injury inflammation is a physiological mechanism of repair of the human body. The athlete must stay away from the field for a variable time depending on the size of the injury, ranging from 1 week to 2 months.[28]

After an injury, the muscle closes the lesion forming a fibrous cicatricial result, that is, a less elastic connective tissue of the muscle. If the muscle is immobilized, the scar is formed with fibers that can go in any direction; if, instead, the muscle is kept moving, the fibers of the connective tissue that is formed have an arrangement according to the lines of force and are more elastic. Instrumental physiotherapy with CO2 laser, ultrasound, and Tecar therapy is indicated; when the lesion is closed, activity is needed to make the muscle more elastic and prevent recurrences. Stretching and eccentric reinforcement are important. In general, the elective exam is muscle-tendon ultrasound, but magnetic resonance may be necessary if the lesion is deep in a very developed muscle.[28]

Rupture of the Calcaneal Tendon

The rehabilitation treatment of calcaneal tendon rupture has a duration of about 12 weeks, at three sessions per week, alternating between the gym and swimming pool, including massage on the scar and the calf muscles, physical therapies for pain and swelling, exercises for the articulation, for restoring strength and correct walking and running.[22]

After about three months, it is possible to perform an isokinetic test to measure the muscular strength of the leg. Based on the result, the therapeutic protocol must adapt to the athlete's sport to speed up the return, refining the recovery of the coordination of the specific technical gesture of the sport practiced.[29]

Kinesio Taping

The application of elastic bands on the muscle tissue could be helpful for some conditions of the gastrocnemius muscle. In the presence of trigger points, taping has proved useful in reducing symptomatology.[30]

It would seem that the use of taping on the calcaneal tendon has a rationale for preventing sports injuries.[31]

Osteopathy

The osteopathic approach with gentle techniques in the presence of scars may help prevent further pain and improve function.[32]

Osteopathy in patients with tendinitis has proven to help reduce symptomatology and improve the active movement of plantar flexion.[33]

Popliteal Fossa: An Important Area for the Function of the Gastrocnemius Muscle

In the popliteal fossa, a fundamental site for the gastrocnemius muscle, it is possible to find tensor fasciae suralis (also known as ischioaponeuroticus), a muscle that could disturb the function of the calf area and is clinically relevant. One of the variants of the tensor fasciae suralis muscle is to extend over the lateral head of the gastrocnemius muscle, innervated by the common peroneal nerve. The abnormal presence of this muscle could alter the function of the gastrocnemius, with dysfunctional problems in the knee and ankle, with repeated traumas in these articular areas. This is because the lines of tension are altered and do not allow the joints to withstand the workloads correctly, from running to jumping.

Another muscle variable that could alter the function of the gastrocnemius muscle is a strip of muscle (it does not have a name) that goes from the medial head transversely to the tendon tissue of the biceps femoris; this anomaly is innervated by the tibial nerve. This muscular structure covers the neurovascular components of the popliteal fossa. Its hypertrophy could cause paresthesia, entrapment of the popliteal artery, and create abnormal tensions for the correct function of the calf area.

A Baker cyst in the popliteal fossa could mimic a problem of deep vein thrombosis. An examination with ultrasound may not be sufficient to understand the reasons for the pain (for example, in the area of insertion of the gastrocnemius muscle when bending the knee). A further consideration is the MRI. If the pharmacological therapy for thrombosis does not remove the pain from the popliteal fossa, one of the etiologies reported in the literature is the presence of this cyst.

Dry Needling

Clinicians and physiotherapists use dry needling to improve the muscular response to previous sports injuries or for the presence of trigger points. They are easy-to-use tools and, according to literature, can increase the muscular performance of the gastrocnemius muscle in sports subjects or the presence of neurological pathology.[34][35]

References

Disclosure: Bruno Bordoni declares no relevant financial relationships with ineligible companies.

Disclosure: Matthew Varacallo declares no relevant financial relationships with ineligible companies.

# data[].attributes.attachment_link.title
Anatomy, Bony Pelvis and Lower Limb, Gastrocnemius Muscle

# data[].attributes.attachment_link.description
The gastrocnemius muscle is a complex muscle that is fundamentally involved in walking and posture. It affects the entire lower limb and the movement of the hip and the lumbar area. It is a muscular district called to work during daily and sports activities and maintain orthostatism.

# data[].attributes.attachment_link.description
Search from 634 Gastrocnemius stock photos, pictures and royalty-free images from iStock. Find high-quality stock photos that you won't find anywhere else.

# data[].attributes.attachment_link.title
Anatomy Of The Gastrocnemius Muscle - Everything You Need To Know - Dr. Nabil Ebraheim

# data[].attributes.attachment_link.title
Soleus Anatomy: Origin, Insertion & Action

# data[].attributes.attachment_link.readable_text
Continuing Education Activity

The gastrocnemius muscle consists of two heads. The medial head originates from the posterior medial femoral condyle while the lateral head arises from the posterior lateral femoral condyle. The gastrocnemius muscle is vulnerable to injury because it crosses 3 joints: the knee, the ankle, and the subtalar joint. While Achilles tendon injury is common, injuries over the posterior calf, including the gastrocnemius, soleus, plantaris, and flexor hallucis longus muscles, are far less prevalent. Proper and timely diagnosis is essential to treat patients with posterior calf injuries. Patients generally recover well if they received appropriate diagnosis and treatment. This activity reviews the etiology, presentation, evaluation, and management of gastrocnemius strain and reviews the role of the interprofessional team in the care of patients with this condition.

Objectives:

Describe the common mechanisms of injury that lead to gastrocnemius strains.

Review the appropriate history, physical, and evaluation of gastrocnemius strain.

Outline the treatment and management options available for gastrocnemius strain.

Summarize the importance of communication amongst the interprofessional team to enhance the care of patients with gastrocnemius strain.

Access free multiple choice questions on this topic.
Introduction

The gastrocnemius muscle consists of 2 heads. The medial head originates from the posterior medial femoral condyle while the lateral head arises from the posterior lateral femoral condyle. The gastrocnemius muscle is vulnerable to injury because it crosses 3 joints: the knee, the ankle, and the subtalar joint.[1][2] The medial and lateral heads of the gastrocnemius muscle arise from 2 separate proximal attachments on the posterior aspect of the femoral condyles. Distal to the myotendinous junction, the gastrocnemius muscle becomes a flat aponeurosis and coalesces with the soleus aponeurosis to form the Achilles tendon. While Achilles tendon injury is common, injuries over the posterior calf, including the gastrocnemius, soleus, plantaris, and flexor hallucis longus muscles, are far less prevalent.[3][4] Proper and timely diagnosis is essential to treat patients with posterior calf injuries. Patients generally recover well if they receive appropriate diagnosis and treatment.

Etiology

Gastrocnemius strain usually develops at the posture of the knee. This is during maximal extension of the knee with the ankle in full dorsiflexion, which maximally stretches the gastrocnemius muscle. The posture increases the tension of the elastic elements of the muscle to let them approach the threshold of muscle tear, especially during eccentric contraction of the muscle.

Epidemiology

Gastrocnemius strain occurs most commonly in middle-aged or older patients. Gastrocnemius strain may also occur in younger athletes and has been termed "tennis leg."[5] The term is derived from the posture to serve tennis, which involves maximal knee extension and ankle dorsiflexion. The medial head of the gastrocnemius muscle is injured more commonly than the lateral head as some studies have shown the muscular activity to be more active at the medial head than the lateral head.[6]

Gastrocnemius injury is common in young athletes practicing exercises such as racquet sports, running, basketball, football, and skiing. One report described that this injury happened following praying namaz.[3] The activity mentioned above requires the participant to kneel with the head touching the ground. The calf muscle is eccentrically stretched when the participant attempts to stand up.

Pathophysiology

Muscle fatigue and impaired coordination are common factors contributing to gastrocnemius muscle strain. Some research has revealed that impaired vascular supply to the gastrocnemius muscle might result in weakness of the musculotendinous unit. Failure of muscle relaxation also plays a role in causing muscle strain. Due to the inability to fully relax, muscle fibers are more vulnerable to stretching injury.[5] The associated symptoms include muscle stiffness, indicating a decline of muscular elasticity. A common prodrome before the injury event comprises of a dull muscle ache.[5]

History and Physical

Often, patients will complain of feeling like something struck them on their calf. Accompanying this feeling is an audible snap or pop, sounding like breaking a twig. At the moment of injury, there may be no pain. After the patient takes a few steps, the pain develops on the posteromedial part of the calf. Intense pain may limit their ability to ambulate.[5] Prodromal symptoms play an interesting role in gastrocnemius muscle injury. Patients may recall similar prodromic discomfort. There might be dull pain first noted on the affected calf. One report states that 20% of patients experience prodromic symptoms.[7]

The physical examination will frequently demonstrate edema, ecchymosis, and tenderness over the muscle tear site and possibly extending to the knee and ankle. Practitioners should palpate the calf along the full length of the muscles. Gastrocnemius strains usually present with pain on palpation of the medial belly or the musculotendinous junction. In soleus strains, the tenderness is frequently more lateral.[1] A subcutaneous gap may be palpated if there is a frank retraction across the injury site. The Achilles tendon should be palpated to determine if it is intact. If the clinician is unable to ascertain this from palpation, the Thompson test can be done.[1] The peripheral pulses should be assessed as well. Provocative muscle testing should be done. The patient may have severe pain during passive ankle dorsiflexion or resistive plantar flexion and this can be indicative of a more severe injury.[3] A neurological examination should include both motor and sensory testing. Some patients with a complete tear may develop a large hematoma. The hematoma may compress the sural nerve and cause loss of sensation in the lateral calf and ankle.

Evaluation

Although the diagnosis of a gastrocnemius strain is mainly clinical, imaging studies can assist in differentiating partial and complete ruptures. The utility of plain films and computed tomography scans in soft tissue injury is limited. Ultrasound offers the advantage of nonionizing radiation and is relatively inexpensive. The examiner should be aware of sonographic findings such as the disruption of the normal fiber arrangement at the myotendinous junction, hematoma, and fluid collection between the gastrocnemius and soleus muscles. Ultrasonography can also differentiate partial from complete tears of the muscle, and determine the size of the hematoma. A bigger hematoma usually indicates a complete gastrocnemius muscle tear rather than a partial tear. Ultrasound can also determine the size of a complete rupture and is helpful in guiding the percutaneous aspiration of the hematoma. Deep vein thrombosis (DVT) can also be scrutinized by using ultrasound, which is sometimes concomitant with gastrocnemius strain.[3][8]

One study of 141 patients with clinically diagnosed tennis leg showed that 67% had a partial tear of medial gastrocnemius, 1.4% had associated plantaris tendon rupture, and 21% had intermuscular fluid collection without medial gastrocnemius muscle tear. Ten percent had deep vein thrombosis without visible gastrocnemius pathology.[9]

Sonography is useful to follow up the healing course. Signs of recovery on sonography include a progressive decrease in the hematoma size, reparative tissue signified by a peripheral hypoechoic area that extends toward the center, and rearrangement of muscle fibers.[10]

Magnetic resonance imaging (MRI) is indispensable in certain conditions as it offers outstanding soft tissue imaging. Findings include rupture or discontinuity of muscle fibers and retraction of the torn muscle fibers. MRI also allows differentiation between gastrocnemius and Achilles tendon injury, which can help to improve direct treatment. MRI can also provide an assessment of surrounding connective tissues in some muscle injury cases. Three MRI studies reported that connective tissue injury plays an important role to evaluate the return to sports after muscle injury.[11][12][13]

Treatment / Management

Early and accurate diagnosis and treatment can have a major impact on patient outcomes. Early treatment emphasizes symptomatic relief. Limiting further bleeding, decreasing pain, and preventing joint contracture are the main goals of early management of the gastrocnemius strain. Rest, ice packing, compressive wrapping, and elevation are common measures to minimize swelling and pain of injured muscle. The application of moist heat and massage are considered contraindicated early in treatment as they are thought to increase the risk of further hemorrhage.[1]

Medications may be necessary to reduce pain and muscle spasms. Early mobilization is also paramount for preventing contracture. Nonsteroidal anti-inflammatory drugs are not recommended within the first 24 to 72 hours of injury due to an increased risk of bleeding from their antiplatelet effects. Celecoxib and possibly other COX-inhibitors are possible options during this period due to their weaker antiplatelet effect. Acetaminophen or narcotic pain medication can also be used.[1] If a patient is not improving despite these measures, reexamination and imaging studies should be considered to look for complications or possible surgical indications.[1]

Once the acute phase of treatment has been successful, that patient should be started on the rehabilitative phase of treatment. It is important to prescribe physical therapy to promote functional recovery. Initially, gentle stretching can help lengthen the intramuscular scar of the injured muscle. Later, strengthening, heel raising, and proprioceptive exercise should be added as well as core muscle strengthening and general reconditioning. The patient is allowed to ambulate as tolerated and to increase the activity level gradually after the pain subsides. In patients suffering from severe injury, weight-bearing of the affected lower extremity should be limited, and a cast or orthosis may be needed for ambulation. Weight-bearing exercise or dorsiflexion stretching should be delayed until the pain subsides.

Most patients recover well under non-operative treatment. Surgical repairing of the muscle tear may be needed in certain cases. However, the procedure is technically challenging due to the difficulty of performing a suture through muscle tissue. Fibrosis and contracture at the incision site appear to be another concern. The absolute surgical indication is still unclear.[1][14]

Differential Diagnosis

The differential diagnosis includes:

Among calf muscle injuries, injury of the medial head of the gastrocnemius muscle remains the most frequent. Plantaris strain is relatively rare. Soleus muscle has a lower risk of injury in contrast to the gastrocnemius muscle. The soleus muscle mainly consists of type 1 slow-twitch muscle fibers and crosses only the ankle joint. Soleus strain tends to be less severe than gastrocnemius strain.[1]

Staging

Grade 1 Injury (Mild)

The patients may feel a sharp pain at the time of injury or pain with activity. They are usually able to continue the activity. There may be no or minimal loss of strength and range of motion. One MRI study exposes bright signals on fluid-sensitive sequences with less than 5% feathery appearance of muscle fiber involved. Pathologically, grade 1 injury indicates less than 10% of muscle fibers disrupted.

Grade 2 Injury (Moderate)

The patient cannot walk at the moment of injury. The patient may complain of weakness during ankle dorsiflexion and plantar flexion. One MRI study concedes the change in myotendinous junction as edema or hemorrhage. Grade 2 injury implies 10% to 50% disruption of muscle fibers.

Grade 3 Injury (Severe)

There may be a palpable defect on the affected calf. The MRI findings include complete disruption of the continuity of muscle, wavy tendon morphology and retraction, and extensive hemorrhage or edema. Dixon et al. suggested that grade 3 injury indicates 50% to 100% disruption of muscle fibers.[1][10]

Prognosis

The majority of studies indicate that gastrocnemius strains mostly have a good prognosis. Most patients can have a marked decrease in pain and return to exercise after adequate management.

Deterrence and Patient Education

Patients should be educated about the importance of maintaining their flexibility in the gastrocnemius muscle. Regular gentle stretching activities for the lower extremities may help lower the risk of gastrocnemius strains and reinjury. Patients should also be encouraged to follow up with their physical therapist as recommended and continue with their prescribed home exercise program during their rehabilitation. Athletes should be educated that return to play should not occur before they are pain-free and have recovered their full range of motion.

Enhancing Healthcare Team Outcomes

Gastrocnemius strain remains a common injury among young athletes and middle to older ages and is best managed with an interprofessional team approach. Many of these patients often first present to the emergency department or the primary care provider. Hence, these professionals need to know about the diagnosis and management of the disorder. Physical therapists will be vital to helping the patient progress through their rehabilitation program. With severe injuries, orthopedic surgeons, sports medicine clinicians, and physiatrists may be required. The key to preventing these injuries is patient education about the importance of warmup and stretching before the formal exercise. For most patients, the prognosis is good. Non-operative treatment often provides optimal symptoms relief and functional recovery. The indication for surgical intervention remains uncertain. [Level 5][15]

References

1.

Bryan Dixon J. Gastrocnemius vs. soleus strain: how to differentiate and deal with calf muscle injuries. Curr Rev Musculoskelet Med. 2009 Jun;2(2):74-7. [ PMC free article: PMC2697334 ] [ PubMed: 19468870 ]

2.

3.

4.

5.

6.

Cibulka M, Wenthe A, Boyle Z, Callier D, Schwerdt A, Jarman D, Strube MJ. VARIATION IN MEDIAL AND LATERAL GASTROCNEMIUS MUSCLE ACTIVITY WITH FOOT POSITION. Int J Sports Phys Ther. 2017 Apr;12(2):233-241. [ PMC free article: PMC5380866 ] [ PubMed: 28515978 ]

7.

8.

9.

10.

Lee JC, Mitchell AW, Healy JC. Imaging of muscle injury in the elite athlete. Br J Radiol. 2012 Aug;85(1016):1173-85. [ PMC free article: PMC3495577 ] [ PubMed: 22496067 ]

11.

12.

13.

14.

15.

# data[].attributes.attachment_link.description
The gastrocnemius muscle consists of 2 heads. The medial head originates from the posterior medial femoral condyle while the lateral head arises from the posterior lateral femoral condyle. The gastrocnemius muscle is vulnerable to injury because it crosses 3 joints: the knee, the ankle, and the subtalar joint.[1][2] The medial and lateral heads of the gastrocnemius muscle arise from 2 separate proximal attachments on the posterior aspect of the femoral condyles.

# data[].attributes.body
This page details the action of the muscle and also includes the origin and insertion

# data[].attributes.attachment_link.readable_text
Author: Emma Jurkovicova, MD * Reviewer: Francesca Salvador, MSc
Last reviewed: September 19, 2023
Reading time: 8 minutes

The soleus muscle is a wide flat leg muscle found on the posterior leg. It runs from just below the knee to the heel and lays immediately deep to the gastrocnemius. These two muscles, along with the plantaris muscle, belong to the group of superficial posterior compartment calf muscles. Soleus' contraction results in strong plantar flexion. It also allows us to maintain an upright posture due to its important role as an antigravity muscle.

Together with the gastrocnemius, they form the three-headed group of muscles referred to as the triceps surae. They both insert on the calcaneus via the calcaneal tendon and act in many basic activities, such as walking, running, and leaping. The size and shape of the triceps surae muscle bellies determine the interindividual differences of human calves' appearance - from slim to rather robust. This article will outline the morphology of the soleus muscle, as well as its functional and clinical anatomy.

Origin

Soleal line, medial border of tibia, head of fibula, posterior border of fibula

Insertion

Posterior surface of calcaneus (via calcaneal tendon)

Innervation

Tibial nerve (S1, S2)

Vascularization

Posterior tibial artery and vein

Function

Talocrural joint: Foot plantar flexion

Clinical relations

Soleus tear, soleus pain

Mnemonic

Stand on your Soles. Gasexplodes!
(refers to the functions of posterior leg muscles; Soleis is for posture, while Gastrocnemius is for explosive movements)

Origin and insertion

The soleus muscle arises from the soleal line on the dorsal surface of the tibia, medial border of the tibia, head of the fibula, and posterior border of the fibula. Part of the fibers arises from the tendinous arch of the soleus, which spans between the tibia and fibula and arches over the popliteal vessels and tibial nerve.

The soleus muscle runs along the gastrocnemius muscle and together they insert onto the posterior surface of the calcaneus via the calcaneal tendon. The calcaneal tendon, commonly called the Achilles tendon, is the strongest tendon of the human body. It is easily visible and palpable at the heel.

Why not make your studies easier with our lower limb muscle chart? These revision tables verified by experts will teach you everything you need to know about the attachments, innervations and functions of the lower extremity muscles.

Learn more about the leg muscles with our learning materials in the following study unit!

Innervation and blood supply

The soleus is innervated by the anterior rami of S1 and S2 spinal nerves, carried by the tibial nerve into the posterior compartment of the leg. Blood supply is provided by two main branches. The superior branch arises from the popliteal artery while the inferior branch arises from peroneal artery (fibular artery) or the posterior tibial artery. The peroneal and posterior tibial arteries are direct branches of the popliteal artery and arise in the popliteal fossa. Minor accessory arteries may also branch off the peroneal, posterior tibial and lateral sural arteries.

A clinically important venous plexus is present in the soleus muscle belly. Physiologically it contributes to the muscle pump of the lower extremity. In pathological circumstances, such as thrombophilia, it constitutes a common site of onset of deep vein thrombosis (DVT). Veins follow the arteries of the same name into the popliteal vein.

Learn more about the leg and knee neurovasculature here and test your knowledge with a quiz!

Function

The soleus' function is closely related to that of the gastrocnemius muscle. Together, they constitute a chief plantar flexor - their contraction results in the plantar flexion of the upper ankle joint, enabling the lifting of the heel against gravity when walking or jumping. The soleus muscle belongs to the group of so-called antigravity muscles (along with the extensors of the leg, gluteus maximus and back muscles), which maintain the upright posture in humans.

As the body's center of gravity is anterior to the ankle joint, the body has a natural tendency to lean forward. This is counteracted by a continuous state of plantar flexion produced mainly by the soleus when standing. For this reason, the soleus consists mostly of type 1 slow, fatigue-resistant fibers. The gastrocnemius muscle is more involved in locomotion where it contributes slightly to the flexion of the knee as it crosses multiple joints. When the knee is flexed, plantar flexion attributes of the gastrocnemius are very limited, making the soleus the main muscle responsible for plantar flexion.

Now it is time to test what you've learned about the soleus and other muscles of the leg with out tailored quiz.

Shorten you studying time with Kenhub 3D muscle anatomy videos! With our videos, you will get to enjoy one of the most detailed 3D models available, designed by a skilled team of anatomy experts and talented 3D artists.

Soleus tear

Torn calf muscles are common injuries seen in primary care and sports medicine clinics. A soleus tear often results from overuse. Injuries occur typically in runners due to fatigue and overtraining. Athletes specialized in long-distance and endurance running or walking, which require constant soleus action, are at higher risk of injury. The symptoms include:

Lateral calf pain on either side of the soleus,

Weakness in the calf and weak plantar flexion, especially with flexed knee

Exercise-related dull calf pain and pain related to foot dorsiflexion or calcaneal tendon pressure

Treatment includes the RICE method: Rest, Icing, Compression, and Elevation. A soleus tear needs to be differentiated from gastrocnemius tear, which is usually a sudden injury caused by quick movements, such as jumping and sprinting. Thanks to its etiology, it is commonly called the tennis leg.

Soleus pain

Soleus pain might be a symptom of a soleus tear or overuse. Affected patients typically report pain onset in the beginning of the exercise, which decreases with time. Once the training is over, the pain usually returns. The muscle pain typically develops in three common areas: calcaneal tendon and muscle belly transition, lateral midsection area, or top of the muscle, which may radiate the pain all the way up into the sacroiliac area. Establishing the diagnosis of a soleus injury might be challenging. Along with clinical symptoms and physical examination, MRI or ultrasound are usually used to determine pathologies within the muscle.

Sudden calf pain may also indicate deep vein thrombosis (DVT), which is a potentially life-threatening medical emergency due to the risk of pulmonary embolism (PE). Besides calf pain, swelling and redness in the calf area are usual symptoms of DVT. Treatment involves anticoagulation with pharmacotherapeutic agents such as heparin, warfarin, and factor Xa inhibitors. Compression stockings are used as a preventative strategy.

Sources

References

Chummy S.Sinnatamby: Last's Anatomy Regional and Applied, 12th Edition, Churchill Livingstone Elsevier.

Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray's Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier.

Frank H.Netter MD: Atlas of Human Anatomy, 5th Edition, Elsevier Saunders.

Layout:

Soleus muscle: want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

What do you prefer to learn with?

"I would honestly say that Kenhub cut my study time in half." - Read more.

Kim Bengochea, Regis University, Denver

© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

# data[].attributes.attachment_link.description
Overview of the origin, insertion and action of soleus muscle. Learn everything about the anatomy of the soleus at Kenhub!

# data[].attributes.headline
Posterior Leg Pain: Understanding Soleus Muscle Injuries

# data[].attributes.subject
Posterior Leg Pain: Understanding Soleus Muscle Injuries

# data[].attributes.headline
Origin, Insertion, Nerve supply and Function

# data[].attributes.subject
Origin, Insertion, Nerve supply and Function

# data[].attributes.attachment_link.title
How to Examine for the Injured Soleus Muscle

# data[].attributes.body
Function of the gastrocnemius and soleus working together to plantar flex and how they look and are set up together

# data[].attributes.attachment_link.title
Institue of Human anatomy gastrocnemius - Google Search

# data[].attributes.attachment_link.description
Search instead for Institue of Human anatomy gastrocnemius

# data[].attributes.attachment_link.title
Anatomy Of The Soleus Muscle - Everything You Need To Know - Dr. Nabil Ebraheim

# data[].attributes.body
Article detailing Gastrocnemius Strains and Treatment

# data[].attributes.attachment_link.readable_text
Abstract

Calf strains are common injuries seen in primary care and sports medicine clinics. Differentiating strains of the gastrocnemius or soleus is important for treatment and prognosis. Simple clinical testing can assist in diagnosis and is aided by knowledge of the anatomy and common clinical presentation.

Keywords: Muscle strain, Calf, Gastrocnemius, Soleus

Introduction

Calf strains are a common injury. The "calf muscle" or triceps surae consists of three separate muscles (the gastrocnemius, soleus, and plantaris) whose aponeuroses unite to form the Achilles tendon. The clinical history and physical exam along with imaging studies allow localization of the injured muscle. Differentiating strains in the gastrocnemius and soleus is particularly important for an accurate prognosis, appropriate treatment, and successful prevention of recurrent injury.

Calf strains are generally regarded as common injuries, particularly in athletes, although specific data on injury rates are sparse [ 1- 5]. In one study of soccer players, calf strains represented 3.6% of injuries over a 5-year period [ 5].

Gastrocnemius strains

Calf strains are most commonly found in the medial head of the gastrocnemius [ 3]. This injury was first described in 1883 in association with tennis and is commonly called tennis leg [ 6]. The classic presentation is of a middle-aged male tennis player who suddenly extends the knee with the foot in dorsiflexion, resulting in immediate pain, disability, and swelling. Pain and disability can last months to years depending on the severity and effectiveness of initial treatment [ 1].

The gastrocnemius is considered at high risk for strains because it crosses two joints (the knee and ankle) and has a high density of type two fast twitch muscle fibers [ 2, 4, 5, 7]. The combination of biarthrodial architecture leading to excessive stretch and rapid forceful contraction of type two muscle fibers results in strain. This mechanism of injury conjures up the image of a cracking whip. Consequently, strains of the gastrocnemius have historically been called coup de fouet or snap of the whip [ 6].

Plantaris strains

The plantaris also crosses the knee and ankle joints prior to its common Achilles tendon insertion on the calcaneus. However, the plantaris is considered largely vestigial and rarely involved in calf strains [ 2, 5]. Isolated strains are difficult to distinguish clinically from strains of the gastrocnemius and can only be identified through imaging [ 5]. If identified recommended treatment is similar to gastrocnemius strains [ 5]. As such, this article will group plantaris strains with strains of the gastrocnemius.

Soleus strains

Strains of the soleus vary in reported occurrence from rare to common [ 3, 5, 8, 9]. Soleus muscle injury may be underreported due to misdiagnosis as thrombophlebitis or lumping of soleus strains with strains of the gastrocnemius [ 9, 10]. Unlike the gastrocnemius the soleus is considered low risk for injury. It crosses only the ankle and is largely comprised of type one slow twitch muscle fibers. Soleus strains also tend to be less dramatic in clinical presentation and more subacute when compared to injuries of the gastrocnemius. The classic presentation is of calf tightness, stiffness, and pain that worsen over days to weeks. Walking or jogging tends to provoke symptoms [ 3]. Swelling and disability are generally mild [ 8].

Differentiating calf muscle strains

Although epidemiology and clinical history can help to distinguish strains of the soleus and gastrocnemius, it is the physical exam that allows us to isolate the site and severity of injury. To localize strains to the gastrocnemius or soleus, a combination of palpation, strength testing, and stretching is required.

Palpation of the calf should occur along the entire length of the muscles and the aponeuroses. It is necessary to identify tenderness, swelling, thickening, defects, and masses if present. Gastrocnemius strains typically present with tenderness in the medial belly or the musculotendinous junction. In soleus strains the pain is often lateral [ 3]. A palpable defect in the muscle helps in localization and suggests more severe injury.

The origin of the gastrocnemius and soleus are anatomically distinct arising from above and below the knee respectively. This allows the examiner to isolate the activation of the muscles by varying the degree of knee flexion. With the knee in maximal flexion the soleus becomes the primary generator of force in plantar flexion. Conversely with the knee in full extension the gastrocnemius provides the greater contribution [ 11]. This relationship allows for more accurate strength testing of the individual calf muscles and enables the clinician to better delineate which muscle has been injured.

A similar approach is used to test pain and flexibility with passive ankle movements and stretching. In this case, the knee is again placed in maximal extension and then subsequently in flexion while the ankle is passively dorsiflexed to cause relative isolated stretch of the gastrocnemius and soleus respectively. Use of this technique for clinical isolation of the gastrocnemius and soleus is key to determining the site of injury and guiding rehabilitate stretching and strengthening exercises as described below.

Additional testing that can be used during evaluation of calf strain includes the Thompson test for complete disruption of the Achilles tendon, circumferential calf measurements to quantify asymmetry and functional movements. These movements may include hopping, running, and jumping in order to illicit more sublet calf muscle dysfunction.

It should be noted that concomitant tears of both the soleus and gastrocnemius are possible. This can complicate the clinical picture. Coexisting strains of the gastrocnemius and soleus were found in 17% of calf strains in one radiology study [ 5].

Although a diagnosis can usually be made on clinical grounds as outlined above, the use of imaging can help if the diagnosis is in doubt. Imaging may also be useful in diagnosis and grading of calf injuries in elite athletes because of unique financial and strategic consequences of return to play decisions [ 5].

Except in rare situations, MRI and musculoskeletal ultrasound (MSK US) are the two choices for imaging. Both can be used to confirm strain, localize the injured muscle and determine extent of injury. Screening MRI usually consists of T1 and T2 series with occasional addition of fluid sensitive fat suppressed sequences. Contrast agents are not routinely recommended [ 5, 12].

MSK US is preferred by some institutions and authors [ 5, 8]. It may be particularly useful when used as part of the initial clinical exam by the sports medicine physician when severe pain and swelling limit clinical testing. Ultrasound may also be valuable in early triage of calf injuries or complaints when a wider differential is in play. Ultrasound has advantages of cost, portability, speed, and ease of use compared to MR when in the hands of an experienced operator.

Grading calf strains

Clinical testing also allows for the grading of calf injuries. All muscle strains are graded 1-3 based on disability, physical finding, and pathologic correlation. Although there is little consistency by authors in the semantics of grading strains, there is consensus on the use of a three-part classification system. This system includes clinical, pathologic, and radiology correlation as noted in the following table [ 1, 7, 13, 14].

Grade 1
1st degree mild

Sharp pain at time of injury or pain with activity. Usually able to continue activity

Mild pain and localized tenderness. Mild spasm and swelling
No or minimal loss of strength and ROM

<10% muscle fiber disruption

Bright signal on fluid-sensitive sequences. Feathery appearance <5% muscle fiber involvement

Grade 2
2nd degree moderate

Unable to continue activity

Clear loss of strength and ROM

>10-50% disruption of muscle fibers

Change in myotendinous junction. Edema and hemorrhage

Grade 3
3rd degree severe

Immediate severe pain, disability

Complete loss of muscle function
Palpable defect or mass. Possible positive Tompson's test

50-100% disruption of muscle fibers

Complete disruption of discontinuity of muscle. Extensive edema and hemorrhage. Wavy tendon morphology and retraction

Treatment for calf strains

Accurate diagnosis and early appropriate treatment can significantly affect duration and amount of disability [ 1]. Complete recovery of strength and flexibility should be achieved prior to return to pre-injury activity. Premature return may result in a prolonged recovery or incomplete return to pre-injury baseline.

Acute treatment is aimed at limiting hemorrhage and pain, as well as preventing complications. Over the first 3-5 days, muscle rest by limiting stretch and contraction, cryotherapy, compressive wrap or tape, and elevation of the leg are generally recommended [ 1, 2, 7, 13, 14]. Simple application of an ACE wrap, heel wedge, and crutch-assisted walking would accomplish these goals. Use of NSAIDs should be restricted in the first 24-72 h due to increased bleeding from antiplatelet effects. Celebrex and possibly other COX-2 inhibitors are an option during this period due to their lack of antiplatelet effect [ 15]. Acetaminophen or narcotic pain medication could also be used. Moist heat and massage early in the healing process are thought to increase the chance of hemorrhage and are generally contra-indicated [ 13]. Although rare, myositis ossifcans and compartment syndrome can complicate acute strains. If symptoms have not improved as expected with acute treatment, reexamination and consideration for imaging studies should be considered to evaluate for complications or surgical indications.

Following successful acute treatment more active rehabilitation strategies can be started. Rehabilitative exercises should isolate the soleus and gastrocnemius by varying knee flexion as described above. Passive stretching of the injured muscle at this stage helps elongate the maturing intermuscular scar and prepares the muscle for strengthening. As range of motion returns, strengthening should begin with unloaded isometric contraction. Ten days after the injury, the developing scar has the same tensile strength as the adjacent muscle and further progression of rehabilitative exercises can begin. Isometric, isotonic, and then dynamic training exercises can be added in a consecutive manner as each type of exercise is completed without pain [ 3, 14]. Application of other physical therapy modalities, including massage, ultrasound and electrical stimulation, could also be considered at this stage.

Surgical consultation should be considered for grade III strains (50-100% disruption of muscle) and for cases of prolonged (4-6 months) pain with evidence of contracture. Contractures suggest the presence of painful and restrictive adhesions that may be amenable to surgical intervention. The presence of large intramuscular hematoma may impair clinical progress and is also an indication for surgical referral [ 14].

Open Access

This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

References

1. Coughlin MJ, Mann RA, Saltzman CL. Surgery of the foot and ankle. 8. Philadelphia: Mosby; 2006. [ Google Scholar]

2. DeLee JC, Drez D, Miller MD, editors. DeLee & Drez's orthopaedic sports medicine; principles and practice. 2. Philadelphia: Saunders; 2003. [ Google Scholar]

3. Brukner P, Khan K. Clinical sports medicine. Revised 2nd ed. Australia: McGraw-Hill; 2002.

6. Fu FH, Stone DA, editors. Sports injuries: mechanisms, prevention, treatment. 2. Philadelphia: Lippincott Williams & Wilkins; 2001. [ Google Scholar]

7. Simon RR, Sherman SC, Koenigsknecht SJ, editors. Emergency orthopedics: the extremities. 5. New York: McGraw-Hill; 2006. [ Google Scholar]

13. Starkey D, Johnson G, editors. Athletic training and sports medicine. Sudbury: Jones and Bartlett Publishers; 2006. [ Google Scholar]

# data[].attributes.attachment_link.title
Gastrocnemius vs. soleus strain: how to differentiate and deal with calf muscle injuries

# data[].attributes.attachment_link.description
Calf strains are common injuries seen in primary care and sports medicine clinics. Differentiating strains of the gastrocnemius or soleus is important for treatment and prognosis. Simple clinical testing can assist in diagnosis and is aided by knowledge of the anatomy and common clinical presentation.

# data[].attributes.attachment_link.title
Gastrocnemius - Actions - Attachments - TeachMeAnatomy

# data[].attributes.attachment_link.description
The gastrocnemius is a muscle within the superficial compartment of the posterior leg. It has medial and lateral heads and forms the characteristic “calf” shape of the leg.

# data[].attributes.attachment_caption
Epidemiology: Gastrocnemius tears & ruptures are the most common injuries. Most likely to present themselves in poorly conditioned male athletes participating in sports that require explosive lower extremity movements where the athlete has not warmed up properly or is fatigued. (Campbell JT. Posterior calf injury. Foot Ankle Clin. 2009 Dec;14(4):761-71. [PubMed] [Reference list])

# data[].attributes.body
Aetiology: Lower risk of injury compared to Gastroc. Most common injuries are strains and ruptures through repetitive sports/ movement patterns. Steady state running is a main cause regardless of training age/ conditioning.

1. Pizzari T. The risks, epidemiology and return to play of calf muscle strain injuries [Internet]. 2021 Mar. Available from: https://www.youtube.com/watch?v=OvC5bn5aGXk

# data[].attributes.attachment_link.readable_text
Abstract

Healthy knees require full range squatting movements. Vastus medialis (VM) muscle regulates and adjusts the extensor apparatus that influences the patellofemoral function. This work was designed to investigate the anatomy and morphometry of vastus medialis oblique (VMO) muscle by widely used imaging techniques and investigate how VMO muscle participates in anterior knee pain. Ten dissected cadaveric specimens were examined, focusing on fiber orientations, origin, insertions and nerve supply of VMO muscle. Magnetic resonance imaging and ultrasound of VMO muscle were recorded. Anatomical cross-sectional areas of VMO muscle were determined in painless and painful knees and statistically analyzed. In cadaveric specimens, there was distinct separation between VM longus and VMO (change in fiber angle or fibro-fascial plane). VMO inserted directly into the medial proximal margin of the patella, capsule of the knee joint and continuous with the patellar tendon. Separate branch of femoral nerve run along the anteromedial border of the muscle. Anatomical cross-sectional area was significantly decreased in painful knee by -17.2%±11.0% at lower end of shaft of femur, -21.1%±6.0% at upper border of patella, -36.7%±11.0% at mid-patellar level. VMO is distinct muscle within quadriceps femoris group. VMO muscle would track the patella medially and participate in last phase of knee extension. Assessment of the VMO muscle anatomical cross-sectional area by ultrasonography may constitute promising and reliable tool to evaluate patellofemoral pain syndrome staging.

Keywords: Vastus medialis, Measures, Anterior knee pain

Introduction

People subjected to excessive impact of physical effort at work are susceptible to knee damages while others complain of anterior knee pain due to conditions like arthritis or simple wear-and-tear with age [ 1]. Patellofemoral pain syndrome (PFPS) is one of the most frequent causes of anterior knee pain in adolescents and adults [ 2, 3]. This illness can much affect patients' capability and quality of life and walk [ 4, 5]. PFPS is often caused by imbalances in the muscles surrounding the knee, which affect the kneecap (patella) and cartilage within the joint. This syndrome most frequently caused by quadriceps weakness or tight lateral structures such as iliotibial band and vastus lateralis (VL). PFPS sometimes gets better on its own without any treatment, though you may have symptoms for several years [ 6].

Clinicians know that injuries to a single part of the musculoskeletal system necessarily impinge on the workings of other (even remotely distant) parts. Thus, weak quadriceps can lead to an unstable knee joint. And an unstable knee joint can lead to weak quadriceps. It's a vicious cycle [ 7]. Such information would be clinically important because it could improve the design of rehabilitation protocols [ 8].

The most distal fibers of vastus medialis (VM) are usually described as a separate part of the muscle, the vastus medialis oblique (VMO) muscle. The primary function of the VMO was medial stabilization of the patella throughout extension. This view has been supported by several authors [ 9- 11]. The quadriceps heads (specifically the VM) are suspected to play a role in pain and degenerative changes in the femoropatellar and femorotibial joints [ 10]. Painful knees frequently had 5.2% lower quadriceps muscle cross-sectional areas than contralateral pain-free knees, whereas thigh flexor and adductor muscles anatomical cross-sectional area (ACSA) didn't differ significantly between painful and pain-free knees [ 12]. Numerous studies have examined the connection amongst muscle strength, structural pathology (e.g. radiographic stage, cartilage loss) and knee symptoms (pain and physical function) [ 12, 13]. Villafañe et al. [ 14] found that muscle strengthening addressed to the symptomatic knee reduced pain.

Magnetic resonance imaging (MRI) of skeletal muscle, allowed investigators to accurately assess muscle mass at an individual time point and its changes over time. However, usage of ultrasound (U/S) has been promoted as a possibly reliable means for the measurement of skeletal muscle mass in young and older healthy volunteers [ 15] and in experimental populations, such as intensive care patients [ 16]. Imaging procedures such as computed tomography (CT) and MRI are expensive means and are not available to the majority of clinicians and researchers. Nevertheless, the literature has not established consensus on the 'best' technique to measure cross sectional muscle area. Because of the need for consensus and standardization for both clinicians and researchers, VMO was investigated in anatomical and morphometric aspects by widely used imaging techniques and was investigated for how VMO participates in anterior knee pain. Within this broader context, we focused on the challenge of early detection of knee problems through radiological measurements of VMO.

Materials and Methods

Cadaveric study

Morphological investigations were performed on ten cadaveric lower limbs, which were provided by the Department of Anatomy, Ain Shams University following the ethical guidelines "On the use of cadavers and parts of cadavers in medical research and for pre-, postgraduate and continued education and research with human subjects". Ten cadaveric specimens (fresh or formalin-based cadaver) were examined. The cadaver parts were investigated using macro-dissection techniques based on a standardized dissection protocol. Each lower limb was placed supine on the dissection table. The femoral nerve and artery were localized and traced distally. With the aid of these neurovascular structures, the rectus femoris, VL, vastus intermedius, and VM were identified. VM was traced from proximal to distal till its insertion. The different parts of VM were studied with special emphasis on the direction of the muscle fibers [ 17].

Participant characteristics

Forty subjects were selected by authors for the present study and divided into two groups. Group (I) (control group); 20 young, healthy males (age=24±3 years, body mass index [BMI]=24.1±2.7 kg/m ²) with no any previous anterior knee pain, trauma, surgery, or other lower extremity disease. The subjects in this group were recruited from the staff and students from Faculty of Medicine, Ain Shams University. Group (II) (study group); 20 males recruited in Ain Shams University Hospital, from 20 to 24 years old (BMI=25.1±1.8 kg/m ²) who presented unilateral PFPS. They were recruited from the waiting list in Rheumatology and Rehabilitation Department. The initial diagnosis was established combining medical records by CT or MRI and the presence of anterior knee pain [ 14, 18]. Patients were included in this study if the following criteria were fulfilled: onset of pain longer than 3 months; positive clinical signs of PFPS (i.e., retro patellar pain, crepitation, pain in patellar grinding, direct patellar compression); no history of physical therapy. The exclusion criteria were previous knee surgery, previous meniscal or ligamentous injuries, or musculoskeletal disorders. Subjects with a lower limbs dissymmetry, with cardiac or vision dysfunctions, or neurologic (whether central or peripheral) which can influence quadriceps circumference, were also excluded from the present study [ 14]. All participants were subjected to MRI and U/S studies of the anterior compartment of the thigh with all experimental procedures conducted in the physical therapy research lab of the hospital from 2017 to 2019 (Retrospective study). Ethical approval for the present study was obtained from the Ethics Committee of Ain Shams University, Faculty of medicine institutional research board (ASU-IRB).

Anatomical Cross-Sectional Area assessments

Magnetic resonance imaging measurements

By means of a MRI scanner (Signa 1.5T; GE Healthcare, Madison, WI, USA), a sequence of axial, sagittal and coronal MRI pictures (echo time: 10 ms, repetition time: 520 ms, matrix: 256×192, field of view: 24 cm, slice thickness: 1 cm) of the entire right thigh was attained after the subject had been lying supine for minimum 20 minutes [ 19]. The subjects lay supine with their legs completely extended and muscles relaxed throughout MRI. From the MRI pictures, the outlines of VMO were manually digitized, and ACSA of the muscles were determined using ImageJ2 software [ 20]. Attention was done to eliminate observable intermuscular adipose and connective tissue. Each image was digitized two times, and the mean values were used for further analysis.

Ultrasonography

ACSA was assessed via the same investigator from images obtained in vivo at rest using US images recorded by a Mindray Z6 Digital Ultrasonic Diagnostic System (Shenzhen Mindray Bio-Medical Electronics Co., Ltd., Shenzhen, China) by using a linear transducer type L4-P with a frequency bandwidth range of 5-10 MHz. ACSA has been assessed by placing the U/S probe transversally relative to the patella and evaluated as the perpendicular space between the skeletal muscle interfaces [ 21]. The transducer was located on the anteromedial thigh along the sagittal axis of the VMO, and clearly visualizes fascicles on the U/S screen. VMO-ACSA was assessed at one fingerbreadth above the patella, upper border of patella and at mid-patellar level (level of femoral condyles). Three images were acquired and stored for offline analysis.

Statistical analysis

The statistical analyses were performed using IBM SPSS Statistics for Windows, Version 20.0 (IBM Co., Armonk, NY, USA). Descriptive statistics (mean and standard deviation [SD]) were used to describe the VMO muscle ACSA in the study groups. Differences between painless and painful knees in addition to differences between MRI and U/S measurements were assessed by two-way analysis of variance (ANOVA) with post-hoc Tukey tests. Furthermore, the difference % in VMO-ACSA (in MRI measurements) for painful vs. painless knee was calculated and expressed [ 10]. Results were considered statistically significant when P-value was<0.05.

Results

Origin, insertion, innervation and architecture of the VMO muscle

VM has a proximal part called the vastus medialis longus (VML) that has vertical fibers. The VMO represents the distal portion of the VM and it is defined by an increased obliquity of the muscle fibers, which originate largely from the same inserting points of pubic part of adductor magnus, the medial lip of linea aspera ( Fig. 1 ) and medial supracondylar line ( Figs. 1 , ​,2). 2 ). The strong muscle belly of VMO finally inserted into the medial proximal margin of the patella and capsule of the knee joint.by fleshy fibers ( Figs. 1 ​1 - 3 ). VMO further has a small region of direct continuity with the patellar tendon ( Fig. 4 ). At the patellar insertion, VMO reached its maximal obliquity. Nine of the 10 limbs analyzed (90%) exhibited a distinct change in muscle fiber direction in VMO. Separate innervation (i.e., a branch that was given off from the posterior division of the femoral nerve was observed in 9 specimens; the VM branch, coursing distally along the anteromedial border of the muscle in a separate fibrotic tunnel ( Fig. 1 ). VMO showed a variable architecture. Seven specimens of the examined cadaveric lower extremities demonstrated fibro-fascial plane of epimysium that would clearly separate the two portions of the VM muscle from one another ( Figs. 1 , ​,3). 3 ). Three specimens showed no separation of VM muscle into two independent parts. Through serial transverse cuts at lower third of femur, VMO covered the anteromedial surface of medial condyle of femur totally at epicondylar level; consequently, the knee joint space was compressed medially ( Fig. 2 ). In addition, in MRI coronal sections of fully extended knee, the medial condyle was pushed posteriorly than the lateral condyle ( Fig. 4 ). To illustrate the VMO muscle in sonogram, we used a diagram from Anatomage. The lower horizontal fibers of VMO were observed to insert in the medial margin of patella as it wraps around the medial femoral condyle. VMO fascicles were transversely oriented near the point of insertion ( Fig. 5 ).

VMO anatomical cross-sectional area (ACSA) in painful vs. painless limbs

The mean values of VMO-ACSA in painful vs. painless knees are summarized in Table 1 . In painless knees, VMO cross sectional area was measured on MRI image from proximal to distal; at lower end of shaft of femur it was 23.2±1.9 cm ² (mean±SD), 16.7±1.7 cm ² at upper border of patella and, 14.1±1.1 cm ² at mid-patellar level (level of femoral condyles). However, no statistically significant difference was found between MRI and U/S measurements in the corresponding levels. In painful knee, all parameters were significantly decreased as compared to painless knee. The noticed decrease in VMO-ACSA in painful knee was expressed as a percentage decrease in relation to painless knee. The differences % in VMO-ACSA were calculated in painful knee vs. painless knee; -17.2%±11.0% at lower end of shaft of femur, -21.1%±6.0% at upper border of patella, -36.7%±11.0% at mid-patellar level.

Table 1

Anatomical level

Painless knee

Painful knee

Difference % in MRI measurements (painful vs. painless knee)

1- Lower end of shaft of femur

MRI measurement

23.2±1.9

19.2±1.6*

-17.2±11.0%

U/S measurement

22.5±0.58

19.5±0.04*

2- Upper border of patella

MRI measurement

16.7±1.7

13.02±1.9*

-21.1±6.0%

U/S measurement

16.5±0.8

12.7±0.62*

3- Mid-patellar level

MRI measurement

14.1±1.1

8.66±0.6*

-36.7±11.0%

U/S measurement

13.5±1.14

9.4±0.08*

Discussion

The quadriceps muscle is a primary contributor to both functional knee joint stability and knee joint loading [ 22]. Dynamic knee joint stability may be compromised when weak quadriceps muscles are unable to provide adequate control of tibial translation during ambulation, increasing risk of damage to joint structures [ 23, 24]. Kim et al. [ 25] found that with respect to knee extensor muscle strength, the more painful knees showed significantly lower strength than the less painful knees at angular velocities and there is a significant association between dynamic balance measurements and quadriceps strength at angular velocity. The current study provides an insight for the morphology of the VMO and its participation in anterior knee pain.

A closer look at the VM reveals that the uppermost fibers (VML) are more vertically oriented while the lower fibers (VMO) are more obliquely oriented [ 11]. VMO has received moderate attention as potentially being more responsible for medial tension on the patella.

Muscles with a larger number of insertion and origin points have a greater impact on the musculoskeletal system when perturbed than muscles with few insertion and origin points [ 26]. The present study showed that VMO originates from medial lip of linea aspera and has attachments to the medial supracondylar line as it wraps around the femur. VMO inserted directly to the upper interior corner of the patella by fleshy fibers. Such finding might reflect the force of movement (applied to the patella) produced by this muscle. Lefebvre et al. [ 9]; Grob et al. [ 17] and Grob et al. [ 27] have demonstrated similar attachment. Motor point stimulation and anatomical dissection clearly showed that VM distal fibers were attached directly to the medial aspect of the patella that contributes to dynamic medial patellar stabilization and opposing its lateral dislocation [ 28]. Altered muscle activity and weaknesses in VMO cause mal-tracking of the patella and subsequent damage to surrounding structures and aching pain [ 29]. In the current research, the oblique fibers of VM are not only involved in the medial border of the patella, but additionally, they have a minor area of direct continuity with the patellar tendon. Also, VMO muscle fibers merge with the knee joint capsule. These observations were in accordance with a previous study [ 17]. Recently, Castanov et al. [ 11] stated that VMO attached to the mid-portion of the patella, the medial patellofemoral ligament and adductor magnus tendon. Its attachment and oblique alignment provide a mechanical advantage to promote medial stabilizing force to the patella. Also, Muhamed et al. [ 30] reported that the patellar thickness is important variable to be considered, while assessing the South Indian female for patellofemoral disorders.

Some of the examined cadavers clearly had a nerve branch entering the distal portion of VM muscle separately from the proximal portion. Although most muscles are innervated from their deep surface, it is not unusual for a nerve to travel superficially in a fascial plane prior to entering a muscle belly. The branch to VM from femoral nerve was the furthermost distally situated, almost half-way down the thigh. The branch was well distinct running lengthwise the antero-medial surface of the muscle with the saphenous nerve and was possibly mobilized well proximally. The nerve to VM is designated as giving a short branch entering the muscle proximally, and a longer nerve fiber wandering in the adductor canal before inflowing the muscle belly in its mid-portion [ 31]. Our nerve branch corresponds to the longer medial branch noted by Page et al. [ 31]. Separate nerve innervation is a sufficient evidence to support the concept that VMO is an anatomically and functionally separate muscle from VML in the quadriceps femoris muscle group.

VM is more powerful and extends more distally than VL and its relative predominance is traditionally described as medial patellar stabilizer [ 10]. The findings of this study indicate that VMO is sufficient to rotate the medial condyle of femur internally and the muscle could act independently. "Proof of medial rotation" has been provided by VMO spiral fiber arrangement that covered the anteromedial aspect of the medial condyle. Moreover, the joint space is more compressed medially. Medial rotation of the femur (locking) occurs during the last 30 degrees of extension and locking is produced by the continued action of the same muscles that produce extension i.e., the quadriceps femoris muscle [ 32]. Moreover, Toumi et al. [ 33] stated that the medial fusing points of the quadriceps tendon in the intermediate layers violates the insertion of the VM with potential consequences on the terminal phase of extension. Recently, Chang et al. [ 34] recorded that an appropriate VMO: VL ratio had beneficial effects on PFPS.

Several different clinical tools have been utilized to objectively report the muscle tension in the clinical and operative settings. The use of MRI is a reasonable and more logical approach, but care must be taken to ensure VMO attachment to the patella is accurate. In the present study, the VMO-ACSA of painless knees was 23.2±1.9 cm ² at lower end of shaft of femur, 16.7±1.7 cm ² at medial epicondyle, 14.1±1.1 cm ² at popliteal groove with no statistically significant difference between MRI and U/S measurements. The relative ACSA taken by VMO varies substantially with anatomical location. In the current investigation, we made an attempt to overcome this limitation by selecting a variable slice level among the lower end of the femur. Sattler et al. [ 10] recorded that in the distal third of the quadriceps femoris muscle, VM took the largest (36%) proportion of the total quadriceps volume. The relative contribution of VM to the total quadriceps increased from proximal to distal. A sum of researches have long-established the trustworthiness of U/S procedure for assessing the dimensions of the quadriceps muscle in health [ 35]. U/S has the benefit of being movable and encompasses no ionizing radiation and can conclude breadth and cross-sectional areas of superficial muscles. It is potential to quantity basic parameters of muscle construction with U/S investigation, like muscle volume, fascicle length (muscle fiber length that indicates the variety of lengths over which the muscle is capable of actively producing force), and pennation angle (muscle fiber angle comparative to the force-generating alliance) [ 36].

ACSA was used to represent muscle size as it has the benefit of being a good predictor of both isometric and isokinetic muscle force [ 37, 38]. In the current work, the VMO-ACSA decreased significantly and similarly in all the experimental groups comparing painless to painful knee (decrements ranged from 17.2% to 36.7%). Painful limbs displayed significantly smaller VMO-ACSA. Therefore, impaired VMO muscle function may lead to injurious loading of articular structures, including menisci, ligaments, cartilage, and bone. Wang et al. [ 39] reported that a low VM-ACSA was associated with pain and with medial tibial cartilage volume loss. Providing support for this hypothesis are data from human studies in which quadriceps muscle dysfunction may precede degenerative changes [ 40]. Patients with PFPS were found to have a lower muscle activity ratio between VM and VL muscles than healthy subjects [ 29]. Engelina et al. [ 2] added that VMO angle may be a contributory factor in a subset of PFPS, especially in patients who have patellar instability. Previous studies have reported a reduction in anterior knee pain following an increase in muscle strength with exercise training in people with symptomatic knee osteoarthritis [ 41, 42]. Further, significant increase in VM-ACSA in response to training over time was found to be associated with a concurrent reduction in anterior knee pain, a reduced medial tibial cartilage loss, and a reduced danger of performing knee replacement [ 39]. So, the current study supports training intervention as a successful measure to induce VMO muscle strength, even at advanced age. This may effectively increase joint health and protect against knee degenerative changes.

Changes in VMO-ACSA as a percentage over a certain time duration using U/S is not widely used for screening and staging. In the near future, it may become a valid method to assess muscle in different settings. Also, further study is recommended to explore the differences in VMO-ACSA as regards age, sex and differences in body weight and to assess whether these correlate to functional findings.

In conclusion, VMO is a distinct muscle within the quadriceps femoris group. VMO would track the patella medially and participating in last phase of knee extension. The assessment of the VMO-ACSA by ultrasonography may constitute promising and reliable tool to evaluate PFPS staging.

Acknowledgements

The authors would like to extend their appreciation to team of Anatomage coordinating center at anatomy department, Faculty of medicine, Ain Shams University for supporting this research. Also, we would like to thank the participants, the study investigators and the staff at the clinical centers for generating the image data sets used in this analysis.

Footnotes

Contributed by
Author Contributions
Conceptualization: MMES. Data acquisition: AMD. Data analysis or interpretation: MMES, SMES. Drafting of the manuscript: DMEEM, AMD. Critical revision of the manuscript: MMES, DMEEM, SMES, AMD. Approval of the final version of the manuscript: all authors.

Conflicts of Interest
No potential conflict of interest relevant to this article was reported.

References

2. Engelina S, Antonios T, Robertson CJ, Killingback A, Adds PJ. Ultrasound investigation of vastus medialis oblique muscle architecture: an [ in vivo study. Clin Anat. 2014;27:1076-84. doi: 10.1002/ca.22413. [ PubMed] [ CrossRef] Google Scholar]

3. Arazpour M, Bahramian F, Abutorabi A, Nourbakhsh ST, Alidousti A, Aslani H. The effect of patellofemoral pain syndrome on gait parameters: a literature review. Arch Bone Jt Surg. 2016;4:298-306. [[ PMC free article] [ PubMed] Google Scholar]

6. Dixit S, DiFiori JP, Burton M, Mines B. Management of patellofemoral pain syndrome. Am Fam Physician. 2007;75:194-202. [[ PubMed] Google Scholar]

28. Belli G, Vitali L, Botteghi M, Vittori LN, Petracci E, Latessa PM. Electromyographic analysis of leg extension exercise during different ankle and knee positions. J Mech Med Biol. 2015;15:1540037. doi: 10.1142/S0219519415400370. [[ CrossRef] Google Scholar]

32. Chaitow L, DeLany J. Clinical application of neuromuscular techniques volume 2, the lower body. 2nd ed. Churchill Livingstone; Edinburgh: 2011. [ Google Scholar]

Articles from Anatomy & Cell Biology are provided here courtesy of Korean Association of Anatomists

# data[].attributes.attachment_link.title
Morphometric analysis of vastus medialis oblique muscle and its influence on anterior knee pain

# data[].attributes.attachment_link.description
Healthy knees require full range squatting movements. Vastus medialis (VM) muscle regulates and adjusts the extensor apparatus that influences the patellofemoral function. This work was designed to investigate the anatomy and morphometry of vastus medialis ...

# data[].attributes.attachment_link.title
Vastus Medialis Anatomy: Origin, Insertion & Action

# data[].attributes.attachment_link.provider_name
ACE Physical Therapy and Sports Medicine Institute

# data[].attributes.attachment_link.readable_text
Young athletes and the overweight sometimes suffer from discomfort in the front of the knee. There can be a many sources for anterior knee pain but the most common cause is a patella that does not remain in the groove of the thigh bone when it coursed up and down during knee motion. Over the past quarter century, healthcare providers have debated the importance of the VMO (one of the quadriceps muscles) in preventing anterior knee pain. Studies now indicate that effective treatment addresses the whole system including the VMO, the core, and the entire leg.

The Vastus Medialis Oblique of VMO is one of the quadriceps muscles and it is located above (proximal) and medially to the patella or kneecap. The knee joint consists of 3 bones, tibia, femur and patella. These bones move in unison during the knee motions of flexion and extension. At the distal end of the femur, there is a groove (trochlear) that houses the patella during motions of the knee. The ideal position of the patella is somewhat centered in the groove which reduces the amount of compressive and sheer forces on the patella and groove.

The patella moves up and down in the groove during knee flexion and extension and the vasti muscles (quadriceps) in thigh are thought to help control the patella motion. The VMO was thought to be the primary controlling dynamic force from the medial side of the patella, but some studies indicate that there are numerous variables that can affect the tracking of the patella.

The VMO is most active in the last 20 degrees of knee extension. When this muscle does not function properly, the patella can track laterally and "rub" against the lateral side of the trochlear groove. If this occurs repeatedly the groove or the patella begins to experience damage to the articular cartilage. The lining within the joint, synovial lining, becomes inflamed and the knee begins to hurt.

Recent studies indicate that the VMO and vasti muscles play a role in controlling the patella and the way it moves within the groove. The method used during these studies involved using a nerve block to eliminate the motor function of the VMO therefore disabling its ability to "pull" the patella medially when it tracks within the groove. In the study the non-symptomatic test subjects experienced more lateral tracking of their patella at 15 degrees of knee flexion than symptomatic test subjects experienced at the same flexion angle. The lateral tracking of both groups is indicative of a non-functioning VMO.

Many healthcare professionals are hesitant to have their patients/clients perform a leg extension exercise due to the increased patella femoral compression force that occurs as the knee moves from flexion to extension. The patella is used as a pulley for the vasti muscles and the patella moves deeper into the groove when the knee is flexed to 90 degrees or more. In most knee extension exercises, the patient/client sits with their knee flexed to 90 degrees and then extends it against resistance that is applies at the ankle level. The increased compressive force of the patella can become detrimental over a period of time.

The results of the recent studies indicate that isolated exercises for the vasti muscle and particularly the VMO are needed for a healthy patella femoral joint. Several exercises can be performed safely that isolate on the VMO and will enhance its capability to control the patella from the medial side (i.e. knee extension exercises, squats and lunges that limit knee flexion to 30 degrees isolates on the VMO and can strengthen it.

If you develop anterior knee pain, you should seek advice and treatment from a Physical Therapist. These licensed healthcare professionals can be visited without a doctor's prescription, but your insurance might require that you secure a referral from your primary care physician. They will evaluate you and prescribe specific exercises that will address the imbalances that exist throughout your lower extremities. Strength development of the vasti muscles is only one part of the treatment protocol that will be implemented to address the symptoms of anterior knee pain.

During the past 25 years the treatment protocols for anterior knee pain have changed quite a bit. The focus has transitioned to include the entire body and not just the muscle structure around the involved knee joint. The VMO and vasti muscles have a role in controlling the patella but the core and entire leg has to be included to fully rehabilitate a knee that has anterior knee pain as a primary symptom.

# data[].attributes.attachment_link.authors[].name
ACE Physical Therapy and Sports Medicine Institute

# data[].attributes.attachment_link.title
WHAT ROLE DOES THE VMO (VASTUS MEDIALIS OBLIQUE) PLAY IN ANTERIOR KNEE PAIN? - ACE Physical Therapy and Sports Medicine Institute

# data[].attributes.attachment_link.description
There can be a many sources for anterior knee pain but the most common cause is a patella that does not remain in the groove...

# data[].attributes.attachment_link.readable_text
Rectus femoris muscle injuries are muscle injuries, which encompass contusions, strains, tears and avulsions of the rectus femoris muscle.

Rectus femoris muscle injuries are a common injury in athletes, especially in football/soccer players ¹. The rectus femoris muscle is most frequently affected in a quadriceps injury and the most common pattern are muscle strains ¹⁻³.

Sports and activities involving a great amount of kicking, jumping and running are considered risk factors for developing a rectus femoris muscle injury and include ¹⁻³:

Additional factors that increase the likelihood of developing a rectus femoris muscle injury include:

Rectus femoris muscle injuries are associated with other quadriceps injuries and hamstring injuries ¹⁻³.

In the event of acute strain, patients frequently present with a sharp pain in the anterior thigh associated with a functional deficit. Tenderness with palpation is a typical finding, which can usually be triggered by resisted muscle activity. Palpable defects might be found ³. Chronic strains might present with vague discomfort, thigh enlargement and variably associated strength deficit ³.

If left untreated rectus femoris muscle injury can lead to the following conditions ⁵:

The rectus femoris muscle crosses two joints, plays an active part in knee extension and hip flexion and features a high proportion of fast-twitch (type II) muscle fibres and is characterised by a complex musculotendinous architecture, which is considered a predisposing factor for strain injury ¹. Muscle injury patterns include strains, contusions and lacerations.

Especially sprinting, jumping and kicking motions are associated with forceful eccentric contractions during counteraction of knee flexion and hip extension leading to the development of high forces across the muscle-tendon units, which can cause strain injury ¹⁻³.

With jumping, hip extension is counteracted during upward propulsion and knee flexion upon landing ¹.

Typical locations of rectus femoris muscle injuries include the origin of the direct and indirect head, the proximal myotendinous junction, the muscle belly, the muscle periphery with the fascia or myofascial unit and the distal myotendinous junction 4,5.

Rectus femoris muscle injuries can be classified based on type and location into the following 4,5 :

Most injuries of the rectus femoris muscle affect the myotendinous junction ⁴⁻¹¹ and are best evaluated with ultrasound or MRI.

Plain radiographs of the pelvis can be used as an initial examination especially in young patients to visualise anterior inferior iliac spine avulsions and to rule out other pathology 4,5. Besides, an avulsion injury of the indirect head of the femoris muscle at the superior acetabular ridge might be detected.

The proximal tendons and the rectus femoris muscle can be nicely visualised on ultrasound. An anterior iliac spine avulsion injury can be visualised with a variably sized fragment separated from the pelvic bone by an anechoic fluid collection. The conjoined tendon and the direct head can be also depicted and easily assessed, evaluation of the indirect head is more difficult due to its depth ⁷⁻⁹.

Typical features of acute muscle injuries are ill-defined hyper or hypoechoic lesions with varying degrees of fibrillar discontinuity or disruption and can be graded accordingly ⁷.

Chronic proximal lesions might appear as hypoechoic tendon thickening and might show calcification or heterotopic ossification ⁸. Scar formation within the muscle displays an irregularly delineated hyperechoic appearance with focal retraction of the adjacent muscle fibres ⁷.

CT can detect and characterise avulsion injuries. Additionally, it can be used to depict large intramuscular haemorrhages. Due to radiation exposure and alternatives with better soft tissue discrimination properties such as ultrasound and MRI, its value in the workup of muscle injuries is only limited.

Typical features of muscle injuries on MRI include fluid signal intensity tracking and surrounding the muscle fibres, myofascial, myotendinous or tendinous units of the rectus femoris muscle and/or discontinuities of the respective muscle components. Location, severity and extent of the muscle injury and extent of muscle retraction can be determined on MRI and even subtle tears can be detected with a suitable MRI protocol and proper knowledge of anatomy.

A further objective of MRI is to predict the time required for rehabilitation, recovery and return to sports, especially in athletes.

A magnetic resonance imaging grading scheme proposed for myotendinous injury 4-6,10:

grade 1: focal or diffuse high signal intensity or feathery muscle oedema without fibre disruption

grade 2: partial tear of the myotendinous junction with the discontinuity of some fibres

grade 3: complete tear of the myotendinous junction with or without tendon retraction

The bull's eye sign seen on axial images indicates a deep intramuscular degloving injury.

Another MR-based classification is the British Athletics muscle injury classification, which also allows for subclassification according to the site.

The radiological report should include a description of the following:

location, type and extent of the lesion

injury grading if possible

the extent of tendon retraction

associated injuries

The vast majority of rectus femoris muscle including avulsion injuries are managed conservatively.

Conservative management includes activity modification with an initial period following the RICE (rest, ice, compression and limb elevation) principle followed by a comprehensive rehabilitation and exercise regimen.

The initial resting period (usually 3-5 days) serves as a measure to prevent further progression of the injury and a more severe strain injury might require the initial use of crutches. Limb elevation and intermittent application of ice and compression are aimed to decrease blood flow and increased amounts of interstitial fluid accumulation. Ice application also serves pain control and can be supplemented by non-steroidal anti-inflammatory drugs (NSAIDs) in the initial period ³.

A rehabilitation protocol should comprise movement, walking, and running exercises as well as stretching, strengthening, range of motion, endurance and agility training. Exercises should be initiated gradually and advanced continuously and should be conducted without increasing pain in the quadriceps ³.

Recovery will depend on the type and extent of the injury and might take from 2-3 weeks in case of a muscle contusion or myofascial injury up to four months or longer in displaced anterior iliac spine avulsion injuries 3,6.

Surgery is an alternative option in professional athletes with full-thickness tendon tears, two tendon head avulsions, patients with recurrent injuries or conservative treatment failure and can be done by surgical resection of the proximal tendon remnants followed by muscular suture repair 12,13.

To return to sports athletes should be pain-free and their range of motion should be normal in the hip and knee. Strength should be near normal compared to the contralateral side ³.

A tear of the deep myotendinous junction of the indirect head of the rectus femoris muscle was first described by Hughes et al. in 1995 ¹⁴.

Conditions that can mimic the presentation and/or the appearance of a rectus femoris muscle injury include:

# data[].attributes.attachment_link.title
Rectus femoris muscle injury | Radiology Reference Article | Radiopaedia.org

# data[].attributes.attachment_link.description
Rectus femoris muscle injuries are muscle injuries, which encompass contusions, strains, tears and avulsions of the rectus femoris muscle. Epidemiology Rectus femoris muscle injuries are a common injury in athletes, especially in football/socc...

# data[].attributes.attachment_link.title
Rectus Femoris Anatomy: Origin, Insertion & Action

# data[].attributes.body
The "teardrop" muscle

# data[].attributes.attachment_link.title
Vastus Medialis | Muscle Anatomy | Joetherapy

# data[].attributes.body
Guidance to strengthening and isolating the VMO and how it may prevent or help Knee injuries.

# data[].attributes.attachment_link.description
VMO Strength exercises can help to improve knee function and decrease pain in the knee. A JOI Rehab Therapist explains the VMO and how to improve it.

# data[].attributes.attachment_link.readable_text
Abstract

Injuries to the quadriceps muscle group occur frequently in sports and athletic activities. Muscle strains and contusions constitute the majority of these injuries. The clinical presentation and assessment of quadriceps strains and contusions are reviewed along with discussion of appropriate imaging used in diagnosis. Treatment protocols for acute injuries are reviewed including rehabilitation techniques frequently utilized during recovery. Special consideration is given to discussing the criteria for return to sports for athletes after injury. Myositis ossificans is a potentially disabling complication from quadriceps contusions and risk factors, prevention, and treatment are reviewed.

Keywords: Quadriceps, Muscle strain, Contusion, Myositis ossificans

Introduction

Injury to the quadriceps muscle group can be painful and debilitating. Strains and contusions of the quadriceps are common in athletics and result in lost time from training and competition. This article will review the presentation, physical examination, diagnostic testing, and treatment of these injuries. In addition, rehabilitation techniques and factors to consider when returning athletes to play after injury will be discussed.

Functional anatomy

The quadriceps muscle group is composed of the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius. The rectus femoris originates at the ilium, thus crossing both the hip and knee joint along its course. This anatomy allows for hip flexion and knee extension. The remaining muscles originate on the femur and function solely as knee extensors. Innervation of these muscles is by the femoral nerve. The quadriceps are primarily active in kicking, jumping, and running.

Quadriceps strains

Acute strain injuries of the quadriceps commonly occur in athletic competitions such as soccer, rugby, and football. These sports regularly require sudden forceful eccentric contraction of the quadriceps during regulation of knee flexion and hip extension. Higher forces across the muscle-tendon units with eccentric contraction can lead to strain injury. Excessive passive stretching or activation of a maximally stretched muscle can also cause strains. Of the quadriceps muscles, the rectus femoris is most frequently strained [ 1- 5]. Several factors predispose this muscle and others to more frequent strain injury. These include muscles crossing two joints, those with a high percentage of Type II fibers, and muscles with complex musculotendinous architecture [ 1, 2, 6, 7]. Muscle fatigue has also been shown to play a role in acute muscle injury [ 8].

Classification of quadriceps strains

Various ways of grading muscle strains have been proposed [ 4, 6, 9]. Factoring in pain, loss of strength, and physical exam findings in a grading system helps provide guidance for treatment, rehabilitation, and eventual return to play. Table 1 provides an outline of a clinical grading system for muscle strains.

Table 1

Grade

Pain

Strength

Physical exam

1

Mild

None or minimal loss of strength

No palpable muscle defect

2

Moderate

Moderate loss of strength

May feel a small palpable muscle defect

3

Severe

Usually complete loss of strength

Often feel a palpable muscle defect

Grade 1 strains represent minor tearing of muscle fibers with only minimal or no loss in strength. Pain is usually mild to moderate with no palpable defect in the muscle tissue on exam.

Grade 2 strains involve more severe disruption to the muscle fibers with significant pain and loss of strength. A defect in the muscle tissue may sometimes be felt.

Grade 3 strains are a result of complete tearing of the muscle with associated severe pain and complete loss of strength. A palpable defect in the muscle tissue can frequently be felt, especially if examined at onset of injury prior to hematoma formation.

History

An accurate history should be obtained in patients presenting with anterior thigh pain. Patients who suffer an acute quadriceps strain will usually know right away. They are typically involved in kicking, jumping, or initiating a sudden change in direction while running. Frequently a sharp pain is felt associated with a loss in function of the quadriceps. Sometimes pain would not fully develop until the end of a game, practice, or sporting activity. Pain may be associated with localized swelling and loss of motion. Location of pain can be anywhere along the quadriceps muscles, but is classically described along the distal portion of the rectus femoris at the musculotendinous junction. However, several studies have shown quadriceps strains commonly occur at the mid to proximal portion of the rectus femoris [ 1, 2, 5].

Physical examination

After obtaining a thorough history, a careful examination should ensue including observation, palpation, strength testing, and evaluation of motion. Strain injuries of the quadriceps may present with an obvious deformity such as a bulge or defect in the muscle belly. Ecchymosis may not develop until 24 h after the injury. Palpation of the anterior thigh should include the length of the injured muscle, locating the area of maximal tenderness and feeling for any defect in the muscle. Strength testing of the quadriceps should include resistance of knee extension and hip flexion. Adequate strength testing of the rectus femoris must include resisted knee extension with the hip flexed and extended. Practically, this is best accomplished by evaluating the patient in both a sitting and prone-lying position. The prone-lying position also allows for optimum assessment of quadriceps motion and flexibility. Pain is typically felt by the patient with resisted muscle activation, passive stretching, and direct palpation over the muscle strain. Assessing tenderness, any palpable defect, and strength at the onset of muscle injury will determine grading of the injury and provide direction for further diagnostic testing and treatment.

Imaging

Most acute injuries to the quadriceps musculature can be diagnosed with an adequate history from the patient and a thorough examination. Imaging can be a useful adjunct in those cases where the diagnosis is uncertain or further detail is needed regarding the type and location of the muscle strain. Radiographs, ultrasound (US), and magnetic resonance imaging (MRI) are the commonly used imaging tools for this area. Radiographs are routinely normal in acute muscle strains, but may be helpful in differentiating between bony (femoral stress fracture, tumor, or myositis ossificans) and muscular etiologies of quadriceps pain in chronic cases. US is an excellent imaging modality for visualizing the quadriceps muscles and tendons, but is highly operator dependent and requires a skilled and experienced clinician [ 6, 10, 11]. US has the ability to image the muscles dynamically and assess for bleeding and hematoma formation via Doppler. MRI provides detailed images of muscle injury and can be quite helpful in characterizing quadriceps injuries [ 5, 12, 13]. It can sometimes be difficult to distinguish between muscular contusion and strain on MRI, which simply re-enforces the importance of clinical history and examination in injury assessment [ 12]. Prognostically, Cross et al. found strains of the central tendon of the rectus femoris, identified on MRI, correlated with a significantly longer rehabilitation period [ 5].

Treatment

Treatment of muscle strain injuries has not changed greatly over the years and there is little scientific basis for the majority of treatment protocols. Despite the relative paucity of literature specific to treatment of muscle injuries, there are certain principles which provide a basis for the currently accepted methods of treatment. When muscle is acutely damaged by strain, contusion, or laceration, there is bleeding and hematoma formation among the ruptured muscles cells followed by an inflammatory reaction [ 6, 14]. Acute phase treatment of quadriceps strains is focused on minimizing bleeding into the muscles by following the RICE principle (rest, ice, compression, and elevation). Allowing the muscle to rest prevents worsening of the initial injury. Ice or cold application is thought to lower intra-muscular temperature and decrease blood flow to the injured area. This may help facilitate faster healing and return to athletics, but has not been proven in the scientific literature [ 15]. This same review did show that cryotherapy is effective in decreasing pain associated with muscle injury [ 15]. Compression may help decrease blood flow and accompanied by elevation will serve to decrease both blood flow and excess interstitial fluid accumulation. Practically, the first 24-72 h after quadriceps strain should be focused on the RICE principles. Cryotherapy, accompanied by compression, should be applied for 15-20 min at a time with 30-60 min between applications. During this time period, the quadriceps should be kept relatively immobile to allow for appropriate healing and prevent further injury [ 6]. A grade 2 or 3 strain may necessitate the use of crutches initially to facilitate rest and immobilization of the quadriceps. Nonsteroidal anti-inflammatory drugs (NSAIDs) can be useful for reducing pain and allowing earlier return to activity. The long-term effects of NSAIDs in muscle strains are unknown and a recent review recommends only a short 3-7 day course after muscle strains [ 16]. In contrast, the use of corticosteroids is definitely discouraged based on research demonstrating delayed healing and reduced biomechanical strength of injured muscle [ 6, 17, 18].

The acute phase of treatment is subsequently followed by an active phase of management once the injured leg is recovering well. This phase usually begins approximately 3-5 days after the initial injury depending on its severity. Stretching, strengthening, range of motion, maintenance of aerobic fitness, proprioceptive exercises, and functional training are the primary components of this phase. Stretching should be done carefully and always to the point of discomfort, but not pain. Various techniques can be utilized including passive, active-passive, dynamic, and proprioceptive neuromuscular facilitation stretching. Generally, ballistic stretching is discouraged due to the risk of re-tearing muscle fibers. An active warm-up should always precede any type of rehabilitation exercises as it has been shown to activate neural pathways in the muscle and reduce muscle viscosity [ 6]. Strengthening exercises can begin gradually and should progress sequentially through isometric, isotonic, isokinetic, and functional exercises. Table 2 provides definitions of these various strengthening techniques.

Table 2

Isometric

Muscle contraction against a fixed object resulting in no change in muscle length

Isotonic

Muscle contraction against a constant resistance, with uncontrolled speed of movement, resulting in muscle shortening or lengthening through a range of motion

Isokinetic

Muscle contraction through a range of motion at a constant angular velocity

Functional

Sport or activity specific strengthening exercises such as sprinting, jumping, or agility drills

All strengthening exercises should be performed through a pain-free range of motion. Advancement through each type of strengthening depends on the level of soreness and pain created by each type of exercise. For example, once isometric straight leg raises at 0°, 20°, and 40° can be completed without any pain or subsequent soreness, isotonics can then be initiated. Maintaining aerobic fitness during rehabilitation is important and can be accomplished by using activities like swimming and biking. Once again, these activities should not increase pain in the injured quadriceps and should be performed in a pain-free range of motion.

Return to sports

Consideration of return to sports criteria is important when managing quadriceps strains in athletic patients. There are no established consensus guidelines or criteria for safe return to sports following muscle strains [ 19]. Practically, athletes should have normal knee range of motion, be pain free, demonstrate near normal strength compared to the contralateral side and perform well on functional field tests [ 13]. Isokinetic muscle strength testing can be a useful tool for assessing strength and guiding return to sports.

Quadriceps contusions

Contusion injuries to the quadriceps are common in athletics. In fact, next to muscle strains, traumatic muscle contusions have been reported as the most frequent type of quadriceps injury in sports [ 20]. A direct blow to the quadriceps causing significant muscle damage is the usual mechanism of this injury. In comparison to strains, contusions will cause rupture to the muscle fibers at or directly adjacent to the area of impact [ 6]. This typically leads to hematoma formation within the muscle causing pain and loss of motion. A contracted muscle will absorb force better and result in a less severe injury [ 6, 18]. Improved protective equipment seems to be decreasing the frequency of this injury in many sports; however, there is no research to support this apparent trend. In sports without padding for the thigh and upper leg, such as soccer and rugby, quadriceps contusions continue to be a major disabling injury.

History and physical examination

Perhaps more so than with muscle strains, athletes will usually report a definite mechanism associated with a contusion injury. A direct blow from an opponent's knee or other piece of equipment frequently is recalled as the traumatic force causing their thigh pain. Localized pain at the site of injury, swelling, decreased range of motion, and tenderness to palpation are usual findings. Depending on the severity of injury, some athletes may be able to continue their activity and present after competition or training for evaluation. Upon examination, the area of injury should be inspected for obvious deformity, swelling, or ecchymosis. Palpation along the injured muscle will help localize the exact site of muscle damage and determine if there is any associated injury. Strength testing of the quadriceps, by resisting knee extension and hip flexion, compared to the uninjured side will aid in assessing severity of injury. Measurement of knee flexion has been used as a prognostic indicator in quadriceps contusions. Jackson and Feagin originally described a classification system, which was further modified by Ryan et al. [ 21, 22]. Mild contusions are characterized by a normal gait, localized tenderness, and active knee flexion of greater than 90°. Moderate contusions are characterized by antalgic gait, swollen, tender mass in the quadriceps, and 45-90° of active knee flexion. Severe contusions are characterized by a severely antalgic gait, noticeably swollen and tender muscle mass, and less than 45° of active knee flexion [ 21]. Table 3 summarizes this classification.

Table 3

Pain

Active knee flexion

Gait

Mild

>90°

Normal

Moderate

45-90°

Antalgic

Severe

<45°

Severely antalgic

The prognostic value of this classification system has been demonstrated as the average disability time is longer for subsequently more severe contusions. Ryan et al. found average disability times of 13 days for mild contusions, 19 days for moderate contusions, and 21 days for severe contusions [ 22].

Imaging

The diagnosis of a quadriceps contusion is, by and large, a clinical diagnosis made by obtaining an accurate history from the patient and completing an appropriate examination. Diagnostic imaging is typically not necessary, but is helpful in a few instances. Occasionally, a patient will present subacutely with anterior thigh pain without a known mechanism of injury. Ultrasound and MRI can be useful in this case to provide additional information regarding the nature of muscle injury. It may help distinguish between strain, contusion, or avulsion injuries [ 12]. Ultrasound can be used to identify a localized hematoma formation caused from a contusion and provide real-time imaging for needle aspiration. If there is concern for bony involvement, radiographs will evaluate for bony injury. Subsequently, radiographs are useful in identifying heterotopic bone formation, known as myositis ossificans (MO), which is a delayed complication of severe muscle contusions.

Treatment

The principles of treatment for quadriceps contusions are essentially the same as for quadriceps strains, with one major exception. It is recommended the injured leg be placed in a position of flexion for the first 24 h post-injury to limit hematoma formation. Practically, this can be done by placing the patient in a hinged knee brace at 120° of knee flexion or using elastic compression wrap to maintain this position of flexion. This needs to be done as soon as possible after injury. Besides maintaining this position of flexion, ice and compression should be applied during this time. Several military studies have demonstrated the efficacy of this acute phase treatment of quadriceps contusions [ 21- 23]. After 24 h, the brace or wrap should be removed and gentle, active, pain-free range of motion at the knee should be instituted along with stretching and isometric quadriceps strengthening. The active phase of treatment, including functional rehabilitation, can begin when pain-free, active knee flexion of at least 120° is attained.

NSAIDs can be useful short term for decreasing pain, but their long-term effect on muscle healing is not known [ 16]. Long-term use of NSAIDs for contusions is usually not necessary and is discouraged. However, NSAIDs have been promoted for prevention of myositis ossificans after severe quadriceps contusions. Evidence for this use is inferred from studies showing a decrease in heterotopic bone formation after total hip replacement in those patients given indomethacin for at least 7 days [ 24]. Similar to muscle strains, corticosteroids are not recommended in the treatment of contusion injuries.

Return to sports

Criteria are similar to muscle strains for return to sports in contusions of the quadriceps. The athlete should be pain free, attain 120° of knee flexion with hip extended, and perform all aspects of functional field testing without limitations [ 13]. Protective thigh padding is recommended prior to resuming sports in order to reduce recurrence.

Myositis ossificans

A complication associated with severe quadriceps contusions; this is a non-neoplastic proliferation of bone and cartilage in the area of contusion injury. In contusions, the reported incidence is between 9 and 17% [ 18, 22]. MO should be suspected if symptoms worsen after 2-3 weeks accompanied by loss of knee flexion and persistent swelling. Ryan et al. identified several risk factors associated with development of MO in their study of West Point cadets. They included knee flexion less than 120°, injury occurring during football, previous quadriceps injury, delay in treatment greater than 3 days, and ipsilateral knee effusion [ 22]. Diagnosis is made by radiographs demonstrating new bone formation in the area of contusion. Radiographic findings may be present within 3 weeks of injury, but frequently lag behind clinical symptoms [ 18]. As mentioned previously, indomethacin is often given for 7 days after severe contusions to prevent development of MO despite lack of direct evidence supporting this treatment. Treatment otherwise consists of stretching, range of motion, and strengthening. Athletes can still participate in sports with MO present, but may find their range of motion restricted with occasional flare-ups of pain and swelling. Unfortunately, many cases do not resolve requiring definitive surgical treatment. Surgical excision should not be performed until the ectopic bone formation has fully matured, which is usually in a range of 12 to 24 months [ 6, 13, 18]. Early excision while bone formation is still active can result in more severe local recurrence [ 18].

References

13. DeLee JC, Drez D, Miller MD. Orthopaedic sports medicine: principles and practice. 2. Philadelphia, PA: Saunders; 2003. [ Google Scholar]

21. Jackson DW, Feagin JA. Quadriceps contusions in young athletes. J Bone Joint Surg. 1973;55A:95-105. [ PubMed]

Articles from Current Reviews in Musculoskeletal Medicine are provided here courtesy of Humana Press

# data[].attributes.attachment_link.title
Diagnosis and management of quadriceps strains and contusions

# data[].attributes.attachment_link.description
Injuries to the quadriceps muscle group occur frequently in sports and athletic activities. Muscle strains and contusions constitute the majority of these injuries. The clinical presentation and assessment of quadriceps strains and contusions are reviewed ...

# data[].attributes.body
Knee Joint Protection: Effect of Foot Pressure Position on Rectus Femoris EMG during Squats

# data[].attributes.body
The VMO is the most distal of muscles fibres within the Vastus Medialis muscle. The difference between the VM & VMO is the VMO mainly helps support and prevent mal-tracking of the patella. Whereas the VM mainly contributes to knee extension.

Common injuries: Sprain, tear, rupture, patella injuries.

# data[].attributes.body
Common injuries: Ruptures & Tears.

# data[].attributes.body
Origin: Straight head from anterior inferior iliac spine; reflected head from groove just above acetabulum
Insertion: Base of patella to form the more central portion of the quadriceps femoris tendon
Action: Extends the knee
Innervation: Muscular branches of femoral nerve
Arterial Supply: Lateral circumflex femoral artery

# data[].attributes.attachment_link.readable_text
Origin: Straight head from anterior inferior iliac spine; reflected head from groove just above acetabulum
Insertion: Base of patella to form the more central portion of the quadriceps femoris tendon
Action: Extends the knee
Innervation: Muscular branches of femoral nerve
Arterial Supply: Lateral circumflex femoral artery

The medical illustrations contained in this online atlas are copyrighted © 1997 by the University of Washington. They may not be utilized, reproduced, stored, or transmitted in any form or by any means, electronic or mechanical, or by any information storage or retrieval system, without permission in writing from the University of Washington.

Receiving a license to use these images is generally quite easy, particularly for academic and scholarly purposes. For more information see the Musculoskeletal Atlas Express Licensing Page.

# data[].attributes.attachment_link.description
Origin: Straight head from anterior inferior iliac spine; reflected head from groove just above acetabulum Insertion: Base of patella to form the more central portion of the quadriceps femoris tendon Action: Extends the knee Innervation: Muscular branches of femoral nerve Arterial Supply: Lateral circumflex femoral artery The medical illustrations contained in this online atlas are copyrighted © 1997 by the University of Washington.

# data[].attributes.body
The effect of gender on vastus medialis oblique/vastus lateralis activation ratio during weight and non-weight bearing activities in recreationally subjects

# data[].attributes.attachment_link.title
Musculoskeletal injuries in British Army recruits: a prospective study of diagnosis-specific incidence and rehabilitation times - BMC Musculoskeletal Disorders

# data[].attributes.attachment_link.description
Background Musculoskeletal injuries during initial military training are a significant medical problem facing military organisations globally. In order to develop an injury management programme, this study aims to quantify the incidence and rehabilitation times for injury specific diagnoses. Methods This was a prospective follow-up study of musculoskeletal injuries in 6608 British Army recruits during a 26-week initial military training programme over a 2-year period. Incidence and rehabilitation times for injury specific diagnoses were recorded and analysed. Results During the study period the overall incidence of musculoskeletal injuries was 48.6%, and the most common diagnosis was iliotibial band syndrome (6.2%). A significant proportion of the injuries occurred during the first 11 weeks of the programme. The longest rehabilitation times were for stress fractures of the femur, calcaneus and tibia (116 ± 17 days, 92 ± 12 days, and 85 ± 11 days, respectively). The combination of high incidence and lengthy rehabilitation indicates that medial tibial stress syndrome had the greatest impact on training, accounting for almost 20% of all days spent in rehabilitation. Conclusion When setting prevention priorities consideration should be given to both the incidence of specific injury diagnoses and their associated time to recovery.

# data[].attributes.attachment_link.readable_text
Introduction Objective outcome measures that can quantify the force generating capacity of the lower limb are required to allow clinicians to accurately measure functional status and treatment adaptations over time. The aim of this prospective observational cohort study is to: (1) evaluate the acceptability of the isometric mid-thigh pull (IMTP) test as a measure of functional strength with military personnel undergoing residential hip pain rehabilitation; (2) compare the peak force values recorded against the updated Army physical employment standards (PES) assessment criteria and (3) assess if the minimum PES required of military personnel has the potential to inform clinical decision making and return to duty criteria within UK Defence Rehabilitation.

Methods Acceptability was assessed against patient's adherence to the testing procedures and test burden. Clinician acceptability was assessed against ease of administration and safety of test procedure. Hip pain was recorded before, immediately following and 1 hour after testing. Net peak force was recorded using portable force plates.

Results Full patient and clinician acceptability to IMTP testing procedures were demonstrated. Minimal changes in visual analogue scale (VAS) pain scores were demonstrated between baseline values at rest and follow-up. Despite being medically downgraded and functionally compromised due to chronic hip pain, 100% of patients met the PES expected on entry to the British Army and 79% met the PES expected at the end of basic training.

Conclusion The IMTP provides rehabilitation clinicians with an objective quantifiable measure of maximum muscle strength that can be used early in the rehabilitation care pathway. Based on our finding, it is unclear if the current British Army PES can be used as a criterion standard in Defence Rehabilitation. Therefore, further research focused on generating clinically relevant patient-specific IMTP score criteria, with a larger sample of diverse diagnostic sub-groups is required.

# data[].attributes.attachment_link.title
Use of an isometric mid-thigh pull test during musculoskeletal rehabilitation: can the criterion values from the updated British Army physical employment standards be used to inform UK Defence Rehabilitation practice?

# data[].attributes.attachment_link.description
Introduction Objective outcome measures that can quantify the force generating capacity of the lower limb are required to allow clinicians to accurately measure functional status and treatment adaptations over time.

# data[].attributes.attachment_link.description
The vastus lateralis muscle is located on the lateral side of the thigh. This muscle is the largest of the quadriceps which includes: rectus femoris, vastus intermedius, and vastus medialis. Together, the quadriceps act on the knee and hip to promote movement as well as strength and stability. They…

# data[].attributes.headline
Journal (Ando et al., 2017). Comparing Vastus intermedius vs vastus lateralis fascicle behaviors during maximal concentric and eccentric contractions

# data[].attributes.subject
Journal (Ando et al., 2017). Comparing Vastus intermedius vs vastus lateralis fascicle behaviors during maximal concentric and eccentric contractions

# data[].attributes.body
"These results indicate that VI fascicles are lengthened 1.4 times more than VL fascicles during eccentric contractions, whereas VI and VL fascicles shorten similarly during concentric contractions. This suggests a possibility that a greater mechanical strain is imposed to VI than VL during eccentric contractions."

# data[].attributes.attachment_link.title
Scandinavian Med Sci Sports - 2017 - Ando - Vastus intermedius vs vastus lateralis fascicle behaviors during maximal

# data[].attributes.headline
Journal (Ando et al., 2017). Comparing Vastus intermedius vs vastus lateralis fascicle behaviors during maximal concentric and eccentric contractions

# data[].attributes.subject
Journal (Ando et al., 2017). Comparing Vastus intermedius vs vastus lateralis fascicle behaviors during maximal concentric and eccentric contractions

# data[].attributes.body
"These results indicate that VI fascicles are lengthened 1.4 times more than VL fascicles during eccentric contractions, whereas VI and VL fascicles shorten similarly during concentric contractions. This suggests a possibility that a greater mechanical strain is imposed to VI than VL during eccentric contractions."

# data[].attributes.attachment_link.title
Scandinavian Med Sci Sports - 2017 - Ando - Vastus intermedius vs vastus lateralis fascicle behaviors during maximal

# data[].attributes.body
HRV Definiton

“The term HRV describes a series of mathematical calculations for modeling psychophysiological responses based on the time between consecutive heart beats, also known as beat-to-beat or inter-beat-intervals (IBI)”

Sympathetic Activity increases HR but decreases IBI/HRV.

Parasympathetic Activity decreases HR and therefore increases IBI and HRV.

# data[].attributes.attachment_link.readable_text
Ellen Glickman, Academic Editor, Filipe Manuel Clemente, Academic Editor, Juan Pedro Fuentes García, Academic Editor, and Rodrigo Ramirez-Campillo, Academic Editor

Abstract

Human performance optimization of tactical personnel requires accurate, meticulous, and effective monitoring of biological adaptations and systemic recovery. Due to an increased understanding of its importance and the commercial availability of assessment tools, the use of heart rate variability (HRV) to address this need is becoming more common in the tactical community. Measuring HRV is a non-invasive, practical method for objectively assessing a performer's readiness, workload, and recovery status; when combined with additional data sources and practitioner input, it provides an affordable and scalable solution for gaining actionable information to support the facilitation and maintenance of operational performance. This narrative review discusses the non-clinical use of HRV for assessing, monitoring, and interpreting autonomic nervous system resource availability, modulation, effectiveness, and efficiency in tactical populations. Broadly, HRV metrics represent a complex series of interactions resulting from internal and external stimuli; therefore, a general overview of HRV applications in tactical personnel is discussed, including the influence of occupational specific demands, interactions between cognitive and physical domains, and recommendations on implementing HRV for training and recovery insights into critical health and performance outcomes.

Keywords: heart rate variability, HRV, autonomic nervous system, military, human performance, physiological monitoring

1. Introduction

The health, well-being, and preparedness of tactical personnel are crucial to mission success [ 1]. In tactical settings, Human Performance Optimization (HPO) programs focus on enhancing and sustaining the individual's ability to perform occupational tasks over time. There are several strategic focus areas that contribute to HPO of tactical personnel, to include physical fitness, health, nutrition, cognitive, and psychosocial. Inherently, military training and operations have remarkably high physical and mental demands that necessitate exceptional levels of fitness [ 2, 3]. To achieve such extreme levels of prowess, bouts of higher training intensities and volume overload are required [ 4], subsequently allowing the operator to develop hypersensitivity to chronic fatigue and overtraining [ 5]. However, incomplete recovery from training may also lead to chronic fatigue, which inevitably hinders operator preparedness [ 6]. Fortunately, scientifically validated approaches in sport and exercise sciences provide the basic concepts and principles for applications of monitoring training loads in tactical settings [ 7].

2. Autonomic Nervous System (ANS)

Since the ANS is intimately connected with other psychological and physiological systems throughout the body, quantifying responsiveness can provide vital information regarding functional occupational performance capacity or adaptions during/after exercise and training iterations [ 38]. This is particularly important to understand in tactical personnel, who are frequently or unpredictably exposed to high acute stressors, bouts of intensive physical activity, extreme environments, and varying occupational schedules (i.e., rotating night shifts, multi-day combative engagements), which may directly influence the cardiovascular system [ 43]. A schematic diagram of SNS and PNS components for various physiological systems is displayed in Figure 1 . The acceleratory responses from the SNS, including elevated heart rates, muscular perfusion, bronchial dilation, and pupil dilation are useful for preparing the body's response to situations requiring physical prowess and precise perceptual motor coupling, which are often noted in tactical settings. Chronically elevated SNS responses can have negative consequences, requiring PNS activation or SNS deactivation to ensure adequate recovery from high intensity events and promote overall health and well-being (i.e., reducing symptoms of burnout, post-traumatic stress disorder, or depression) [ 2, 6, 32]. Further, sympathetic responses can impede activity and efficiency in specific brain regions, such as the pre-frontal cortex, which is responsible for higher level executive functions, and the motor cortices [ 34, 35, 44]. Inhibition of the pre-frontal cortex can negatively affect attention, emotion, and decision-making, while further impeding the fine motor skills needed for occupational tasks of tactical personnel [ 11, 12, 34, 44, 45]. In tactical populations, measuring and monitoring ANS responses, such as HRV, will likely be useful for informing skill efficiency and expertise, individualized training programs, occurrence or likelihood of injury or illness, risks of overtraining, training and recovery statuses, and operational preparedness. An individual's HRV may be described through numerous calculations of raw heart rate data signals, which are deconstructed to quantify the underlying signal components, resonant frequency bands, and glean more value, greater inference, and actionability in the underlying data.

3. Defining Heart Rate Variability (HRV) and Respective Metrics

With respect to training loads and recovery, the most frequently implemented and practically useful HRV parameters are analyzed using time- and frequency-domain methods [ 5, 20, 26]. These commonly used methods represent various ways to view the central tendency (i.e., mean, median, mode), variability, and distribution (i.e., standard deviation) of heart rate over time [ 5]. HRV parameters consider the average values and overall magnitudes of fluctuations to quantify control of heart rate over time. However, two individuals may have the same average R-R interval and heart rate (HR) responses to an event, but vastly different variability of R-R intervals (see Figure 2 for graphic example). The two most commonly accessible/used time-domain parameters are the standard deviation of the N-N intervals (SDNN) and the root mean square of the differences in adjacent N-N intervals (rMSSD). The SDNN represents a coarse quantification of HRV via autonomic regulation from sympathetic and parasympathetic inputs [ 21, 54], while rMSSD represents parasympathetic activity [ 20, 21, 27, 54, 55]. Unlike SDNN, rMSSD is void of HR slow-wave components, resulting in minimal respiratory influence and a more accurate representation of parasympathetic activity [ 46, 54].

Consequently, in a resting state, increases in HF and TP can indicate desired training adaptations and complete systemic recovery, whereas a decrease in HF and TP typically indicates unwanted training effects, maladaptation, and incomplete recovery [ 26, 34, 35]. Mathematically, the modulation of HF to TP (HP/TP) may index the proportion of parasympathetic activity within the entire ANS frequency-domain spectrum. Overall, many HRV metrics exist ( Table 1 ) and represent specific ANS domains, but their applicability and accuracy in specific settings need to be considered prior to their use. Typically, the three variables often considered the most meaningful and actionable are rMSSD, HF, and TP. The normative values for each of these components are dependent on many factors, such as age, gender, fitness, and various demographics. Therefore, context is required when drawing conclusions based on these HRV metrics. For all of these reasons, HRV metrics should be utilized as within-individual changes. Comparing one individual's HF score to another's would not provide any personalized insights. Overall, these HRV metrics will initially decrease under persistent conditions of high stress environment, poor sleep hygiene, poor nutrition, inadequate exercise, durations of isolation or harmful relationships. On the contrary, higher HRV values are often associated with positive health states and functionally beneficial adaptations, including cardiovascular fitness and resiliency to training loads or other various stressors.

Table 1

Time Domains in Short-Term Recordings

Index

Definition

Interpretation

Correlates

SDNN (ms)

Standard deviation of all R-R intervals

Global quantification of HRV

Total Power

rMSSD (ms)

Root-mean square of successive differences between R-R intervals in a specified time segment

Vagal tone

High Frequency, Parasympathetic activity

Basic Frequency Domains

Index

Definition

Interpretation

Correlates

VLF (ms ²)

Power in the very-low frequency range
(<0.04 Hz)

Hormonal factors and peripheral thermoregulation origination

Parasympathetic activity

LF (ms ²)

Power in the low frequency range
(0.04-0.15 Hz)

Baroreflex, arousal

Sympathetic activity,
Parasympathetic activity

HF (ms ²)

Power in the high frequency range (0.15-0.4 Hz)

Cardiopulmonary reflex, cognitive regulatory state, dependent on resource availability and interpretation of environmental demands

Parasympathetic activity

LF/HF

Low frequency/high frequency ratio

Sympathetic-parasympathetic balance (assuming known LF)

Sympathetic activity,
Parasympathetic activity

Total Power

Total power in the entire frequency range (<0.4 Hz)

General autonomic resource allocation

Sympathetic activity.
Parasympathetic activity

HF/Total Power

High frequency/total power ratio

Proportion of parasympathetic to total autonomic resources

Parasympathetic activity

4. Implementation and Analysis of Heart Rate Variability (HRV) Measures

4.1. How to Measure HRV

Field based HRV monitoring is considered user friendly, objective, and a particularly reliable tool, which can be easily used to manage training loads and facilitate proper recovery [ 5, 53]. Additionally, HRV assessment and monitoring tools have become more affordable and practical to use [ 26, 27]. With a continuously growing number of commercial devices capable of measuring and reporting HRV, it is first important to understand device accuracy [ 70], then equally necessary to understand their compatibility for use in the field with tactical populations. For example, the placement of the sensors, wiring requirements, duration of battery life, internal memory capacities, device durability (i.e., handling weather conditions and impacts), and screen displays (i.e., ability to be shut off on night missions) should all be considered when selecting devices. Detailed information on wearable sensors capable of measuring HRV in military settings can be found in a recent review article [ 71], which highlights the pros and cons of several devices. Overall, practical, commercially available wearable sensors tend to lack adequate battery life and internal memory for long duration recordings (24 h cycles), while clinical devices (3-12 lead electrocardiography) provide the most accurate results for long and short durations, particularly at rest [ 72]. Despite better signal to noise ratios, clinical electrocardiography (ECG) devices may still be susceptible to motion artifact or electrode impedance and lack practicality due to their bulkiness and wiring requirements, making them less suitable for tactical environments.

4.2. When to Measure HRV

A measurement's purpose dictates the appropriate schedule of data collection timepoints (e.g., time of day, duration, and frequency) [ 5]. Monitoring HRV may involve periodic resting measures and pre- and post-training assessments across key timepoints to capture autonomic resource distribution, consumption, and restoration before, during, and following specific task demands. Following training, the time required for HRV to return near baseline values may demonstrate the relative ability to recover from training sessions of various intensities and volumes. However, to fully understand the demands of specific occupational tasks (i.e., clearing a house in military or law enforcement) it is important to include HRV monitoring during the task, in addition to baseline and post-event measures. Since time of day, similar to body position, will significantly influence HRV [ 38, 76], it should be matched across repetitive assessments, where able. Otherwise, comparisons across time-points will be difficult to interpret (i.e., unsure if change is due to training loads or fluctuations in diurnal variation). For example, HF increases at night and decreases during the day, emphasizing the importance of both nocturnal and resting awake readings [ 26, 38]. Figure 3 offers a graphical depiction of the opportunities for HRV measurements throughout the day along with context considerations at each time point.

4.2.1. Establishing Baseline HRV Measures

Establishing a resting state, baseline, HRV is important for assessing overall ANS modulation and cardiac vagal tone. Following the principles of resource theory, baseline HRV identifies the pool of available ANS resources affecting the cardiac cycle [ 77]. The volume of ANS resources utilized while preparing for, completing, and recovering from psychophysiological demands is related to the imposed workload and functional efficiency to accomplish the task [ 78]. Comparing the HRV during tasks to HRV before and after tasks can be accomplished through normalization, difference scores, and percentage values for indices of training and recovery loads, respectively. When collecting baseline or resting HRV measures, the supine position is recommended, although sitting or standing may be warranted in field exercises (keeping in mind that methodological consistency is critical) [ 5, 79]. Lastly, confounding factors from external (e.g., temperature, noise, and lighting) and internal (e.g., physical and emotional behaviors leading up to testing) need to be controlled and kept consistent as they will affect HRV and the ability to record "true" baseline values.

To establish an individual baseline for consistent daily and actionable assessments, HRV measurements should be recorded over a minimum 1-2 week period [ 46], before making any adjustments to training load or recovery. The utilization of within-individual HRV metric changes are further improved by quantifying rolling 28-day averages [ 80] and assessing the individual's deviation from this average. Using the 28-day rolling average, an individual's change in status is identified as the difference between daily mean (or specific event) HRV and the prior 28-day average HRV. Typically, raw or standard scores (Z-score, percentage change) are compared to rolling average measures. With this approach, acute stress or incomplete recovery states may be denoted by reduced rMSSD as compared to that individual's 28-day average, as shown in Figure 4 below where Z-score values are less than zero. Conversely, completed recovery could be indicated by maintained or increased HRV [ 81] shown in Figure 4 where Z-score vales are greater than zero. However, it should be noted that excessive lengths of incomplete recovery can result in an exaggerated increase in parasympathetic modulation, as an attempt to force systemic recuperation efforts [ 82, 83].

4.2.2. Nocturnal and Morning HRV Measures

Nocturnal and morning HRV indicate different psychophysiological processes; however, both inform recovery and readiness status. In long-term monitoring, HRV quantification represents the amount of recovery in relation to average baseline HRV pre-awakening (i.e., during sleep) [ 84]. Nocturnal HRV, although not completely understood, is commonly used to assess sleep quality, resource recovery, and systemic adaptations [ 85]. Moreover, assessments pre/post sleep onset and pre/post awakening provide insights to the systemic resources leftover from yesterday's workload and the subsequent night's recovery, without and with conscious cognitive processes involved [ 85]. As stated previously, HRV measurements must be done in a consistent fashion for proper utilization for training load and recovery monitoring. Whether an end user chooses nocturnal or post wake measures, they should be done consistently.

4.2.3. Pre-, During-, and Post-Event HRV Measures

Measuring HRV pre-, intra-, and post-task (e.g., physical or cognitive) may indicate autonomic resources roused in the anticipation of completing a task, the resources required to execute said task, and the rate of resource recovery following completion [ 35]. HRV responses to events, including training simulations, exercise, or real-world applications, may require a resting HRV measurement 5-10 min prior and a recovery HRV measurement 5-10 min after the event (or at least a known baseline value for difference score calculations). Depending on the circumstances, the first 5 min of a 10 min resting assessment may be discarded as part of an acclimation phase, while the remaining 5 min more accurately represents baseline or recovery HRV. Regardless, the timing of pre- and post- event measures should be similar (i.e., resting measurement 5 min before = resting measurement 5 min after). Furthermore, additional timepoints (e.g., 5-10-, 10-15-, and 15-20 min post-event) may provide further information on the time-course of recovery following the event. The same original temporal duration (e.g., 5-min) should be used for all time blocks and may require parsing into multiple sample windows. Body position (e.g., supine, seated, standing) should also be the same for any pre- and post-measurements.

The modulation of HRV measures can be used to assess various tasks, such as the observe-orient-decide-act cycle of an advanced marksmanship/close quarter battle (CQB) assessment or intense training session, but require more precise time stamping and data parsing compared to pre- and post-task measures [ 35]. Further, during longer duration sessions, behavioral events (e.g., start/stop, breach, contact, endex, exfil) should be used to determine the context-dependent time windows of specific activities being investigated. Delta scores among time periods and rest/baseline represent the autonomic modulation allocated to process changes in environmental states and accomplish relevant behavioral responses. For example, the difference between baseline and pre-task HRV provides insights to biological priming and arousal/anxiety management, while the difference between baseline to during-task HRV indicates the required ANS resources to achieve the cognitive/motor performances [ 34, 69, 86]. As a novice develops expertise, the shift toward automaticity of underlying control processes frees up systemic resources, including HRV [ 87]. Thus, the aforementioned use of HRV may inform the tasks' relative difficulty or unfamiliarity, as well as the individual's level of efficiency and resiliency (i.e., ease of task completion and ability to quickly recover between tasks). For example, smaller changes in HF HRV (i.e., parasympathetic tone), compared to baseline, would suggest greater autonomic resource efficiency and heightened resiliency [ 11, 35, 88, 89]. This type of HRV perception-action coupling analysis provides a psychophysiological measure of resource allocation, which permits the comparison of performance effectiveness (e.g., resulting outcomes) with resource efficiency (e.g., HRV modulation from baseline) to determine skill automaticity and expertise. Additionally, it enables the tracking of individual progression over time.

4.2.4. Recording 24 h HRV

Continuous measurements over a 24 h period may represent HRV responses to all the various stimuli encountered during typical, daily occupational endeavors. In daily HRV, values below an individual's normative rolling baseline could indicate incomplete recovery from high training volume, high training intensity, onset of illness, chronic stress, or poor sleep [ 84]. However, more research is necessary to understand the complexities of daily HRV, which can fluctuate greatly due to diurnal variations in circadian sleep patterns/rhythms, body temperature, metabolism, activity [ 26]. Still, by encapsulating 24 h HRV recordings a more holistic measure of HRV is permitted. The circadian rhythms may best be accounted for by segmenting the HRV data into daily events, such as nocturnal, morning, training, and occupational events. Nonetheless, a single short duration value should not be used interchangeably with 24 h recordings. The 24 h recordings have been shown to be more reliable than 5 min resting measures, which led some to suggest the use of 24 h recordings for intervention studies [ 26]. Using the 24 h recording may also allow the quantification of overall ANS responses to a range of stimuli, providing more granularity into wholesome psychophysiological states. However, the execution of collecting 24 h HRV data also requires more buy-in from individuals as they will be tasked with wearing the sensor constantly and ensuring adequate battery charging. Thus, it is necessary to consider whether a method will result in high attrition rates, since large quantities of missing data will not permit reliable, actionable inferences.

5. Monitoring Heart Rate Variability (HRV) in Tactical Populations

5.1. Relations between HRV and Stress

5.2. Relations between HRV and Physical Occupational Performance and Training Loads

Conventionally, HRV is mainly directed at capturing internal cardiovascular strain resulting from (or required to accomplish) a given external training load. However, for the tactical athlete, quantifying responses to external workloads and cardiovascular homeostatic balance is only one of many actionable insights HRV can afford. Theoretically, running faster and lifting heavier require more resources through PNS withdrawal and SNS stimulation, which influences cardiac output to become more forceful, rapid, and less variable. The scientific literature supports the link between HRV metrics, aerobic adaptations, and general fitness levels [ 95, 96, 97], although these relationships may be dictated by the individual's characteristics and training intensities and volumes. HRV may be useful for understanding the current state of aerobic fitness and monitoring aerobic training adaptations. However, it is important to note that the HRV is mainly a marker of neurocardiac ANS and its direct relation to neuromuscular performances of maximal strength and power output are unclear [ 97]. Training programs which base workload adjustments on HRV feedback have demonstrated greater increases in countermovement jump and sprinting abilities [ 98]. Therefore, despite a lack of direct associations between HRV and neuromuscular capabilities, HRV monitoring may improve training program design by prescribing adequate recovery and training volumes. Thus, it is recommended that HRV monitoring is used in conjunction with other monitoring tools including external and internal loads, as well as measures of neuromuscular capacities such as those described in prior literature reviews [ 99].

5.3. Relations between HRV and Cognitive/Motor Skill Performance

5.4. Recovering to Restore HRV

Post-exercise recovery has gained a great deal of attention in the past decade, leading to a rise in scientific understanding of accelerated recovery techniques to elicit restored autonomic and cardiovascular homeostasis [ 124, 125]. Flotation using R.E.S.T. (Restricted Environmental Stimulation Therapy) has been effective in increasing sleep quality [ 126] and HRV, specifically the parasympathetic marker HF [ 127]; thus, being effective in facilitating ANS balance, regardless of pre-float autonomic dominance. Whole-body cryostimulation has an immediate and lasting effect on ANS balance that may be observed via improved HRV [ 128, 129]. For example, 3 min whole-body cryostimulation has increased HF, decreased LF, and decreased LF/HF ratios for upwards of 6 h post-cryotherapy [ 130, 131, 132]. Cryotherapy in the evening, after training or competition, has also shown positive effects on sleep quality [ 129, 131, 132], which subsequently would improve HRV considering the positive influence of quality sleep hygiene on HRV.

Photobiomodulation (PBM), formerly known as Low-Level Laser Therapy (LLLT) [ 133], uses red and near-infrared light spectrum wavelengths (600 nm-1100 nm) through light emitting diodes (LED), lasers, or a combination of both, to deeply penetrate the skin and be absorbed by mitochondria of underlying structures (e.g., musculoskeletal tissue) to induce recovery [ 134, 135]. Although limited, the emerging research has shown that whole body PBM may increase sleep quality and the HF of HRV [ 133, 136, 137].

Many of the aforementioned methods show promise for improving sleep, HRV, and recovery, but require further investigation as to their true efficacy and practicality. However, through the use of a holistic physiological monitoring program, including consistent HRV assessments, it is likely that individual prescription of recovery can be established for the tactical athlete.

6. Conclusions

The key parameters of HRV to be concerned with are rMSSD, HF, and TP. Each provides essential information regarding ANS regulation, making HRV a valuable tool to assess the overall health, wellness, and fitness of tactical personnel [ 5, 27]. Training load is affected by intensity and physiological impact of training, which are represented by the changes in HRV during exercise and recovery [ 79]. HRV is also influenced by cognitive load, physiological and emotional stress, and environment [ 18, 31, 123]. In long-term monitoring, it is a non-invasive method to assess the ANS and make appropriate changes to training and recovery [ 5]. In short-term monitoring, HRV is useful in assessing responses to acute stimuli, such as a physical or cognitive training events [ 138]. Since, age, body position, and time of day all influence HRV measurements [ 5, 38, 52, 139, 140], HRV should be highly individualized and assessed relative to the individual. Statistically, time periods before, during, and after activity are compared between one another (and as deltas from accurate baselines or rolling averages) to determine resource modulation in relationship to task performance.

Within high performing populations, HRV assessment and monitoring has recently risen in popularity as one method to foster awareness and avoid overtraining or non-functional overreaching [ 18, 53]. The HRV measures may also be used to understand the differences in occupational requirements among occupational specialties. For example, occupational tasks with high demands for concentration and precision, such as paratroopers, dynamic precision shooters, or medics, require significantly more resources and SNS activity than low-stress, office-based occupational tasks [ 43]. More traditionally, HRV is clinically used to assess both illness and wellness [ 27, 138], primarily cardiac morbidity and the progression of cardiac death, as well as diabetic neuropathy [ 5, 20], all of which should be confirmed by qualified physicians. More recently, HRV metrics are used to garner further understanding of training workload and physical exercise effects on the body [ 5, 20, 46, 53]. The information gathered is extremely valuable when assessing expertise or preparedness to train, planning training loads, and prescribing the utilization of recovery tools or techniques [ 5]. For example, those accustomed to greater levels of physical activity and more experienced in given occupational tasks will have improved overall stress management processes, which may indicate improved capacities for handling certain occupational specialties [ 43]. The likelihood for facilitating greater awareness of one's own training, recovery, and lifestyle habits, as well as the effect of these habits on their psychophysiology (i.e., HRV), provides another direct benefit of monitoring HRV. Ultimately, HRV measures can be useful for indicating overall health and physical and cognitive preparedness of all tactical personnel, which may be particularly informative for occupational specialties that possess extremely low margins for error in high stress environments (e.g., special forces). Thus, physiological feedback via monitoring HRV would prove useful for addressing the individual's present capability to handle occupational tasks, be it during selection processes or returning to duty following injury or illness but should not be used to diagnose any pathological conditions.

Author Contributions

The review was conceptualized by M.D.S., A.G.T. and J.A.H. The literature search and data collection were performed by M.D.S., A.G.T. and J.D.S. The manuscript was written by M.D.S. and A.G.T. The manuscript was critically reviewed and edited by A.G.T., J.D.S., J.J.M. and J.A.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Air Force Research Laboratory (AFRL) and Special Operations Forces Acquisition, Technology, and Logistics (SOCOMSOF AT&L) via KBR contract # LX06000011.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors claim no conflict of interest.

Footnotes

Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

16. Fields J.B., Esco M.R., Merrigan J.J., White J.B., Jones M.T. Internal training load measures during a competitive season in collegiate women lacrosse athletes. Int. J. Exerc. Sci. 2020;13:778-788. [[ PMC free article] [ PubMed] Google Scholar]

21. Vigo D.E., Siri L.N., Cardinali D.P. Psychiatry and Neuroscience Update. Springer; Berlin/Heidelberg, Germany: 2019. Heart Rate Variability: A tool to explore autonomic nervous system activity in health and disease; pp. 113-126. [ Google Scholar]

22. Hoareau V., Godin C., Dutheil F., Trousselard M. The Effect of stress management programs on physiological and psychological components of stress: The influence of baseline physiological state. Appl. Psychophys. Biofeedback. 2021:1-8. doi: 10.1007/s10484-021-09508-0. [[ PMC free article] [ PubMed] [ CrossRef] Google Scholar]

25. Taylor M.K., Gould D.R., Adams B.D., Potterat E.G., Dial Ward M.D., Padilla G.A., Evans K.E., Markham A.E. Age-Matched Comparison of Elite and Non-Elite Military Performers during Free Living and Intense Operational Stress. Naval Health Research Center; San Diego, CA, USA: 2009. [(accessed on 3 February 2021)]. Available online: [ https://apps.dtic.mil/sti/citations/ADA499987 Google Scholar]

47. Williams S., West S., Howells D., Kemp S.P., Flatt A.A., Stokes K. Modelling the HRV response to training loads in elite rugby sevens players. J. Sports Sci. Med. 2018;17:402. [[ PMC free article] [ PubMed] Google Scholar]

63. Fantozzi M.T., Artoni F., Faraguna U. Heart rate variability at bedtime predicts subsequent sleep features; Proceedings of the 2019 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC); Berlin, Germany. 23-27 July 2019; pp. 6784-6788. [[ PubMed] Google Scholar]

77. Kahneman D. Attention and Effort. Prentice-Hall; Upper Saddle River, NJ, USA: 1973. [ Google Scholar]

90. Maupin D., Wills T., Orr R., Schram B. Fitness profiles in elite tactical units: A critical review. Int. J. Exerc. Sci. 2018;11:1041. [[ PMC free article] [ PubMed] Google Scholar]

108. Pojman N., Behneman A., Kintz N., Johnson R., Chung G., Nagashima S., Espinosa P., Berka C. Characterizing the psychophysiological profile of expert and novice marksmen; Proceedings of the International Conference on Foundations of Augmented Cognition; San Diego, CA, USA. 19-24 July 2009; pp. 524-532. [ Google Scholar]

123. Kim J.W., Dancy C., Goldberg B., Sottilare R. International Conference on Augmented Cognition. Springer; Cham, Switzerland: 2017. A cognitive modeling approach-does tactical breathing in a psychomotor task influence skill development during adaptive instruction? pp. 162-174. [ Google Scholar]

Articles from International Journal of Environmental Research and Public Health are provided here courtesy of Multidisciplinary Digital Publishing Institute (MDPI)

# data[].attributes.attachment_link.title
Applying Heart Rate Variability to Monitor Health and Performance in Tactical Personnel: A Narrative Review

# data[].attributes.attachment_link.description
Human performance optimization of tactical personnel requires accurate, meticulous, and effective monitoring of biological adaptations and systemic recovery. Due to an increased understanding of its importance and the commercial availability of assessment ...

# data[].attributes.attachment_link.title
Everything You Should Know About Heart Rate Variability (HRV)

# data[].attributes.headline
Systematic Review of HRV for Tactical Athletes 

# data[].attributes.subject
Systematic Review of HRV for Tactical Athletes 

# data[].attributes.attachment_link.readable_text
Abstract

Objectives: Heart Rate Variability has gained substantial interest in both clinical and athletic settings as a measurement tool for quantifying autonomic nervous system activity and psychophysiological stress. However, its uses in tactical work settings, such as military, police, and firefighting environments, remain controversial. Given the physical, mental, and emotional stress public safety personnel face both operationally and in training, heart rate variability measurement may be key in promoting their health, safety and operational effectiveness.

Methods: This study identified, critically appraised, and summarized primary studies investigating relationships between heart rate variability and outcomes of interest to tactical personnel. Key literature databases were searched, and quality assessment checklists were applied to analyze retained literature. The results of the screening and assessment processes, along with key data extracted from each study were summarized and tabulated. Research gaps were also identified to facilitate improvements to how tactical personnel and health or performance providers may best utilize heart rate variability to monitor or promote personnel health and performance, and thereby facilitate public safety.

Results: Twenty studies were included and were all of generally high quality. Cohort size, length of follow-up, measurement objectives, data acquisition, and data analysis all varied considerably across studies, precluding meta-analysis. However, study results correlating heart rate variability and relevant outcomes indicated that overall, heart rate variability is an effective indicator of key fitness and performance elements in the tactical work setting.

Conclusions: Heart rate variability can be an effective health and performance tool in tactical work environments. However, measurement methods must be carefully selected and applied. Further research is required to understand causal relationships. Specifically, larger cohort inclusion and the isolation and study of specific variables unique to public safety work and training may improve the effectiveness of heart rate variability measurement to provide meaningful information to end users and providers.

Keywords: injury prevention and reduction, occupational stress and mental-physical health, physiological monitoring data, military, police, firefighting, first responder, occupational fitness

Introduction

Heart rate is typically described as the number of beats of the heart per minute, however, the time between beats in a healthy heart is highly variable; the heart is under constantly varying regulation modulating its activity from the two complimentary branches of the autonomic nervous system (ANS): the sympathetic and parasympathetic nervous systems ( 1, 2). The autonomic nervous system is the efferent branch of the peripheral nervous system responsible for regulating most unconscious bodily activities; respiration, distribution of blood flow, digestion, pupillary response, waste excretion, arousal, and heart rate ( 1). The ANS is itself composed of two complimentary pathways that typically act antagonistically to maintain homeostasis. The sympathetic nervous system originates from the thoracic and upper lumbar vertebrae, and governs response that require immediate action, such as the "fight or flight" response to acute stress. The parasympathetic nervous system is characterized as a more slow-moving, dampening response, and governs actions that do not typically require immediate action ( 1).

With respect to the heart specifically, the sympathetic nervous system increases heart rate, which will decrease the variation in time between beats. This is accomplished not only through direct innervation, in which the muscle is stimulated to contract more quickly and more forcefully, but also through catecholamines secreted via the adrenal glands ( 1). The parasympathetic nervous system regulates the heart through the vagus nerve, slowing conduction of the sinoatrial node and contributing to respiratory sinus arrhythmia, the natural increase in heart during inspiration and decrease during expiration ( 3).

Disruption of the balance between sympathetic and parasympathetic influences on the cardiovascular system can lead to devastating health consequences ( 4, 5). These disruptions have been described as excess allostatic load ( 6). Allostatic loading occurs in situations of either energy insufficiency, in which energy demand exceeds supply, and focus must be diverted wholly to survival and the maintenance of a positive energy balance, or in situations with energy abundance, but social conflict or disruption ( 6). While the two types of allostatic load differ to some extent, both disrupt cardiovascular autonomic balance, and are associated with the development of chronic diseases, such as excessive inflammation, chronic pain, diabetes, asthma, fatigue, depression, and anxiety ( 5). Acute injuries, such as concussion, may also contribute to elevated allostatic load ( 7), and consequent disruptions to cardiac autonomic balance. Conversely, higher levels of physical fitness are associated with a more optimal cardiovascular autonomic balance ( 8).

HRV has been identified as a valid measurement of autonomic nervous system regulation and has both time and cost advantages over other methods, such as biomarker testing, while also being noninvasive. With recent technological developments, HRV data are now available and interpretable by coaches and individual athletes, not requiring specialist appointments to read or understand results ( 9). Correlations between HRV measurements and process or outcome measures are allowing for readily accessible insights to valuable health and fitness data ( 10, 11) and have been associated with the prediction of morbidity from the chronic and acute disorders mentioned above. Individuals at high risk for the development of ANS dysregulation or allostatic loading and consequent morbidity may therefore benefit from HRV monitoring. In the athletic performance setting, HRV data has been proven useful in improving the precision of energy expenditure estimates ( 12). More recently, HRV has been investigated as a means of quantifying fatigue and recovery levels, detecting overuse injuries, and calibrating training loads ( 11, 13, 14). Research continues to emerge linking HRV domains to human performance optimization efforts, which have themselves recently gained interest within the tactical community.

Tactical personnel, individuals who have sworn to protect and serve their communities ( 15), and who may place their own health and safety at risk in execution of those duties, are often faced with a multitude of unique challenges that often result in stresses to their autonomic nervous systems ( 16, 17), and may benefit from access to the fitness and health data HRV analyses can provide. For example, police officers and other first responders (firefighters, emergency medical responders) may be sedentary for the majority of their time on duty but could be called with little to no warning into situations of extreme danger and physiological stress ( 18). The ability to monitor physiological response during these events may help protect their health. Likewise, military personnel, while often prepared in advance for deployment, are exposed to high levels of physical, mental, and emotional stress for prolonged periods of time ( 19), and may thus also benefit from physiological data measurement and analysis. Furthermore, tactical personnel are also regularly subjected to operations in austere environments, dysregulated sleep and poor nutrition ( 20, 21). As a result of these occupational demands, literature has reported the risk of cardiovascular disease, especially in police officers, may be greater than that of the general population ( 22). Conversely, many tactical personnel perform at extraordinary levels of physical performance and may seek to optimize their training, avoid overtraining and maximize the balance between training and operational demands ( 23).

As a result, HRV applications developed in both clinical and athletic settings may be of interest to tactical organizations seeking an inexpensive and noninvasive means of monitoring the health and fitness of their personnel, both in training and in operations. However, the unique requirements of working as a tactical professional may dictate that functional inferences derived from HRV measurements originally developed in athletic or clinical populations may not always generalize to tactical populations ( 15). To date, no comprehensive, systematic reviews of the literature examining associations between occupational fitness, operator or trainee health, or occupational performance and heart rate variability analyses for tactical personnel have been conducted. Therefore, the aim of this systematic review was to identify primary studies examining relationships between HRV and relevant health and operational outcomes specifically in tactical personnel, critically appraise the methodological quality of the identified studies, and summarize the results to inform tactical professionals and those in the health and performance fields supporting tactical professionals.

Survey Methodology

Search Method

Prior to conducting the initial search, this study was registered with PROSPERO (ID: 153293). The PRISMA guidelines for systematic reviews were followed and the outcome of each step can be found in Figure 1 ( 24). A rapid search using a set of predetermined keywords was conducted to determine relevant subject heading terms and develop a sample of salient articles to guide a detailed search strategy.

The PICO strategy developed to construct the organization and syntax of search terms was as follows: Patient(s); tactical personnel, and derivative, related, or more specific terms that would capture research conducted with public safety operators or trainees, Intervention; while no interventional studies were expected, given the observational nature of HRV analysis and the research aim, if studies compared HRV to another health or performance monitoring modality, results could be considered provided details of the HRV analysis were included. The control/comparison was not limited or specified; any comparison method could be included, provided the specific details of HRV monitoring were reported. For outcomes, a wide variety of health and occupational outcomes were considered, including physical fitness/injury, mental/emotional stress, or recovery, resilience or job performance.

Each database (Pubmed, CINAHL, Embase, and SportDiscus) was then sequentially searched by a single author (CT) using database heading terms, Boolean operators and available filters. A second author independently verified the search results (RO). The Defense Technical Information Center (DTIC) was also searched to capture gray literature not indexed in traditional academic databases. The DTIC was utilized as an expedient means of capturing information relevant to the research question and population of interest that was not peer-reviewed but was still likely to contribute to the aim of the study. The final search was executed and verified on 9 October 2019. Details of the finalized search strategy and initial results can be seen in Table 1 . Duplicates were screened using EndNote software (Clarivate Analytics, Philadelphia, PA, USA), while additional duplicates were removed manually.

Table 1

Database

Population

Target variable

Outcome

Results

Article Screening

Review studies, conference proceedings and other literature that was not primary, not peer-reviewed, not available in English or had no available abstract was also rejected manually if not captured by search filters. Article titles clearly irrelevant to the topic (e.g., drug intervention studies, in vitro studies) were also removed. Articles reporting results from animal studies, a human population that did not include active or veteran tactical personnel, and studies using HRV as an intervention rather than a measurement (e.g., biofeedback device studies) were also screened out for the purposes of this review.

Inclusion and Exclusion Criteria

Developed inclusion criteria were applied to the remaining titles: tactical personnel or trainees (well or unwell), veterans, measurement of HRV, assessment of a health/fitness/occupational outcome, use of HRV analysis findings to predict, measure or monitor a state of disease, physical or occupational performance. All study design types were considered, provided that HRV analyses were reported in relation to another factor. Article relevance to the question was first screened by CT. An additional author verified inclusions and exclusions (RO). Disagreements were adjudicated by a third author, BS.

Quality Assessment

The Critical Appraisal Skills Programme (CASP) toolkit (Middle Way, Oxford, UK) ( 25) was selected for methodological assessment of the included studies. This assessment system has been used in previous systematic reviews and allows for fair and equitable assessment of a variety of study types ( 26). Three CASP checklists were ultimately necessary to evaluate the selected studies; the cohort study checklist, comprised of 13 total questions, the case-control checklist, comprised of 11 questions, and the randomized control trial checklist, also of comprised 11 questions. The randomized control trial checklist was the most suitable for quasi-experimental studies included in this review. The first two questions of each checklist screen for validity, and the following 9-11 questions guide reviewers through the assessment of study results, relevance of the results, methodology, and applicability. On the cohort study checklist, Question 7 simply asks for a summary of the results. This question was omitted, resulting in a final maximum score of 12. For each question that could be answered dichotomously, a publication was awarded with 1 point for answers of "yes" and 0 points for answers of "no" or "can't tell." The case-control and randomized-control trial checklists required no modification beyond score quantification. For any questions which were not answered dichotomously within the checklist, an objective parameter was fit to the question. For example, on all checklists, there is a question pertaining to the precision of the results, phrased, "how precise were the results?" This question was awarded a "1" (yes) if exact measurements of significance, rather than inequalities, were reported. For other questions asking for treatment effect sizes, publications were awarded "1" point (yes) for including power or effect size analysis, and "0" points (no) if no determination was made as to the effect size. Disagreements in CASP scoring between authors were resolved by consensus. A referee (RO) was appointed prior to initiation of scoring. Screening and quality assessment were completed 2 February 2020.

Results

The results of the search, screening, and selection processes are summarized in the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) flow diagram ( Figure 1 ). A total of 296 citations were captured in the finalized search, executed on 9 October 2019. At that time, all eligible results were downloaded, retained and screened in accordance with the screening methodology. A total of 56 duplicates were removed, leaving 233 titles. Of these, 194 were identified as irrelevant to the research question. From the remaining studies, the following exclusion criteria were developed: post-traumatic stress disorder (PTSD) or depression were the condition of interest ( n = 10), an environmental condition was a key variable in the study ( n = 2), HRV was measured, but not linked to a health and fitness or occupational outcome ( n = 7). The remaining 20 publications were retained for this review. Further screening for quantitative review was considered, but ultimately not possible due to the heterogeneity of the included studies. The review was completed on 14 March 2020.

Critical Appraisal

The mean CASP of the quasi-experimental studies was 9.67 ± 0.58 of 11 maximum points. The mean CASP of the case-control studies was 10.5 ± 0.7 of 11 maximum points, and the mean CASP of the cohort studies was 10.73 ± 0.78 of 12 maximum points.

Data Extraction

Details of all relevant data retrieved from each included study can be found in Table 2 . The Author(s), title, year, participants, demographics as available, anthropometrics as available, the performance metric(s) and key results, significant or not, as well as the final CASP score are included. The HRV analysis or analyses selected and their results are also included if succinct summarizations of the data were possible. Studies are organized first by design methodology, then alphabetically.

Table 2

References/Title

Participants

HRV analysis

Performance measurement

Results

CASP Score

Gamble et al. ( 27)
Different profiles of decision making and physiology under varying levels of stress in trained military personnel

Male US infantry soldiers/SRT operators ( n = 26)
Age: 30.73 ± 7.71 yrs

Low Frequency domain (LF)
High Frequency domain (HF)
LF/HF Ratio

Threat identification simulation: accuracy, sensitivity
Low stress and high stress shoot conditions

Basal HF-Accuracy: positive relationship, p = 0.0379
Basal HF-sensitivity: positive relationship, p = 0.0379
LF/HF-stress condition: negative relationship, p = 0.0379

10/11
Quasi-Experimental

Marcel-Millet et al. ( 28)
Physiological responses and parasympathetic reactivation in rescue interventions: The effect of the breathing apparatus

French Firefighters
( n = 28 males)
Age: 37 ± 7 yrs
Height: 179 ± 6 cm Weight: 76 ± 9 kg BMI: 24 ± 2
( n = 6 females) Age: 29 ± 3 yrs
Height: 171 ± 4 cm Weight: 64 ± 3 kg BMI: 22 ± 1

SDNN
LnRMSSD

Three load conditions: personal protective clothing only (PPC); PPC and the full self-contained breathing apparatus (SCBA); and with PPC and only the cylinder of the SCBA (SCBAc)
One baseline unloaded condition: Intermittent Fitness Test (IFT)
VO 2max:
54.3 ± 4.9 ml/kg/min

PPC-SCBA, SDNN:
27.8 ± 14.1-21.4 ± 9.2 ( p <0.05)
IFT-PPC, LnRMSSD: 2.4 ± 0.5-2.1 ± 0.5 ms p <0.01
IFT-SCBAc, LnRMSSD: 2.4 ± 0.5-2.0 ± 0.5 p <0.01
IFT-SCBA, Ln RMSSD: 2.4 ± 0.5-2.0 ± 0.5 p <0.01

10/11
Quasi-Experimental

Sanchez-Molina et al. ( 29)
Effect of Parachute Jump in the Psychophysiological Response of Soldiers in Urban Combat

Male Spanish Army Soldiers ( n = 19)
Age: 31.9 ± 6.2 yrs
Height: 173.6 ± 5.3 cm
Weight: 73.8 ± 8.3 kg
BMI: 24.2 ± 2.3
Experience: 12.8 ± 7 yrs

Blood Lactate increased significantly at the end of the simulation regardless of infiltration method ( p <0.05)
RMSSD and HF domain decreased while LF increased regardless of infiltration method ( p <0.05)
RMSSD-ΔLactate: r = −0.504 ( p = 0.039)
LFpost-ΔLactate: 0.589( p = 0.013)
HFpost-ΔLactate: r = −0.589 ( p = 0.013)
ΔHF-BOS: r = 0.493 ( p = 0.044)

9/11
Quasi-Experimental

Porto et al. ( 30)
Firefighters' basal cardiac autonomic function and its associations with cardiorespiratory fitness

Two groups of Male Brazilian Firefighters
( n = 38, on-duty)
Age: 41 ± 11 yrs
BMI: 26.1 ± 7.8
( n = 26, off-duty)
Age: 40 ± 12 yrs
BMI: 26 ± 13

pNN50:
CRF <12METs: 1.3
CRF>12METs: 3.6
RMSSD: CRF <12METs: 18.3
CRF>12METs 23.8
LF/HF ratio: CRF <12METs: 5.2
CRF>12METs:
3.3

Maximal metabolic equivalent capacity (METs):
VO 2max (on-duty): 42.4
VO 2max (off-duty): 40.0
Groups divided into <12METs or >12METs
On duty status

Significant differences between those with METs <12 and >12:
PNN50 ( p = 0.07)
RMSSD ( p = 0.03)
LF/HF Ratio ( p = 0.01)
No difference between duty status

11/11
Case-Control

Sanchez-Molina et al. ( 31)
Assessment of Psychophysiological Response and Specific Fine Motor Skills in Combat Units

Two groups of male Spanish Army Soldiers
( n = 19 Light Infantry)
Age: 30.2 ± 5.25 yrs
Height: 176.15 ± 8.31 cm
Weight: 77.93 ± 10.25 kg

RMSSD
LF
HF

Simulated Urban Combat
Competitive State Anxiety Inventory (CSAI-2R)

10/11
Case-Control

BMI: 25.00 ± 3.11
Experience: 9.95 ± 5.17 yrs
( n = 12 Heavy Infantry)
Age: 34.5 ± 4.85 yrs
Height: 177.42 ± 7.28 cm
Weight: 79.21 ± 9.57 kg
BMI: 25.10 ± 1.8
Experience: 14.58 ± 4.87 yrs

Light:76.60 ± 10.75
Heavy: 19.25 ± 13.11
HFpre:
Light: 35.10 ± 25.51
Heavy: 80.59 ± 13.41
RMSSDpost:
Light: 7.62 ± 5.62
Heavy: 88.68 ± 4.45
CSAI-2R correlated with all HRV measures:
HFpre: r = 0.382( p = 0.034)
LFpre: r = 0.406 ( p = 0.24)
RMSSDpre: r = 0.416 ( p = 0.022)
HFpost: r = −0.487 ( p = 0.006)
LFpost: r = 0.424( p = 0.020)
RMSSDpost: r = 0.433 ( p = 0.017)

Andrew et al. ( 32)
Adiposity, muscle and physical activity: Predictors of perturbations in hear rate variability

Buffalo, NY (USA) Police Officers ( n = 360)
Age: 42.1 ± 7.66 yrs
BMI: 29.0 ± 4.40
Resting HR: 64.0 ± 8.82 bpm

Low Frequency domain natural log (LnLF): 5.31 ± 0.91
High frequency domain natural log (LnHF):
4.96 ± 1.13

Physical Activity Recall Questionnaire
(Ln Physical Activity Index): 2.66 ± 1.04

Significant ( p <0.05) correlation between LnPA and HRV:
LnLF: r = 0.071
LnHF: r = −0.035

11/12
Cohort

Andrew et al. ( 33)
Police work stressors and cardiac vagal control

Buffalo, NY (USA) Police Officers
( n = 259 males)
Age: 41.2 ± 6.8 yrs
( n = 87 females)
Age: 40.7 ± 5.5 yrs

Low Frequency domain natural log (LnLF)
High frequency domain natural log (LnHF)

Spielberger Police Stress Survey

No significant correlations for men
Significant ( p = 0.024) inverse relationship between the "lack of support" stressor and LnHF for females

10/12
Cohort

Delgado-Moreno et al. ( 34)
Combat Stress Decreases Memory of Warfighters in Action

Urban Combat simulation
Post-mission event recall questionnaire
Body Temperature, Pre: 37.8 ± 1.2°C, Post: 37.5 ± 1.3°C

Significant correlations between the following:
LFpost, temp post: p = 0.035, r = 0.473
LFpost, correct sound recall: p = 0.020,
r = −0.516
HFpost, temp post: p = 0.035, r = −0.474
HFpost, correct sound recall: p = 0.019, r = 0.517

10/12
Cohort

Diaz-Manzano et al. ( 35)
Higher Use of Techniques Studied and performance in melee combat produce a higher psychophysiological stress response

RMSSD
pNN50
HF (nu)
LF (nu)
HF/LF Ratio
SD1
SD2

Hand-to-hand combat training drill
Analyses divided by higher performing half vs. lower performing half (HPG, LPG)

Significant pre-post differences for all HRV measures in the HPG
Significant differences for all HRV measures in the LPG, except HF ( p = 0.880), LF ( p = 0.164) and HF/LF ratio ( p = -.140)

10/12
Cohort

Duarte et al. ( 36)
Efforts of Patrol Operation on Hydration Status and Autonomic Modulation of Heart Rate of Brazilian Peacekeepers in Haiti

Estimated Energy Expenditure (EE)
Estimated VO 2max (Cooper's Test)
Estimated Hydration Status (%Body Mass Change)
Pre: 74.6 ± 7.9, Post: 72.9 ± 7.6

EE-LF/HF ratio: r = 0.49 p = 0.02
VO 2max-LF/HF ratio: r = −0.42 p = 0.04
%Body Mass Change: r = 0.53 p = 0.02

12/12
Cohort

Grant et al. ( 37)
The difference Between Exercise-Induced Autonomic Fitness Changes Measured After 12 and 20 weeks of medium to high intensity military training

South African Defense Force Recruits
( n = 89 males)
( n = 65 females)
Age: 20.91 ± 1.29 yrs
BMI: 22.85 ± 2.78

AVNN
SDNN
RMSSD
pNN50
LF (nu)
HF (nu)
LF/HF ratio
SD1
SD2

Basic Military training over 12 and 20 weeks
VO 2max:
Basal: 49.54 ± 8.79
12 weeks: 54.14 ± 7.09
20 weeks: 54.15 ± 7.16

Significant differences in Mean RRI ( p = 0.007), SDNN ( p = 0.024), RMSSD ( p <0.001) and SD1 ( p <0.001) between weeks 12 and 20, but no changes in LF, HF or LF/HF Ratio

10/12
Cohort

Johnsen et al. ( 38)
Heart Rate Variability and cortisol responses during attentional and working memory tasks in naval cadets

Royal Norwegian Naval Academy Cadets ( n = 49)
Age: 23.3 ± 8 yrs

RMSSD

Basal RMSSD-cortisol: r = −0.35, p <0.04
Basal RMSSD-post calcap: p <0.04
Basal RMSSD-post pop-out attention: p <0.04
Basal RMSSD-recovery: p <0.04

10/12
Cohort

Jouanin et al. ( 39)
Analysis of heart rate variability after a ranger training course

Male French Military Academy Students
( n = 23)
Age: 21.7 ± 0.2 yrs
Height: 176.5 ± 1.1 cm
Weight: 74.0 ± 1.3 kg

AVNN
Pre: 1008.4 ± 33.1
Post: 1177.5 ± 33.8
SDRR:
Pre: 763.1 ± 18.8 ms
Post: 837.5 ± 20.5 ms
LF (nu)
HF (nu)
LF/HF ratio
Total Power (TP)

French Ranger Training Course

Pre-Post training: Significant change in Mean RRI ( p <0.001), TP ( p <0.01), LF, HF and LF/HF ratio, ( p <0.05)

10/12
Cohort

Kaikkonen et al. ( 40)
Physiological Load and Psychological Stress During a 24-h Work Shift Among Finnish Firefighters

24-h shift
6-h firefighting shift
6-h ambulance shift

Significant Difference between shift conditions ( p <0.01)

10/12
Cohort

Lyytikäinen et al. ( 41)
Recovery of rescuers from a 24-h shift and its association with aerobic fitness

Professional Finnish Fire/Rescue personnel ( n = 14)
Age: 34 ± 9 yrs
Height: 178 ± 7 cm
Weight: 80.8 ± 11.4 kg

SDNN
RMSSD
VLF
LF
HF

24-h rescue shift
VO 2max:
51 ± 9 ml/kg/min

Significant difference in SDNN: On shift-2nd day: p <0.05
On shift-third day: p <0.001
Significant difference in LF/HF ratio:

10/12
Cohort

LF/HF ratio
Total Power

on shift-2nd day: p <0.05
On shift-third day: p <0.01
No significant effect of VO 2max on HRV during recovery

Oliveira-Silva et al. ( 42)
Physical Fitness and Dehydration Influences on the Cardiac Autonomic Control of Fighter Pilots

RMSSD:
Pre: 24.2 ± 8.2 ms
Post: 20.8 ± 10.7 ms
SDNN:
Pre: 67.0 ± 29.1 ms
Post: 77.3 ± 27.5 ms
SD1:
17.5 ± 5.6 ms
16.4 ± 7.8 ms
SD2:
79.3 ± 18.3 ms
74.7 ± 28.1 ms
Sample Entropy
(SampEn):
Pre: 0.94 ± 0.22
Post: 0.86 ± 0.28
Alpha1 (fractal scaling)

Flight training
Hydration status (hematocrit)
VO 2max (Montreal University Track Test): 46.4 ± 5.66
Bench Press: 68.7 ± 10.8 kg
Pull-Down: 70.0 ± 8.36 kg
Leg Press: 201.3 ± 42.49

Flight Training day-rest day: RMSSD ( p = 0.036), SDNN ( p = 0.001), SD1 ( p = 0.031)
Significant negative correlation between hematocrit and RMSSD, SD1: r = −0.61-−0.81, p = 0.044-0.002
Significant relationship between VO 2max-SampEn: r = 0.777, p = 0.001

12/12
Cohort

Rodrigues et al. ( 43)
Stress among on-duty firefighters: An ambulatory assessment

Professional Portuguese Firefighters
( n = 15 males)
( n = 2 females)
Age: 29.35 ± 8.85 yrs
Experience: 9.41 ± 7.3 yrs

Perceived Stress Scale (PSS)
Work events
Normative ECG values:
AVNN: 930 ± 133 ms
LF/HF Ratio: 3.33 ± 3.47

Significant differences between fire, pre-hospital assistance, and accidents:
LF/HF ratio ( p <0.01)
Significant differences between firefighter and normative ECG values:
Mean NNI (82% of firefighters lower)
LF/HF ratio (71% of firefighters above)

11/12
Cohort

Rodrigues et al. ( 44)
Wearable biomonitoring platform for the assessment of stress and its impact on cognitive performance of firefighters: An experimental study

Professional Portuguese Firefighters ( n = 19 males)
( n = 2 females)
Age: 29.90 ± 8.83 yrs
Experience: 8.33 ± 8.21 yrs

AVNN
SDNN
RMSSD
pNN20
pNN50
LF/HF ratio

Trier Social Stress Test (TSST)
2-choice reaction time task (CRTT)

CRTT1-Mean NNI: significant decrease, p <0.005
TSST-Mean NNI: significant decrease, p <0.005
TSST-SDNN: significant increase, p <0.001
LF/HF ratio-CRRT2: significant increase, p <0.001
LF/HF ratio-TSST: significant increase, p <0.001

12/12
Cohort

Shin et al. ( 45)
Factors related to heart rate variability among firefighters

Professional Male South Korean Firefighters
( n = 645)

SDNN
RMSSD
LnLF
LnHF
LF/HF Ratio

Korean Occupational Stress Scale
(yes or no answers to a variety of occupational variables)

Smoking-RMSSD
Yes: 31.96 ± 16.92
No: 28.69 ± 15.31 ( p <0.05)
Smoking-LnLF
Yes: 5.73 ± 0.91
No: 5.54 ± 0.98 ( p <0.05)
Shift work-RMSSD:

12/12
Cohort

Yes: 30.17 ± 16.54
No: 25.78 ± 11.11 ( p <0.05)
Job Demand-RMSSD:
Low: 30.96 ± 16.41
High: 28.04 ± 14.97 ( p <0.05)

Souza et al. ( 46)
Resting vagal control and resilience as predictors of cardiovascular allostasis in peacekeepers

Male Brazilian Army Soldiers
( n = 50)
Age: 25.4 ± 5.99 yrs
BMI: 23.2 ± 4.34

AVNN
RMSSD 2 min
RMSSD 5 min

Trier Social Stress Test
Ergo Resiliency Scale (ER-89)

Significant Spearman's Correlations for the following:
Basal Mean RRI-speech task: r = 0.50, p = 0.0003
Basal Mean RRI-arithmetic task: r = 0.54, p <0.0001
Basal Mean RRI-recovery (speech): r = 0.45, p = 0.001
Basal Mean RRI-recovery (arithmetic): r = 0.53, p <0.0001
ER-89, RMSSD: r = 0.28, p = 0.05

11/12
Cohort

Research Metadata

Study Designs

Three study design methodologies were utilized in the selected studies. Quasi-experimental studies ( 27- 29) and case-control studies ( 30, 31) represented a total of four included studies. All other studies were of a cohort design. The follow-up period for the cohort studies varied from single instances of data collection activity, such as administration of a questionnaire and a single ECG trace, to as long as 20 weeks of follow-up in one study ( 37).

Demographics

Selected publications included males only in 11 studies, both males and females in seven studies and the gender distribution was not reported in three studies. Seven tactical subpopulations were represented; general law enforcement officers ( 32, 33), Army ( 27, 29, 31, 34- 36, 46), Defense Force trainees ( 37), Naval cadets ( 38), Army Ranger trainees ( 39) firefighters/rescue personnel ( 28, 30, 40, 41, 43, 45), and Air Force fighter pilots ( 42). Specifically, the Army personnel were members of either the US, Brazilian or Spanish militaries. For the Fire/Rescue personnel, members were serving in French, Brazilian, Finnish, Portuguese, or South Korean communities. A total of nine countries were represented: USA ( 27, 32, 33), Spain ( 29, 31, 34, 35), Brazil ( 30, 36, 42, 46), South Africa ( 37), Norway ( 38), France ( 28, 39), Finland ( 40, 41), Portugal ( 43, 44), and South Korea ( 45).

HRV Analyses

Analytical methods vary widely in their mathematical approach and provide different information on different autonomic processes. Table 3 describes the HRV analysis methods discussed in this review and provides an expansion of each acronym, a brief explanation of the measurement, and its clinical implications.

Table 3

Measurement

Units

Expansion

Application

Interpretation

RMSSD

ms

Root-mean square of successive differences

Primary measurement for short-term vagally mediated changes ( 2)

Time-Domain Measurements generally increase with increased aerobic capacity ( 2, 47)

pNN20

%

Percentage of adjacent intervals that differ by more than 20 ms

Assessment of vagal activity ( 2)
Thresholds of ~6.8% or lower for disease states have been posited ( 48, 49)

VLF

ms ²

Very-Low Frequency Band power (0.0033-0.04 Hz)

Intrinsically generated by the heart ( 2)
Modulated by sympathetic activity ( 2)

Low VLF power has been associated with cardiac death, PTSD and inflammation ( 2, 51)

LF

ms ²

Low Frequency Band power (0.04-0.15 Hz)

Represents baroreceptor activity during resting measurements ( 2)

Can be used to estimate vagal tone ( 2, 52)

HF

ms ²

High Frequency Band power (0.15-0.4 Hz)

Parasympathetic and respiratory activity ( 2)

Low HF values are associated with high stress ( 50)
Correlated with RMSSD ( 2)

LF/HF

%

Ratio of LF to HF power

For resting seated measurements, likely measures PNS and baroreceptor activity ( 2)

Highly dependent on measurement conditions ( 2)

TP

ms ²

Total Spectral power

Useful for determining relative power measurements ( 2)

Broad estimate of total autonomic activity ( 2)

SD1

ms

Standard Deviation from y-axis on Poincare plot

Nonlinear measurement, useful for short-term analysis without trend sensitivity ( 2)

Predicts RMSSD, pNN50, SDNN, and power in the LF and HF bands, and total power during 5 min recordings ( 2, 52)

SD2

ms

Standard Deviation from x-axis on Poincare plot

Nonlinear measurement, useful for long-term analysis without trend sensitivity ( 2)

Correlates with LF power ( 2)

SampEn

-

Sample entropy or approximate entropy

Quantification of the unpredictability of a time series ( 2)

Useful for short recordings which may have noise ( 2)

Alpha1

-

Short-term detrended fluctuation analysis
Extracts the correlations between successive RR intervals over different time scales

Reflects baroreflex activity ( 2)

Useful when longer recordings are not available ( 2)

Frequency-Domain and Nonlinear Analyses

The low-frequency band (LF) and high frequency band (HF) spectral power were the most popular analytical methods, with 18 studies documenting these HRV characteristics. Other spectral analyses included LF/HF ratio ( 27, 30, 36, 37, 39, 41, 44, 45, 54), total power (TP) ( 39), and the very-low frequency band (VLF) ( 41). The nonlinear analyses included alpha1 (α1), a measure of signal self-similarity ( 42), sample entropy (SampEn) ( 42), SD1, and SD2 ( 35, 37, 42).

Time-Domain Analyses

The root-mean square of successive differences (RMSSD) was the most popular time-domain analysis, with 11 studies documenting this HRV characteristic. Other time-domain analyses included pNN20 ( 54), pNN50 ( 30, 35, 37, 54), mean interbeat interval, or AVNN ( 36, 37, 39, 43, 46, 54), or SDNN/SDRR ( 28, 37, 39, 41, 42, 45, 54).

Aerobic Fitness

A total of four studies examined the relationship between one or more HRV metrics and aerobic fitness (VO 2max) ( 30, 36, 41, 42). Three of the four were cohort study designs ( 36, 41, 42), and one was quasi-experimental ( 30). Three ( 30, 42), including the experimental study ( 36), found significant relationships between one or more HRV values and aerobic fitness, and one did not ( 41). The study finding no significant relationship examined only the LF/HF ratio and SDNN HRV indices. Of the studies reporting significant relationships, LF/HF ratio ( 36, 41), sample entropy, PNN50, and RMSSD were examined. The pooled mean VO 2max across all four studies was 45.4 ± 6.2 ml/kg/min, with the lowest VO 2max in the cohort of off-duty male Brazilian firefighters (40.0 ml/kg/min) ( 30) and the highest in the cohort of male Brazilian peacekeepers (52.9 ml/kg/min) ( 36).

Other Fitness Measurements

Two studies ( 32, 42) examined fitness variables that were not estimates of aerobic capacity. One recorded one-rep maximum strength measurements from Brazilian fighter pilots (bench press, pull-down and leg press) and found no significant relationships between HRV indices measured on a rest day or on a flight training day and those strength tests. The other study ( 32) found a significant correlation between self-reported physical activity levels and the LF and HF signal strengths.

Tactical Scenario Simulations

Six studies utilized a simulated combat scenario for participant testing ( 27, 29, 31, 34, 35) or observed an actual tactical engagement ( 36). Of these studies, all found significant pre-event to post-event changes in HRV. Specifically, one study ( 35) comparing differences between high-performing and lower-performing soldiers in hand-to-hand combat training drills found significant time-domain differences (RMSSD, PNN50, SD1, SD2) between groups, but no differences in spectral-domain analyses. In the other studies, spectral analyses correlated with weapon accuracy and threat discrimination sensitivity ( 27), estimated energy expenditure, and hydration status as measured by percentage of body weight change of the course of the observation period ( 36). One study in this category compared a light infantry unit to a heavy unit in a simulated ground combat scenario ( 31). The light infantry unit, acclimated to such events and training, showed significantly different spectral responses when compared against the heavy infantry unit.

Cognition Testing

Three studies included a measurement of cognitive performance ( 34, 38, 43). Of these three, one found significant correlations between sound recall and the LF domain following a combat simulation (see section above) ( 34). One found significant relationships between greater basal RMSSD and two computerized cognitive stress tests, as well as during recovery from those tasks ( 38). The last study found a significant decrease in AVNN at the start of a two-choice critical reaction time task and a significant change in LF/HF ratio following the task ( 43).

Occupational Stress

Three studies ( 33, 45, 46) measured occupational stress directly through surveys or through a tactically relevant evaluation tool. Andrew et al. ( 55) found an inverse correlation between lnLF and the "lack of support" stressor; meaning that greater lack of support correlated with less vagal control as measured by LF power. Shin ( 45), in a sample of 645 professional South Korean firefighters, found significantly reduced RMSSD values in those that reported smoking, reported "high" vs. "low" job demand, and those that reported shift work. Significantly higher lnLF values were also reported in smokers. Souza et al. ( 46) measured trait resilience in a cohort of Brazilian Army soldiers using the ER-89 scale and found a correlation between trait resilience and RMSSD.

First Response Shift Work

Three studies assessed cohorts of fire and rescue personnel working 24-h shifts ( 40, 41, 43). Kaikkonen et al. ( 40) found a significant difference in RMSSD between the 6-h firefighting and 6-h ambulance phases of professional Finnish firefighters' shifts, with the rescue phase of the shift resulting in a decrease of RMSSD. Lyytikäinen et al. ( 41), previously mentioned above, examined HRV changes during the recovery days following a 24-h shift. They found significant differences in the SDNN and LF/HF ratio from days 2 to 3 of recovery. Rodrigues et al. ( 44) found significant differences in LF/HF ratio during a variety of fire and rescue tasks. Specifically, the LF/HF ratio increased most during response to accidents. They also compared their mean N-N interval data to normative healthy adult data and found the rescue personnel lower AVNN values for rescuers, as well as higher LF/HF values.

Training Events

Two studies ( 37, 39) followed participants through a period of tactical training. One ( 39) measured differences in AVNN, TP, LF, HF, and LF/HF ratio between the start and end of French Ranger training. They found significant changes in all measured values. Grant et al. ( 37) assessed the differences in aerobic capacity and HRV values at one 12 and 20 weeks during South African Defense Force initial entry training. They found that while the mean AVNN, SDNN, and RMSSD continued to improve between 12 and 20 weeks, the LF/HF ratio was not significantly different over the same time period ( 37).

Discussion

The aim of this review was to identify primary studies examining relationships between HRV and relevant health, fitness and operational outcomes in tactical personnel, critically appraise the methodological quality of the identified studies, and summarize the results. The quality of the 20 included studies was generally high and included a variety of observational and quasi-experimental designs over a wide range of follow-up periods. While observational research was necessary for the aims of most studies, and variations in study designs and research questions covered a variety of topic in which single instance recordings were appropriate, few of the studies included in this review were longitudinal, and larger, more comprehensive datasets describing the HRV characteristics of tactical personnel in a variety occupational fields and settings would be beneficial. Further, the CASP checklists do not account for sample sizes, which were generally small, with a few notable exceptions, so while the methodology of the included studies may have been sound, caution must be taken when interpreting the results. Qualitatively, however, the studies included in this review generally agreed, and did indicate that HRV is an effective tool for measuring psychophysiological stress across professional environments and often correlates effectively with a wide variety of outcomes of interest. The included studies indicate that HRV measurements can be effectively applied across a wide variety of tactical settings.

Physical Fitness

Of the four studies investigating the relationships between HRV measures and aerobic fitness, only one found no significant association, and the mean VO 2max of 51 ml/kg/min was higher than all but one of the other studies. This could suggest a ceiling effect, indicating that aerobic fitness above a certain capacity within a certain profession lends no additional benefits to recovery, but all considered studies used an indirect, estimated method to determine VO 2max. Additionally, a variety of HRV analysis methods were used, and while frequency domain values did not trend significantly with physical fitness in all studies, the study by Oliveira-Silva et al. examining the cohort with the highest aerobic capacity did find a significant relationship when examining sample entropy ( 42), suggesting that non-linear methods may be the most suitable method for these study designs.

Further research in this area might focus on directly measuring aerobic capacity of the cohort, and conducting similar research investigating VO 2max, HRV and recovery in different tactical professionals. Larger sample sizes may also clarify these emerging relationships. So far, only one study has examined any measurements of strength, and no measures of power have been linked to HRV indices. While no significant associations were found in a cohort of fighter pilots, it may be possible that fitness within the strength and power domains contributes to individual perception and response to stress, as well as recovery from stressful exposures in other tactical settings.

Tactical Scenarios

All six studies measuring HRV during simulated or actual combat situations found significant changes. While not unexpected, these results nonetheless provide a foundation on which organizations can begin to consider implementing additional psychophysiological monitoring of personnel to assist in training and deployment decisions. While these studies achieved high levels of external validity, by exposing personnel to rigorously designed and realistic simulations, questions remain as to which factors within these scenarios most significantly affected the HRV indices of operators, and therefore, which components require further validation or incorporation into regular training. For example, it is possible that load carriage, a common task for tactical personnel, may influence chest biomechanics and perceived stress, independently altering HRV. Previous research has indeed demonstrated that even experienced tactical personnel report increased exertion when carrying a load, even if physiological measures, such as estimated energy expenditure, do not significantly change ( 56). Spectral and time domain analyses of HRV may be able to quantify and explain these anomalies. Furthermore, while one study has conducted a threat identification scenario, live firearms were not used, and further research into the specific effect of live firearms operation on HRV may be necessary, and may provide additional information on underlying mechanisms that contribute to the effectiveness of deliberate HRV modulation as an intervention for improving tactical decision making ( 57). For field measurements and analyses of HRV in these scenarios and similar scenarios, selecting more robust analyses, such as nonlinear and fractal-scaling methods may again be the most suitable.

Cognition Testing

Making decisions under pressure and other cognitive work is critical to the success of many tactical operations, and a key skill for ensuring operator and public safety. While the studies examining cognitive stress and HRV values are limited to only three, which represented the military and firefighting professions, the results are promising. The value for organizations may come primarily from the ability of HRV to discriminate and quantify the severity of responses to cognitive demands without relying solely on subjective feedback. The results from one study in particular by Delgado-Moreno et al., which assessed recall capability and spectral HRV analysis ( 34), may demonstrate that personnel who adapt to stressful stimuli struggle less with higher-order tasks. This type of analysis and its integration into training, especially in law enforcement, may be beneficial for improving decision-making training and promoting improved public safety.

Occupational Stress

The study by Shin et al. ( 45) was the largest cohort studied by a considerable margin, with a total of 645 participants. The Korean Occupational Stress Scale, while a subjective tool, is a widely utilized measurement of occupational stress and was used as the primary instrument ( 58). They found that the organization system and occupational climate were correlated with lnHF changes. Time-domain changes were also found in firefighters that reported high job stress. This agrees with another large cohort of manual laborers in Korea and suggests that organizational structures and employee support can have significant changes in the allostatic load of personnel ( 59). Although the study by Shin et al. did not account for physical activity outside the workplace, the study by Kang et al. did, and determined that social support was an independent risk factor for adverse HRV changes. Therefore, it cannot be assumed that improvements in physical conditioning will be able to fully counteract the potential negative effects of job stress induced by organizational, job support or occupational climate concerns. As such, tactical organizations may benefit from monitoring not only external occupational stressors and hazards but may potentially benefit from assessing the effects of internal organizational and support impacts. Further research accounting for physical conditioning in tactical personnel, and enhanced qualitative methods assessing occupational organization, support, and climate may clarify some of the unique causal relationships between organization-induced occupational stress and allostatic load in tactical operations.

Likewise, in the study by Andrew considering police work stressors and cardiac autonomic balance, a significant association between a lack of organizational support and frequency-domain changes in HRV was found, but for female officers only ( 33). The authors concluded that chronic insufficient organizational support may lead to a loss of the cardioprotective effect females typically experience as a result of greater vagal control compared to men. Their results were consistent with another study that found relationships between organizational occupational stress and metabolic disorder in women, but not men ( 60). Given that females will typically form a minority within a tactical organization, specific regard to their occupational support may be especially necessary to support their health and effectiveness. One solution Andrew and his colleagues offer is for police organizations to provide for support coping opportunities, a method of stress coping more typically adopted by females. Further research may investigate the effect of occupational organization, support methods and climate with specific regard to female operators or trainees to determine optimal strategies for maximizing their performance.

Shift Work

While shift work is endemic to many tactical organizations, the studies in this review represented exclusively fire and rescue personnel, and the results indicated that the autonomic impacts of shift work in tactical personnel may manifest differently than in other populations. For example, two separate studies in two very different geographical locations (Finland and Portugal), both found that fire and rescue personnel who were assigned to rescue duties, which included motor vehicle accident response, experienced significantly greater HRV changes than when they worked regular fire suppression duties ( 40, 43). Research in healthcare workers found limited differences between physicians and nurses who worked rotating shifts and those who worked day shifts only ( 61), indicating that research considering longer-term HRV measurements, such as the VLF component of the frequency-domain and the nonlinear SD2, may require careful interpretation in tactical settings.

Other work in this area to aimed at using HRV indices to measure physiological strain, which may then be used when determining work/rest cycles and other scheduling concerns. Time-domain analyses were the most common across these studies, but spectral analyses were also represented and were significant. While no significant associations were found between VO 2max and shift recovery (discussed above), it should be noted that the autonomic effects of shift work lasted up to three days ( 41). Given that males may be more susceptible to adverse HRV changes induced by circadian rhythm disruptions ( 62), and that males comprise the majority of most tactical organizations, the effects of shift work on autonomic regulation should be investigated further in a wide variety of tactical operations to develop stronger evidence to guide policy decisions. Further studies or reanalysis efforts may also aim to apply novel indices, such as sample entropy or other signal-self similarity assessments to strengthen the relationships between HRV measurement, physiological strain and operator deployment decisions. Given the differences in HRV responses to different duties even within a single unit, other populations that rely on rotating shifts, such as law enforcement and military organizations should also be considered separately to determine the unique influences present between professions and how to proceed with long-term HRV measurement interpretations.

Training Events

Two studies followed trainees through a period of accession training, one a Ranger training course in France, and the other basic military training in South Africa. Both found that tactical training results in HRV changes across both time and spectral domains, and one was specifically using HRV to assess for the presence of overtraining syndrome (OTS) between the 12th and 20th week of training ( 37), as OTS is known to contribute to injury risk ( 63). A link between HRV and chronic musculoskeletal injury has been proposed in endurance athletes ( 13), and if proven effective, tactical organizations may be able to screen and triage trainees before clinical symptoms develop, reducing the burden of injury during training. One recent study in Crossfit™ athletes found significant associations when investigating the relationships between workload and HRV in the time domain ( 64). Given the similarities between Crossfit™ and tactical conditioning activities ( 14), this association may prove especially valuable for tactical training programs and warrants further investigation in larger cohorts of tactical personnel. As previously stated, more robust HRV analytical methods may also be beneficial for inclusion in further studies.

Conclusions

The measurement and application of HRV indices to monitor psychological, physiological and more specifically, autonomic stress, encountered by the tactical operator continues to be developed and has made significant strides over the past decade in terms of utilization by a wide range of organizations, comparison to normative populations and quantification of the stress of tactical work. While strong evidence is still emerging, based on the quality and general agreement of studies included in this review, HRV monitoring appears to provide valuable insight into the psychophysiological responses of tactical personnel during occupationally relevant activities and recovery from those activities. However, substantial further research is still necessary. Specifically, the recruitment of larger cohorts and the collection of normative data specific to healthy tactical personnel should enhance result interpretation. Additionally, further isolation and analysis of specific variables relevant to the end user, such as the independent effects of load carriage, may help determine causal relationships, strengthen the reliability of applications and ultimately provide personnel with an additional tool to maximize health and performance. Finally, HRV may prove to be highly effective for mitigation of chronic musculoskeletal injury in tactical operators and trainees as a screening tool, but such associations require further exploration, in terms of both cohort selection and recruitment and HRV analysis method(s).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

CT and RO: conceptualization, methodology, and writing-original draft preparation. CT: software, formal analysis, investigation, writing-review and editing, and supervision. RO and BS: resources, data curation, visualization, project administration, and validation. All authors have read, reviewed, and approved the current manuscript for submission.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to thank the editorial and review personnel for their efforts, and all first responders protecting our communities throughout the world.

Footnotes

Funding. This research was funded in part through a Ph.D. scholarship provided for the lead author. No other funding or grant from any agency in the public, commercial or not-for-profit sectors was provided or obtained.

References

17. Zefferino R, L'Abbate N, Facciorusso A, Potenza A, Lasalvia M, Nuzzaco A, et al.. [Assessment of heart rate variability (HRV) as a stress index in an emergency team of urban police]. G Ital Med Lav Ergon. (2003) 25 Suppl3:167-9. [[ PubMed] Google Scholar]

23. Schwartz MD. SMART (Sports Medicine and Rehabilitation Team) Centers: An Empirical Analysis. Pendleton CA: Naval Hospital Camp; (2007). 10.21236/ADA477206 [[ CrossRef] Google Scholar]

64. Williams S, Booton T, Watson M, Rowland D, Altini M. Heart rate variability is a moderating factor in the workload-Injury relationship of competitive crossFit™ athletes. J Sports Sci Med. (2017) 16:443-9. [[ PubMed] PMC free article] [ Google Scholar]

Articles from Frontiers in Public Health are provided here courtesy of Frontiers Media SA

# data[].attributes.attachment_link.title
Relationships Between Heart Rate Variability, Occupational Performance, and Fitness for Tactical Personnel: A Systematic Review

# data[].attributes.attachment_link.description
Objectives: Heart Rate Variability has gained substantial interest in both clinical and athletic settings as a measurement tool for quantifying autonomic nervous system activity and psychophysiological stress. However, its uses in tactical work settings, ...

# data[].attributes.attachment_link.readable_text
Origin: Superior portion of intertrochanteric line, anterior and inferior borders of greater trochanter, superior portion of lateral lip of linea aspera, and lateral portion of gluteal tuberosity of femur
Insertion: Lateral base and border of patella; also forms the lateral patellar retinaculum and lateral side of quadriceps femoris tendon
Action: Extends the knee
Innervation: Muscular branches of femoral nerve
Arterial Supply: Lateral circumflex femoral artery

The medical illustrations contained in this online atlas are copyrighted © 1997 by the University of Washington. They may not be utilized, reproduced, stored, or transmitted in any form or by any means, electronic or mechanical, or by any information storage or retrieval system, without permission in writing from the University of Washington.

Receiving a license to use these images is generally quite easy, particularly for academic and scholarly purposes. For more information see the Musculoskeletal Atlas Express Licensing Page.

# data[].attributes.attachment_link.description
Origin: Superior portion of intertrochanteric line, anterior and inferior borders of greater trochanter, superior portion of lateral lip of linea aspera, and lateral portion of gluteal tuberosity of femur Insertion: Lateral base and border of patella; also forms the lateral patellar retinaculum and lateral side of quadriceps femoris tendon Action: Extends the knee Innervation: Muscular branches of femoral nerve Arterial Supply: Lateral circumflex femoral artery The medical illustrations contained in this online atlas are copyrighted © 1997 by the University of Washington.

# data[].attributes.attachment_link.description
Original Editor - User:Uchechukwu Chukwuemeka

# data[].attributes.attachment_link.title
Anatomy Of The Vastus Lateralis Muscle - Everything You Need To Know - Dr. Nabil Ebraheim

# data[].attributes.attachment_link.title
Vastus Intermedius Anatomy: Origin, Insertion & Action

# data[].attributes.headline
Quadriceps Vastus Lateralis (3D Animation)

# data[].attributes.subject
Quadriceps Vastus Lateralis (3D Animation)

# data[].attributes.attachment_link.title
Quadriceps Vastus Lateralis (3D Animation)

# data[].attributes.attachment_link.title
Functions of the vastus lateralis muscle (preview) - Human 3D Anatomy | Kenhub

# data[].attributes.body
Common injuries: Contusions (bruising) & Strains.

# data[].attributes.body
Common injuries: Contusions, strains, tears. Most of these injuries occur without a proper warm up or trauma impact.

# data[].attributes.attachment_link.readable_text
The .gov means it's official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you're on a federal government site.

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Affiliations

¹ Institute for Nutrition, Exercise and Sports, Department of Integrative Physiology, University of Copenhagen, Copenhagen, Denmark.

² Stenhus Physical Test Center, Holbæk, Denmark.

³ Football Club, NorthZealand, Denmark.

⁴ Univ Savoie Mont Blanc, Inter-university Laboratory of Human Movement Biology, Chambéry, France.

⁵ Univ Lyon, UJM-Saint-Etienne, Inter-university Laboratory of Human Movement Biology, Saint-Etienne, France.

⁶ Sport Performance Research Institute, Auckland University of Technology, New Zealand.

⁷ Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Denmark.

Affiliations

¹ Institute for Nutrition, Exercise and Sports, Department of Integrative Physiology, University of Copenhagen, Copenhagen, Denmark.

² Stenhus Physical Test Center, Holbæk, Denmark.

³ Football Club, NorthZealand, Denmark.

⁴ Univ Savoie Mont Blanc, Inter-university Laboratory of Human Movement Biology, Chambéry, France.

⁵ Univ Lyon, UJM-Saint-Etienne, Inter-university Laboratory of Human Movement Biology, Saint-Etienne, France.

⁶ Sport Performance Research Institute, Auckland University of Technology, New Zealand.

⁷ Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Denmark.

Abstract

Comprehensive information regarding neuromuscular function, as assessed through force-velocity-power (FVP) profiling, is of importance for training optimization in athletes. However, neuromuscular function is highly task-specific, potentially governed by dissimilarity of the overall orientation of forceapplication. The hip thrust (HT) exercise is thought to be of relevance for sprinting considering its antero-posterior force orientation and considerable hip-extensor recruitment, however, the association between their respective FVP profiles remains unexplored. Therefore, to address the concept of force orientation specificity within FVP profiling, the maximal theoretical neuromuscular capabilities of 41 professional male footballers (22.1 ± 4.1 years, 181.8 ± 6.4 cm, 76.4 ± 5.5 kg) were assessed during sprint acceleration, squat jumping (SJ) and the HT exercise. No significant associations were observed for maximal theoretical force or velocity between the three FVP profiling modalities, however, maximal theoretical power (Pmax) was correlated between sprinting and SJ (r = 0.73, P < 0.001) and HT and SJ (r = 0.44, P = 0.01), but not between sprinting and HT (r = 0.18, P = 0.36). In conclusion, although Pmax may be considered a somewhat universal lower-extremity capability, neuromuscular function is associated with substantial task-specificity not solely governed by the overall direction of force orientation.

Keywords: Neuromuscular; force-vector; hip thrust; sprint; squat jump.

PubMed Disclaimer

Cited by

Differences in Force-Velocity Profiles During Countermovement Jump and Flywheel Squats and Associations With a Different Change of Direction Tests in Elite Karatekas.

Smajla D, Spudić D, Kozinc Ž, Šarabon N. Smajla D, et al. Front Physiol. 2022 Jun 21;13:828394. doi: 10.3389/fphys.2022.828394. eCollection 2022. Front Physiol. 2022. PMID: 35800347 Free PMC article.

# data[].attributes.attachment_link.keywords[]
national center for biotechnology information

# data[].attributes.attachment_link.title
Force-velocity-power profiling of maximal effort sprinting, jumping and hip thrusting: Exploring the importance of force orientation specificity for assessing neuromuscular function - PubMed

# data[].attributes.attachment_link.description
Comprehensive information regarding neuromuscular function, as assessed through force-velocity-power (FVP) profiling, is of importance for training optimization in athletes. However, neuromuscular function is highly task-specific, potentially governed by dissimilarity of the overall orientation of f ...

# data[].attributes.attachment_link.readable_text
Abstract

The gluteus maximus (GMax) is one of the primary hip extensors. Several exercises have been performed by strength and conditioning practitioners aiming to increase GMax strength and size. This systematic review aimed to describe the GMax activation levels during strength exercises that incorporate hip extension and use of external load. A search of the current literature was performed using PubMed/Medline, SportDiscuss, Scopus, Google Scholar, and Science Direct electronic databases. Sixteen articles met the inclusion criteria and reported muscle activation levels as a percentage of a maximal voluntary isometric contraction (MVIC). The exercises classified as very high level of GMax activation (>60% MVIC) were step-up, lateral step-up, diagonal step-up, cross over step-up, hex bar deadlift, rotational barbell hip thrust, traditional barbell hip thrust, American barbell hip thrust, belt squat, split squat, in-line lunge, traditional lunge, pull barbell hip thrust, modified single-leg squat, conventional deadlift, and band hip thrust. We concluded that several exercises could induce very high levels of GMax activation. The step-up exercise and its variations present the highest levels of GMax activation followed by several loaded exercises and its variations, such as deadlifts, hip thrusts, lunges, and squats. The results of this systematic review may assist practitioners in selecting exercised for strengthening GMax.

The step-up and its variations may elicit the highest level of Gmax activation possibly to the stabilization requirement of the exercise.

Several bilateral exercises (e.g. hip thrusts, squats, deadlifts, and lunges) can provide very high level of GMax activation.

The external load, movement velocity, level of fatigue, the mechanical complexity of the exercise, and the need for joint stabilization, might directly influence GMax activation.

Further research may investigate the best practices for normalizing GMax activation.

Keywords: Skeletal muscle, gluteus maximus, electromyography, strength training

Introduction

Hip extension is a fundamental movement in daily life and athletic activities. Previous research has proposed an increasing role of hip extensor musculature with heavier lower body exercises (e.g., squats, lunges, and deadlifts) and explosive sports actions (e.g., jumping, sprinting and change of direction) (Beardsley and Contreras, 2014). The primary muscles responsible for this movement are gluteus maximus (GMax), long head of biceps femoris, semimembranosus, semitendinosus, and the ischiocondylar portion of the adductor magnus (Broski et al., 2015; Neumann, 2010; Youdas et al., 2017). Despite the involvement of all these muscles, GMax has been identified as the primary muscle responsible for hip extension, specifically on loaded exercises that typically do not sufficiently activate the hamstrings in tasks involving simultaneous hip and knee extension, such as the squat and the leg press (Krause Neto et al., 2019, McCurdy et al., 2018; Williams et al., 2018; Sugisaki et al., 2014). There is a significant number of studies comparing GMax activation levels between several loaded and bodyweight exercises (Bishop et al., 2018; Boren et al., 2011; Macadam et al., 2015; Macadam and Feser, 2019; Selkowitz et al., 2016).

Electromyography (EMG) is a technique for measuring the electric potential field generated by the depolarization of the sarcolemma (Merletti and Parker, 2004). Despite limitations and common misinterpretations (Vigostky et al., 2015; 2016), under controlled conditions, the EMG signal comprises the summation of motor unit action potentials and provides an index of muscle activation (Enoka and Duchateau, 2015). Therefore, EMG has been widely used to compare the muscle activation between exercises, which can assist the strength and conditioning coach on selecting and systematically progressing exercise intensity (Vigostky et al., 2015, Macadam and Feser, 2019).

Previous studies have systematically reviewed the gluteal muscle activity, measured by EMG, in a variety of lower body exercises (Macadam et al., 2015; Macadam and Feser, 2019). The systematic review conducted by Macadam et al. ( 2015) showed that exercises with dynamic hip abduction and external rotation elicited high levels of GMax activation (ranging from 79% to 113% of a maximal voluntary isometric contraction [MVIC]). Recently, Macadam and Feser ( 2019) have found that it is still possible to achieve high levels of GMax activation (>60% of MVIC) by performing exercises with bodyweight as resistance. However, due to the inclusion/exclusion criteria chosen by the authors to answer their research questions, both studies eventually excluded more ecologically valid studies for strength and conditioning coaches that investigated exercises with higher intensity (external load) and neuromuscular demand. As external load may affect exercise mechanics and the resultant muscular activation (Bryanton et al., 2012; Da Silva et al., 2008; Riemann et al., 2012; Swinton et al., 2011), currently there is ambiguity on which exercises that incorporate hip extension and use of external load achieve the most significant Gmax activation.

Several factors, including relative external load, movement velocity, level of fatigue, the mechanical complexity of the exercise (open or closed kinetic chain, weight bearing or non-weight bearing), and the need for joint stabilization, may directly influence GMax activation. The purpose of this systematic review was to describe the GMax activation levels during dynamic exercises that incorporate hip extension and use of external load. To assist strength and conditioning coaches in selecting exercises for the GMax, we categorized the exercises as low level of activation (0 to 20% of MVIC), moderate level of activation (21 to 40% of MVIC), high level of activation (41 to 60% of MVIC), and very high level of activation (greater than 60% of MVIC) accordingly to the recommendations of Macadam and Feser ( 2019).

Methods

Literature research strategies

The preferred item declaration guide for systematic review and meta-analysis reports (PRISMA) was used to conduct this systematic review (Liberati et al., 2009).

On February 15 th, 2019, a systematic review was conducted using the PubMed/Medline, SportDiscuss, Scopus, Google Scholar, and Science Direct electronic databases. The MeSH descriptors, along with the related terms and keywords, were used as follows: ((((resistance training OR resistance exercise OR training, resistance OR strength training OR training, strength OR weight-lifting strengthening program OR strengthening program, weight-lifting OR strengthening programs, weight-lifting OR weight lifting strengthening program OR weight-lifting strengthening programs OR weight-lifting exercise program OR exercise program, weight-lifting OR exercise programs, weight-lifting OR weight lifting exercise program OR weight-lifting exercise programs OR weight-bearing strengthening program OR strengthening program, weight-bearing OR strengthening programs, weight-bearing OR weight bearing strengthening program OR weight-bearing strengthening programs OR weight-bearing exercise program OR exercise program, weight-bearing OR exercise programs, weight-bearing OR weight bearing exercise program OR weight-bearing exercise programs OR isometric OR exercise OR rehab OR physical therapy OR load OR training))) AND ((muscle development OR development, muscle OR muscular development OR development, muscular OR myogenesis OR myofibrillogenesis OR muscle hypertrophy OR hypertrophy OR hypertrophies OR electromyography OR electromyographies OR surface electromyography OR electromyographies, surface OR electromyography, surface OR surface electromyographies OR electromyogram OR electromyograms OR muscle strength OR power output OR force OR strength OR muscular excitation OR excitation OR EMG OR muscle activation OR activation))) AND ((gluteus maximus OR gluteus OR hip extensor OR hip extensors)).

After reading the titles and abstracts, all eligible full text was assessed for methodological quality using the PEDro methodological quality scale. This scale is composed of eleven questions and scores proportional to the number of items. However, due to the inability to "blind" coaches and practitioners, we excluded three questions, setting the eight as the maximum score. Thus, studies with scores equal to or higher than five were considered of good methodological quality, excluding those with scores equal to or less than 4 (Krause Neto et al., 2019).

Inclusion and exclusion criteria

The inclusion criteria were: (a) original articles; (b) descriptive studies (in case of no raw description of the data, an e-mail was sent to the authors requesting the raw data); (c) studies with physically trained participants; (d) studies that measured surface EMG and reported muscle activation as a percentage of maximal voluntary isometric contraction (MVIC); (e) studies which analyzed the muscle activation of the GMax using strength exercises with external load and (f) English language. Studies with insufficient data, review articles, conference papers, student thesis, samples from metabolic patients, patients with musculoskeletal trauma and older people, poor presentation of data, unclear or vague descriptions of the protocols applied, and articles evaluating isometric, plyometrics, and calisthenics exercises were excluded.

Studies selection

Authors WKN, RA, and TAC independently performed the data analysis with two subsequent meetings to decide on the inclusion of eligible articles in the final text. After each article was read, the following information was extracted: (1) exercise performed, (2) EMG normalization procedure, (3) electrode placement, (4) external load used in the exercise, (5) main findings and (6) mean %MVIC values achieved in each exercise. If two or more studies evaluated the same exercises, the data were pooled as an average of the mean % MVIC of each exercise. Only the mean %MVIC data from each study was used here.

To classify the Gmax activation measured, we used the following levels: 0-20% MVIC, low muscle activation; 21-40% MVIC, moderate muscle activation; 41-60% MVIC, high muscle activation; >60% MVIC, very high muscle activation (Escamilla et al., 2010; Youdas et al., 2014, Cacchio et al., 2008). According to Macadam and Feser (p. 17, 2019), " this classification scheme provides a means by which the practitioner can select exercises, that match the capabilities of their client/athlete thus targeting neuromuscular, endurance, or strength type training, and provides a means by which the GMax can be progressively overloaded in a systematic fashion."

Results

Search results

A total of 1963 articles were identified in the initial survey. After the analysis of the titles/abstracts, 1853 articles were eliminated, leaving 110 articles selected for full-text examination. After two meetings and discussion of the data, 61 items were included and evaluated by the methodological quality scale and inclusion/exclusion criteria, of which 16 articles were eligible for this systematic review ().

In total, 231 participants (90 women and 141 men) underwent 24 strength exercises variations. describes the exercises investigated, methods of EMG normalization, testing load, and the main findings. Of these, ten studies investigated the back squat exercise and its variations [partial, parallel and full] (Aspe and Swinton, 2014; Contreras et al., 2015b; 2016a; Da Silva et al., 2017; Evans et al., 2019; Gomes et al., 2015; McCurdy et al., 2018; Williams et al., 2018; Yavuz et al., 2015; Yavuz and Erdag, 2017), five studies investigated the barbell hip thrust and its variations [American and traditional styles and different feet positions] (Andersen et al., 2018; Collazo Garcia et al., 2018; Contreras et al., 2015b; 2016b; Williams et al., 2018), three studies investigated the deadlift, and its variations [traditional and hex bar] (Andersen et al., 2018; Escamilla et al. 2002; McCurdy et al., 2018) and two studies investigated the front squat (Contreras et al., 2016a; Yavuz et al., 2015). Other studies investigated the overhead squat (Aspe and Swinton, 2014), split squat (Williams et al., 2018), modified single-leg squat (McCurdy et al., 2018), belt squat (Evans et al., 2019), lunges (Marchetti et al., 2018), and step-ups (Simenz et al., 2012). External loads were prescribed either by % of 1RM (varied from 40 to 100% of 1RM) or repetition maximum (varied from 3 to 12RM). The methods for normalizing EMG levels varied among the studies; the positions glute squeeze, standing glute squeeze, and prone with 90° flexion being the most common (). Interestingly, three studies evaluated the lower and upper GMax portions separately (Contreras et al., 2015b; Contreras et al., 2016a; Contreras et al., 2016b).

Table 1.

References

Exercises

EMG normalization method

Electrode placement

Testing Load

Main Findings

Although there was no time limit as an inclusion criterion, all the articles included in this review were published between the years of 2002 and 2019. After the methodological quality analysis, all included studies were classified as excellent (mean score 7).

Muscle activation levels

describes the pooled average muscle activation levels and the minimum and maximum EMG values for each exercise. In general, the step-up exercise and its variations [lateral, diagonal, and cross-over] showed the highest GMax activation (average 125.09% MVIC, ranging from 104.19-169.22% MVIC).

Table 2.

Exercise

Number of studies

Number of subjects

Average (mean %MVIC)

Minimum-maximum (%MVIC)

In , it is possible to verify that 24 variations related to the ten main exercises included in this study were investigated. In this analysis, the classification of the exercises regarding the activation of GMax ranged from moderate to very high. Among all, the step-up exercise demonstrated the highest Gmax activation. However, possibly due to the wide variation of methods used for EMG normalization, at least 16 exercises variations presented similar maximum Gmax excitatory levels (step-up, lateral step-up, diagonal step-up, crossover step-up, hex bar deadlift, rotation barbell hip thrust, traditional barbell hip thrust, American barbell hip thrust, belt squat, split squat, in-line lunge, traditional lunge, pull barbell hip thrust, modified single-leg squat, band hip thrust and conventional deadlift []).

Table 3.

Discussion

The results of this systematic review have shown that GMax activation varied among the exercises investigated. In general, the step-up exercise and its variations present the highest levels of GMax activation (>100% of MVIC) followed by several loaded exercises and its variations, such as deadlifts, hip thrusts, lunges, and squats, that presented a very high level of GMax activation (>60% of 1RM). It was observed that several factors, including relative external load, movement velocity, level of fatigue, the mechanical complexity of the exercise, and the need for joint stabilization, might directly influence GMax activation.

The exercise that elicited the highest activation levels of the GMax was the step-up and its variations [lateral, diagonal, and cross-over step-up] (Simenz et al., 2012). All four exercises are unilateral and require weight-bearing from the practitioner; therefore, during these exercises, the GMax is responsible for extending the hip joint, while simultaneously maintaining the pelvis level controlling excessive femur adduction and medial rotation (Baker et al., 2014; Blemker and Delp, 2005; Macadam et al., 2015). According to Macadam et al. ( 2015), the higher excitatory demand for step-up and its variations are associated with the need to stabilize the knees and hip during the upward and downward movement (the more significant synergistic activity of the gluteus medius). However, these exercises are considered difficult to perform and have a high stabilizing demand for most beginning and intermediate practitioners; even for the experienced practitioner, the higher stability demand may limit the load used, and therefore, may hinder maximal strength and hypertrophy development (Behm and Anderson, 2006).

The back squat exercise and its variations are widely used in strength training with goals of increasing strength and lower limb muscle hypertrophy (Clark et al., 2012). This fact was demonstrated here by a large number of studies included, which investigated different variations of the squat (10 articles). In our results, squats were classified as high GMax. However, we found significant variations in the classification between the different types of squats (ranging from low [13% of MVIC] to very high GMax activation [92.7% of MVIC]). Several factors, such as barbell position (front, high/low bar back squat), stance width, and the depth of squat, are the main factors affecting GMax activation during the squat. For example, Paoli et al. ( 2009) suggested that larger stance widths (1,5 and 2x great trochanter distance) are necessary for greater activation of the GMax during the back squat. Regarding the effect of squat depth on GMax activity, the results are contradicting. Caterisano et al. ( 2002) compared three different squat depths (partial: ~45° of knee flexion; parallel: ~90° of knee flexion, and full: ~135° of knee flexion) using 100 to 125% of subject's body weight as external resistance. Their results suggested that the full squat elicited greater GMax activation than the parallel and partial back squat. However, their main limitation was the lack of equalization of external load by the depth investigated. Contreras et al. ( 2016a) found no significant difference between full and parallel back squats for any of the GMax portions evaluated. More recently, Da Silva et al. ( 2017) demonstrated that the partial squat elicited higher GMax activation than the full squat variation when external loads are equated to squat depth. GMax relative contribution to hip extensor moment may be reduced in positions of greater squat depth (Vigotsky et al., 2016; Hoy et al., 1990; Neumann, D. A. 2010). Nevertheless, chronic studies have suggested that deeper squats, or a combination of different ranges of motion, induce the most substantial functional and muscular gains, possibly due to more considerable time under tension, mechanical tension, and longer muscle length (Bloomquist et al., 2013; Kubo et al., 2019; Bazyler et al., 2014).

The barbell hip thrust exercise and its variations are expected to demonstrate higher GMax excitation levels when compared to any exercise that includes simultaneous knee and hip flexion/extension movement, such as squats and their variations (Contreras et al., 2015b; Contreras et al., 2016b). Regarding the hip thrust and its variations, GMax activation varied between 49.2 and 105% of MVIC. These results are similar to a recent review performed by our group (Krause Neto et al., 2019), where mean GMax activity ranged between 55 and 105% of MVIC. The foot position is the main factor affecting GMax activation during the barbell hip thrust. For example, Collazo Garcia et al. ( 2018) compared the GMax activation between the different variations of barbell hip thrust. They observed the highest GMax activation when subjects were oriented to intend to rotate the foot outward. Additionally, Kang et al. ( 2016) found placing the foot at 30° of hip abduction presented higher GMax activation than 15 and 0° of hip abduction during a bodyweight hip bridge. Another interesting fact is that barbell hip thrusts elicit high and very high GMax activation even when relative low loads are lifted. Collazo Garcia et al. ( 2018) used 40% of 1RM and obtained high and very high levels of GMax activation in the variations of hip thrusts investigated. Delgado et al. ( 2019) observed that barbell hip thrust performed at 60 kg (~36% of 1RM) elicited similar GMax activation than Romanian deadlift and back squat at 1RM.

The reader should be aware of the number of methodological limitations present in the studies included in this systematic review: (1) the electrode placement, the EMG signal processing, movement phase analyzed and normalization varied between studies, therefore, may have influenced the results obtained in the systematic review; (2) a heterogeneous sample composed of studies that investigate women and/or men may suffer different influences; (3) the variation of the loads used (40% to 100% maximum) may alter the activity levels of GMax as presented by Yavuz and Erdag ( 2017); and (4) different levels of training experience and familiarization with the exercises tested may have influenced the EMG levels that were investigated.

Conclusion

Despite the limitations of the present review, we observed that several exercises and variations elicited very high levels of GMax activity. Therefore, it is reasonable to suggest that the strength and conditioning coach should select in a variety of exercises, the one that most fit-on clients' individual needs.

Other factors such as exercise kinetics and kinematics, relative external load, movement velocity, range of motion, level of fatigue, the mechanical complexity of the exercise (open or closed kinetic chain; weight bearing or non-weight bearing) should be considered when selecting an appropriate exercise for strengthening the GMax.

Therefore, this systematic review demonstrated that the step-up exercise and its variations present the highest levels of muscle excitation of GMax followed by several bilateral exercises and its variations, such as deadlifts, hip thrusts, and squats. GMax activity may vary significantly according to changes in technique during the exercise.

Acknowledgments

The authors have no conflicts of interest.

Biographies
Vinicius de Lima SAMPAIO
Employment

Personal trainer

Degree

Bachelor of Physical Education.
Eliane Florencio GAMA
Employment

Master's and PhD Advisor at São Judas Tadeu University

References

Beardsley C., Contreras B. (2014) The increasing role of the hip extensor musculature with heavier compound lower-body movements and more explosive sports actions. Journal of Strength and Conditioning Research 36(2), 49-55. [Google Scholar]

Collazo García C.L., Rueda J., Suárez Luginick B., Navarro E. (2018) Differences in the Electromyographic Activity of Lower-Body Muscles in Hip Thrust Variations. Journal of Strength and Conditioning Research. In press. [[PubMed] Google Scholar]

Contreras B., Vigotsky A.D., Schoenfeld B.J., Beardsley C., Cronin J. (2015a) A comparison of two gluteus maximus EMG maximum voluntary isometric contraction positions. PeerJ 3:e1261; [[PubMed] PMC free article] [Google Scholar]

Haff G. G., Triplett N. T. (2015) Essentials of strength training and conditioning. 4th edition Human kinetics. [Google Scholar]

Krause Neto W., Vieira T.L., Gama E.F. (2019) Barbell Hip Thrust, Muscular Activation and Performance: A Systematic Review. Journal of Sports Science and Medicine 18, 198-206. [[PubMed] PMC free article] [Google Scholar]

Merletti R., Parker P. A. (2004) Electromyography: physiology, engineering, and non-invasive applications. Vol. 11 John Wiley & Sons. [Google Scholar]

Ramis T.R., Muller C.H.L., Boeno F.P., Teixeira B.C., Rech A., Pompermayer M.G., Medeiros N.D.S., Oliveira Á.R., Pinto R.S., Ribeiro J.L. (2018) Effects of traditional and vascular restricted strength training program with equalized volume on isometric and dynamic strength, muscle thickness, electromyographic activity, and endothelial function adaptations in young adults. Journal of Strength and Conditioning Research 30. [[PubMed] Google Scholar]

Riemann B.L., Lapinski S., Smith L., Davies. G. (2012) Biomechanical analysis of the anterior lunge during 4 external-load conditions. Journal of Athletic Training 47, 372-378. [[PubMed] PMC free article] [Google Scholar]

Williams M.J., Gibson N., Sorbie G.G., Ugbolue U.C., Brouner J., Easton C. (2018) Activation of the gluteus maximus during performance of the back squat, split squat, and barbell hip thrust and the relationship with maximal sprinting. Journal of Strength and Conditioning Research 9, 309. [Google Scholar]

Yavuz H.U., Erdag D. (2017). Kinematic and electromyographic activity changes during back squat with submaximal and maximal loading. Applied Bionics Biomechanics 9084725. [[PubMed] PMC free article] [Google Scholar]

# data[].attributes.attachment_link.title
Gluteus Maximus Activation during Common Strength and Hypertrophy Exercises: A Systematic Review

# data[].attributes.attachment_link.description
The gluteus maximus (GMax) is one of the primary hip extensors. Several exercises have been performed by strength and conditioning practitioners aiming to increase GMax strength and size. This systematic review aimed to describe the GMax activation levels during strength exercises that incorporate hip extension and use of external load.

# data[].attributes.headline
Updated link for Force-velocity-power profiling

# data[].attributes.subject
Updated link for Force-velocity-power profiling

# data[].attributes.body
HT = Squat for glutes.

Squat better for Quads and Adductors

# data[].attributes.attachment_link.readable_text
School of Kinesiology, Auburn University, Auburn, AL, United States

Department of Psychological Sciences, Auburn, AL, United States

Departments of Biomedical Engineering and Statistics, Evanston, IL, United States

Department of Neuroscience, Northwestern University, Chicago, IL, United States

College of Nursing, Auburn University, Auburn, AL, United States

MRI Research Center, Auburn University, Auburn, AL, United States

International Scientific Research Foundation for Fitness and Nutrition, Amsterdam, Netherlands

BC Strength, San Diego, CA, United States

We examined how set-volume equated resistance training using either the back squat (SQ) or hip thrust (HT) affected hypertrophy and various strength outcomes. Untrained college-aged participants were randomized into HT (n = 18) or SQ (n = 16) groups. Surface electromyograms (sEMG) from the right gluteus maximus and medius muscles were obtained during the first training session. Participants completed 9 weeks of supervised training (15-17 sessions), before and after which gluteus and leg muscle cross-sectional area (mCSA) was assessed via magnetic resonance imaging. Strength was also assessed prior to and after the training intervention via three-repetition maximum (3RM) testing and an isometric wall push test. Gluteus mCSA increases were similar across both groups. Specifically, estimates [(−) favors HT (+) favors SQ] modestly favored the HT versus SQ for lower [effect ±SE, −1.6 ± 2.1 cm ²; CI 95% (−6.1, 2.0)], mid [−0.5 ± 1.7 cm ²; CI 95% (−4.0, 2.6)], and upper [−0.5 ± 2.6 cm ²; CI 95% (−5.8, 4.1)] gluteal mCSAs but with appreciable variance. Gluteus medius + minimus [−1.8 ± 1.5 cm ²; CI 95% (−4.6, 1.4)] and hamstrings [0.1 ± 0.6 cm ²; CI 95% (−0.9, 1.4)] mCSA demonstrated little to no growth with small differences between groups. mCSA changes were greater in SQ for the quadriceps [3.6 ± 1.5 cm ²; CI 95% (0.7, 6.4)] and adductors [2.5 ± 0.7 cm ²; CI 95% (1.2, 3.9)]. Squat 3RM increases favored SQ [14 ± 2 kg; CI 95% (9, 18),] and hip thrust 3RM favored HT [−26 ± 5 kg; CI 95% (−34, −16)]. 3RM deadlift [0 ± 2 kg; CI 95% (−4, 3)] and wall push strength [−7 ± 12N; CI 95% (−32, 17)] similarly improved. All measured gluteal sites showed greater mean sEMG amplitudes during the first bout hip thrust versus squat set, but this did not consistently predict gluteal hypertrophy outcomes. Squat and hip thrust training elicited similar gluteal hypertrophy, greater thigh hypertrophy in SQ, strength increases that favored exercise allocation, and similar deadlift and wall push strength increases.

Introduction

Resistance training (RT) presents potent mechanical stimuli that produce robust biological responses ( Egan and Sharples, 2023; Roberts et al., 2023). However, RT responses vary considerably depending on several training variables. One such variable is exercise selection; specifically, different exercises have varying mechanical demands that can lead to differences in muscle growth, strength, and other related outcomes ( Waters et al., 1974; Maeo et al., 2021; Zabaleta-Korta et al., 2021; Maeo et al., 2022). Practitioners and researchers often rely on functional anatomy, basic biomechanics, and acute physiological measurements to surmise what adaptations different exercises may elicit. The degree to which such surmises can meaningfully predict outcomes remains an open question, and recent work casts some doubt on their fidelity.

The reliance on theory and acute measures to guide exercise selection is especially evident in the hip extension exercise literature, an area of particular interest with applications in rehabilitation ( Collings et al., 2023a), aging ( Kulmala et al., 2014), performance ( Miller et al., 2021), and bodybuilding. The roles of various hip extensor muscles during different hip extension tasks have been studied in several ways, including surface electromyography (sEMG), nerve blocks, and musculoskeletal modeling ( Boren et al., 2011; Brazil et al., 2021; Collings et al., 2023b). Based on these acute measures, investigators infer stimulus potency or exercise superiority. For instance, previous work investigated sEMG amplitudes during two common and contentiously contrasted hip extension exercises-the hip thrust and squat-to compare muscle function, implying that this relates to subsequent adaptations ( Contreras et al., 2015; Delgado et al., 2019; Williams et al., 2021). Although mean and peak sEMG amplitudes favored hip thrusts, the ability of sEMG to predict longitudinal strength and hypertrophy outcomes from resistance training interventions was recently challenged ( Vigotsky et al., 2022). To help overcome some sEMG limitations, more sophisticated investigations integrate excitation into musculoskeletal models ( Collings et al., 2023b). Yet, more comprehensive analyses of muscle contributions are still limited by their underlying assumptions ( Herzog and Leonard, 1991), and even perfect modeling of muscle contributions presumes a one-to-one relationship between tension and adaptations.

Muscle tension is the primary driver of muscle hypertrophy but is unlikely to be its sole determinant. Recent evidence demonstrates that RT at long muscle lengths and long-duration static stretching can augment hypertrophic outcomes ( Warneke et al., 2023), suggesting other factors may modulate anabolic signaling. It is unknown to what extent muscle tension may interact with position-specific anabolic signaling and other variables to contribute to the anabolic response and how this interaction may change under different conditions. Regarding the squat and hip thrust, the former has a steeper hip extension resistance curve with a relatively greater emphasis in hip flexion ( Lahti et al., 2019; Brazil et al., 2021), which may confer a more potent gluteal training stimulus. However, this notion assumes proportional force sharing among the hip extensors, but contributions shift throughout the range of motion, clouding inferences. This highlights that longitudinal predictions necessitate assumptions about how motor systems satisfy the mechanical constraints imposed by each exercise and subsequent biological responses, it is difficult to infer the potency of the hypertrophic stimulus using indirect measures. We ultimately need longitudinal data to understand and accurately forecast longitudinal outcomes from individual movements.

Direct evidence is presently needed to compare the outcomes of various exercises. Therefore, the purpose of this study was to examine how RT using either the barbell squat or barbell hip thrust on a set-volume equated basis affected gluteus maximus, medius, and minimus muscle hypertrophy (determined by MRI) and various strength outcomes including the back squat, hip thrust, deadlift, and isometric wall push. As a secondary outcome, we sought to determine how these exercises affected gluteus maximus/medius muscle excitation patterns using sEMG and if sEMG amplitudes forecasted hypertrophy.

Materials and methods

Ethical considerations and participant recruitment

Before commencing study procedures with human participants, this study was approved by the Auburn University Institutional Review Board (protocol #: 22-588). All approved study procedures followed the latest revisions to the Declaration of Helsinki (2013) except for being pre-registered as a clinical trial on an online repository. Inclusion criteria were as follows: (a) between the ages of 18-30 years old with a body mass index (body mass/height ²) of less than 30, have minimal experience with resistance training, averaging less than or equivalent to 1 day per week for the last 5 years; (c) have not been actively participating in any structured endurance training program (e.g., running or cycling) for more than 2 days per week over the past 6 months; (d) free of any known overt cardiovascular or metabolic disease; (e) have not consumed supplemental creatine, and/or agents that affect hormones (testosterone boosters, growth hormone boosters, etc.) within the past 2 months, f) free of any medical condition that would contraindicate participation in an exercise program, (g) do not have conditions which preclude performing an MRI scan (e.g., medically-implanted devices), h) and free of allergies to lactose or intolerances to milk derived products that would contraindicate ingestion of whey protein. Eligible participants who provided verbal and written consent partook in the testing and training procedures outlined in the following paragraphs.

Study design overview

An overview of the study design can be found in Figure 1. Participants performed two pre-intervention testing visits, one in a fasted state for body composition and MRI assessments and the other in a non-fasted state for strength assessments. These visits occurred in this sequence ∼48 h apart; after the pre-intervention strength visit, participants were randomly assigned to one of two experimental groups, including the barbell back squat (SQ) or barbell hip thrust (HT) groups. Two days following the pre-intervention strength testing, all participants partook in their first workout, which served to record right gluteal muscle excitation via sEMG during one set of 10 repetitions for both the SQ and HT exercises. Thereafter, participants engaged in 9 weeks of resistance training (2 days per week). Seventy-2 hours following the last training bout, participants performed two post-intervention testing visits with identical timing and protocols as pre-testing.

FIGURE 1. Study design overview Legend: Figure depicts study design overview described in-text. Abbreviations: PRE, pre-intervention testing visit; POST, post-intervention testing visit; HT, barbell hip thrust; SQ, barbell squat; body comp., body composition testing using bioelectrical impedance spectroscopy; MRI, magnetic resonance imaging; sEMG, surface electromyography.

Body composition and MRI assessments

Body composition

Participants were told to refrain from eating for 8 h prior to testing, eliminate alcohol consumption for 24 h, abstain from strenuous exercise for 24 h, and to be well hydrated for testing. Upon arrival participants submitted a urine sample (∼50 mL) for urine specific gravity assessment (USG). Measurements were performed using a handheld refractometer (ATAGO; Bellevue, WA, United States), and USG levels in all participants were ≤1.020, indicating sufficient hydration. Participants' heights were measured using a stadiometer and body mass was assessed using a calibrated scale (Seca 769; Hanover, MD, United States) with body mass being collected to the nearest 0.1 kg and height to the nearest 0.5 cm. Body composition was then measured by bioelectrical impedance spectroscopy (BIS) using a 4-lead (two hands, two feet) SOZO device (ImpediMed Limited, Queensland, Australia) according to the methods described by Moon et al. ( Moon et al., 2008). Our laboratory has previously shown these methods to produce test-retest intraclass correlation coefficients (ICC 3,1) >0.990 for whole body intracellular and extracellular water metrics on 24 participants ( Haun et al., 2018), and this device provided estimates of fat free mass, skeletal muscle mass, and fat mass.

MRI measurements

MRI testing assessed the muscle cross-sectional area (mCSA) of both glutei maximi. Upon arriving to the Auburn University MRI Research Center, participants were placed onto the patient table of the MRI scanner (3T SkyraFit system; Siemens, Erlangen, Germany) in a prone position with a ∼5-min latency period before scanning was implemented. A T1-weighted turbo spin echo pulse sequence (1,400 m repetition time, 23 ms echo time, in-plane resolution of 0.9 × 0.9 mm ²) was used to obtain transverse image sets. 71 slices were obtained with a slice thickness of 4 mm with no gap between slices. Measurements were taken by the same investigator (R.J.B.) for all scans who did not possess knowledge of the training conditions for each participant.

Following the conclusion of the study, MRI DICOM files were preprocessed using Osirix MD software (Pixmeo, Geneva, Switzerland), and these images were imported into ImageJ (National Institutes of Health; Bethesda, MD, United States) whereby the polygon function was used to manually trace the borders of muscles of interest to obtain mCSA. For all participants, image standardization was as follows: (a) the middle of the gluteus maximus was standardized at the image revealing the top of the femur, (b) the image that was 10 slices upward from this mark was considered to be the upper gluteus maximus, (c) the image that was 18 slices downward from the top of the femur was considered lower gluteus maximus, (d) gluteus medius and minimus mCSAs were ascertained at the upper gluteus maximus image, and (e) combined quadriceps (vastii and rectus femoris), adductors (brevis, longus, and magnus), and combined hamstrings (biceps femoris, semitendinosus, semimembranosus) mCSAs were ascertained at the first transverse slice distal to the last portion of the lower gluteus maximus. When drawing borders to quantify muscles of interest, care was taken to avoid fat and connective tissue. Certain muscles were grouped (i.e., gluteus medius + minimus, combined quadriceps muscles, combined adductor muscles, combined hamstrings muscles) due to inconsistent and poorly delineated muscle borders within participants. All left- and right-side gluteus muscles were summed to provide bilateral mCSA values at each site. Alternatively, thigh musculature mCSA values were yielded from the averages of the left and right legs. This method was performed on the thigh because ∼10% of participants yielded either left or right thigh images that presented visual artifacts from the edge of the MRI receiving coil. In these situations, thigh musculature from only one of the two legs was quantified.

Strength assessments

Isometric muscle strength (wall push)

Participants reported to the laboratory (non-fasted) having refrained from any exercise other than activities of daily living for at least 48 h before baseline testing. A tri-axial force plate (Bertec FP4060-10-2000; Columbus, OH, United States) with an accompanying amplifier (Bertec model # AM6800) sampling at 1,000 Hz was used to measure horizontal force production in newtons N) during a wall push test. The distance from the force plate to the wall was positioned such that when the subjects' forearms were parallel with the ground, the torso was at a ∼45° angle with the ground and one rear foot was in contact with the force plate. Hand placement was standardized by distance from the ground and foot placement was standardized by distance from the wall. The subject was instructed to push, using the dominant leg, as hard as possible into the wall while keeping the torso at 45° ( Figure 2). Two wall pushes were performed for 3 s each, with each repetition being separated by 2 m of rest. The highest peak horizontal force from these two tests was used for analysis.

FIGURE 2. Wall push demonstration. Legend: Figure depicts the wall push test with one of the co-authors (M.D.R.) and shows force tracing.

Dynamic muscle strength

Following wall push testing, dynamic lower body strength was assessed by three-repetition maximum (3RM) testing for the barbell back squat, barbell hip thrust, and barbell deadlift exercises. Notably, our laboratory has extensively performed 3RM dynamic strength testing on numerous occasions in untrained and trained participants ( Mobley et al., 2018; Vann et al., 2020; Godwin et al., 2023; Smith et al., 2023). Briefly, specific warm-up sets for each exercise consisted of coaching participants through the movement patterns and gauging comfort and movement proficiency. Subsequent warm-ups for each exercise were chosen with an attempt at approximating 5 repetitions at ∼50% 1RM for one set and 2-3 repetitions at ∼60-80% 1RM for two additional sets. Participants then performed sets of three repetitions with incremental increases in load for 3RM determinations for each exercise and 3 m of rest was given between each successive attempt. For all exercises, participants were instructed to perform repetitions in a controlled fashion, with a concentric action of approximately 1 s and an eccentric action of approximately 2 s. All three exercises were performed with feet spaced 1-1.5-times shoulder width apart. For the barbell squat, depth was set to when the femur was parallel to the floor, with all but one participant achieving a depth at or below this point. For the barbell hip thrust, the hip thrust apparatus (Thruster 3.0, BC Strength; San Diego, CA, United States) was set to a height at which participants could make brief contact with the ground with the weight plate (21") and hips at the bottom of each repetition. Repetitions were considered properly executed when the participant's tibia was perpendicular to the floor and the femur was parallel to the floor. Torso position was sufficiently maintained to avoid excessive motion through the pelvis. For the barbell deadlift, participants began repetitions from the floor and were prompted to maintain the torso position throughout the execution of the lift. A lift was deemed successful once participants stood upright with full knee and hip extensions.

sEMG measurements during the first training bout

Subjects were asked to wear loose athletic attire to access the EMG electrode placement sites. Before placing the electrodes on the skin, if necessary, excess hair was removed with a razor, and the skin was cleaned and abraded using an alcohol swab. After preparation, double-sided adhesives were attached to wireless sEMG electrodes (Trigno system; Delsys, Natick, MA, United States), where were placed in parallel to the fibers of the right upper gluteus maximus, mid gluteus maximus, lower gluteus maximus, and gluteus medius (see Figure 4A in Results). Upper and middle gluteus maximus electrodes were placed based on the recommendations of Fujisawa and others ( Fujisawa et al., 2014), albeit we considered the lower gluteus maximus as middle. The upper gluteus maximus electrodes were placed superior and lateral to the shortest distance between the posterior superior iliac spine (PSIS) and the posterior greater trochanter, and the middle gluteus maximus electrodes were placed inferior and medial to the shortest distance between the PSIS and the posterior greater trochanter. Lower gluteus maximus electrodes were placed one inch (2.54 cm) above the most medial presentation of the gluteal fold. If it was ambiguous as to whether an appreciable amount of muscle tissue existed in this lower region, the participant was asked to contract the area and palpation was used to confirm proper placement. Gluteus medius electrodes were placed over the proximal third of the distance between the iliac crest and the greater trochanter. After the electrodes were secured, a quality check was performed to ensure sEMG signal validity. Following electrode placement, maximum voluntary isometric contraction (MVIC) testing was performed immediately prior to 10RM testing. For the gluteus maximus, the MVIC reference was a prone bent-leg hip extension against manual resistance applied to the distal thigh, as used by Boren and others ( Boren et al., 2011). For the gluteus medius MVIC, participants laid on their side with a straight leg and abducted against manual resistance. Care was taken not to depress the joint of interest during manual testing. In all MVIC positions, participants were instructed to contract the tested muscle as hard as possible. After 5 m of rest following MVIC testing, all participants performed one set of ten repetitions utilizing estimated 10RM loads for both the barbell back squat and the barbell hip thrust exercises. The exercise form and tempo used were the same as described in the strength testing section above. During both sets, muscle excitation of the upper/middle/lower gluteus maximus and gluteus medius were recorded with the wireless sEMG system whereby electrodes were sampled at 1,000 Hz. Participants allocated to HT training performed the squat set first followed by the hip thrust set. Participants allocated to SQ training performed the hip thrust set first followed by the back squat set. Following these two sEMG sets, the wireless sEMG electrodes were removed. Participants finished the session with two more sets of 8-12 repetitions using the calculated 10RM load for the exercise allocated to them for the intervention.

Signal processing was performed using software associated with the sEMG system (Delsys EMGworks Analysis v4.7.3.0; Delsys). sEMG signals from the MVICs and 10RM sets of back squat and hip thrust were first rectified. Signals were then processed with a second-order digital low-pass Butterworth filter, with a cutoff frequency of 10 Hz, and further smoothed using a root mean square moving window of 250 m. The average of the middle 3 s of the filtered MVIC time series was then used to normalize the squat and hip thrust data for each site. Data were then visually inspected for fidelity before calculating the mean and peak sEMG values. Partial sequences of sEMG data were removed in the rare event that tempo was irregular or not maintained, or if a brief artifact was introduced. Final EMG data are presented as mean and peak sEMG amplitudes during the hip thrust and back squat 10RM sets. sEMG issues were only evident for a small portion (see Results) of the 34 participants who finished the intervention. Data were dropped from analyses due to artifacts produced through either electrode slippage or sEMG electrode jarring during the 10RM sets, leading to persistent distortion. In this regard, sample sizes for each muscle site are presented in the results section.

Resistance training procedures

The RT protocol consisted of 3-6 sets per session of barbell hip thrusts for HT participants or barbell back squats for SQ participants. Excluding the first week, which consisted of one session, all remaining weeks consisted of two sessions per week on non-consecutive days for 9 weeks. Week-to-week set schemes per session were as follows: week 1, 3 sets; week 2, 4 sets; weeks 3-6, 5 sets; weeks 7-9, 6 sets. The repetition range was set to 8-12 repetitions; if a participant performed less than 8 repetitions or more than 12 repetitions, the load was adjusted accordingly. D.L.P. and 1-2 other co-authors supervised all sessions, during which participants were verbally encouraged to perform all sets to the point of volitional muscular failure, herein defined as the participants being unable to volitionally perform another concentric repetition while maintaining proper form. Again, the exercise form and tempo used were the same as described in the strength testing section above; however, squat repetitions were not limited to a depth corresponding to the femur parallel to the floor but rather the lowest depth achievable. Outside of these supervised training sessions, participants were instructed to refrain from performing any other lower-body RT for the duration of the study. Participants could miss a maximum of 2 sessions and still be included in the analysis.

Dietary instructions during the study

Participants were given containers of a whey protein supplement (Built with Science; Richmond, BC, Canada) and were instructed to consume one serving per day (per serving: 29 g protein, 1 g carbohydrate, 0.5 g fat, 130 kcal). This was done in the hope of diminishing inadequate protein intake as a confounding variable. Other than this guidance, participants were advised to maintain their customary nutritional regimens to avoid other potential dietary confounders.

All participants were instructed to provide 4-day food logs (2 weeks days, 2 weekend days) during the first and last weeks of the intervention. A registered dietician (A.D.R.) oversaw food log analyses using The Nutrition Data System for Research (NDSR; NDSR 2014; University of Minnesota) food log entry and analyses ( Schakel, 2001). Calories and macronutrients from each time point represent the 4-day average for the respective food log dates.

Notes on randomization and blinding

Investigators were blinded to group allocation during the MRI scan and its analysis. Participants were not blinded to group allocation as exercise comparisons were not amenable to blinding. Due to logistical constraints investigators were not blinded to group allocation during strength testing and, thus, bias cannot be completely ruled out in this context. Randomization into SQ and HT groups was performed via a random number generator in blocks of 2 or 4 as participants consented.

Statistics and figure construction

Data were analyzed in Jamovi v2.3 ( https://www.jamovi.org) and R (version 4.3.0). We performed three different sets of analyses.

First, we compared mean and peak HT and SQ sEMG amplitudes from the first training session, for which we performed paired t-tests.

Second, we compared the longitudinal effects of HT and SQ training on mCSA and strength. Notably, baseline and within-group inferential statistics were not calculated, as baseline significance testing is inconsequential ( Senn, 1994) and within-group outcomes are not the subject of our research question ( Bland and Altman, 2011). However, we descriptively present within-group changes to help contextualize our findings. The effect of group (SQ versus HT) on each outcome variable was estimated using linear regression, in which post-intervention scores were the response variable, group was dummy-coded 0 for SQ and 1 for HT, and the pre-intervention score was included as a covariate of no interest ( Vickers and Altman, 2001). The model output can thus be interpreted as the expected difference in post-intervention (or mathematically equivalently, change) scores between the SQ and HT groups for a given pre-intervention score. We used the bias-corrected and accelerated stratified bootstrap with 10,000 replicates to calculate 95% compatibility intervals (CIs).

Third, we investigated the extent to which sEMG amplitudes from the first session forecasted growth. There are multiple ways this question could be posed, and since claims surrounding sEMG amplitude's predictive power are ambiguous, we addressed each of the following questions: i) Do individuals with greater sEMG amplitudes grow more than individuals with lower sEMG amplitudes? For this, we calculated a Pearson correlation for each muscle using changes in mCSA and the sEMG amplitudes. ii) Do regions or muscles with greater sEMG amplitudes grow more than regions or muscles with lower sEMG amplitudes? For this, we used a linear mixed-effects model in which ln (mCSA post/mCSA pre) was the response variable; sEMG amplitude, group, and their interaction were fixed effects; and we permitted intercepts and slopes for sEMG amplitude to vary across subjects. Since we are interested in generalizable predictions, we calculated prediction intervals for the slopes by calculating a Wald interval using the sum of the parameter variance and random effects variance. iii) Can the differences in growth elicited from different exercises be accounted for by sEMG amplitude? For this, we calculated the so-called "indirect effect" of sEMG amplitude, which represents the extent to which the group effect on hypertrophy can be explained by sEMG amplitudes. This was done the same way a typical "mediation analysis" is done (although, this should not be viewed as causal here)-we bootstrapped the difference between the group effect (SQ vs HT) when sEMG was not in the model and when sEMG was added to the model. If group-based sEMG differences accounted for group-based hypertrophy differences, then the effect of group on growth would shrink towards 0 and sEMG would absorb the variance in growth.

Finally, self-reported food log data (kcal/d, protein g/d, carbohydrate g/d, and fat g/d) were analyzed using two-way repeated measures ANOVAs.

Figures were constructed using Microsoft PowerPoint and through paid site licenses for BioRender ( https://www.biorender.com), GraphPad Prism v9.2.0 (San Diego, CA, United States), and ggplot2.

Results

Consort and general baseline participant characteristics

The CONSORT diagram is presented in Figure 3. In total, 18 HT and 16 SQ participants completed the study and were included in data analyses unless there were technical issues precluding the inclusion of data (e.g., sEMG signal distortion).

FIGURE 3. CONSORT diagram Figure depicts participant numbers through various stages of the intervention. All participants were included in data analysis unless there were technical issues precluding the inclusion of data (e.g., EMG distortion).

General baseline characteristics of the 18 HT participants who finished the intervention were as follows: age: 22 ± 3 years old, 24 ± 3 kg/m ², 5 M and 13 F. Baseline characteristics of the 16 SQ participants who finished the intervention were as follows: age: 24 ± 4 years old, 23 ± 3 kg/m ², 6 M and 10 F. Also notable, the HT participants missed an average of 0.8 ± 0.4 workouts during the study, and the SQ participants missed 0.8 ± 0.5.

Self-reported food log data between groups

Most participants reported 4 days at during the first and last weeks of the study with the following exceptions: i) two SQ participants reported 3 days only with the first food log, ii) two SQ participants were missing last-week food logs, and iii) one HT participant did not turn in the first food log. Thus, food log data were analyzed for n = 15 SQ and n = 16 HT participants. No group x time (G x T) interactions were evident for any of the assessed variables (data not shown).

First bout sEMG results

sEMG data obtained from the right gluteus muscles during the first workout bout, based on one set of 10RM hip thrust and one set of 10RM squat, are presented in Figure 4. All sites showed greater mean sEMG values during the hip thrust versus squat set ( p < 0.01 for all; Figure 4B). Peak sEMG values were greater for the upper and middle gluteus maximus ( p < 0.001 and p = 0.015, respectively), whereas small differences existed for the lower gluteus maximus or gluteus medius sites ( Figure 4B). The number of repetitions completed during the 10RM sets used for sEMG recordings were not different between exercises (back squat: 9 ± 1 repetitions, hip thrust: 9 ± 2 repetitions).

FIGURE 4. Surface electromyography (sEMG) amplitudes during the back squat and barbell hip thrust. Legend: During the first session, all participants performed both back squats and barbell hip thrusts while we recorded sEMG amplitudes. (A) Representative sEMG electrode placement is depicted on a co-author in panel. (B) Data depict mean (left) and peak (right) sEMG amplitudes during one 10RM set of hip thrusts and one 10RM set of back squats. As 34 participants partook in this test, sample sizes vary due to incomplete data from electrode slippage or distortion. Bars are mean ± SD, and individual participant values are depicted as dots. (C) Representative data from one participant.

Gluteus musculature mCSAs according to MRI

The effect of SQ relative to HT for left + right mCSA was negligible across gluteal muscles ( Figures 5A-D). Point estimates modestly favored HT for lower [effect ±SE, −1.6 ± 2.1 cm ²; CI 95% (−6.1, 2.0)], mid [−0.5 ± 1.7 cm ²; CI 95% (−4.0, 2.6)], and upper [−0.5 ± 2.6 cm ²; CI 95% (−5.8, 4.1)] gluteal mCSAs; these point estimates were dwarfed by the variance. Left + right mCSA values for the gluteus medius + minimus demonstrated a lesser magnitude of growth, with a point estimate that also modestly favored HT albeit with appreciable variance [−1.8 ± 1.5 cm ²; CI 95% (−4.6, 1.4)].

FIGURE 5. Gluteus musculature mCSA changes following back squat and barbell hip thrust training, assessed using MRI. Legend: Figure depicts change adjusted for pre-intervention scores for MRI-derived muscle cross-sectional area (mCSA). (A) left + right (L + R) upper gluteus maximus, (B) L + R middle gluteus maximus, (C) L + R lower gluteus maximus, and (D) L + R gluteus medius + minimus. Data include 18 participants in the hip thrust group and 16 participants in the back squat group. Graphs contain change scores with individual participant values depicted as dots. (E) Three pre and post representative MRI images are presented from the same participant with white polygon tracings of the L + R upper gluteus maximus and gluteus medius + minimus (top), L + R middle gluteus maximus (middle), and L + R lower gluteus maximus (bottom).

Thigh musculature mCSAs according to MRI

Compared to HT, SQ produced greater mCSA growth for quadriceps [3.6 ± 1.5 cm ²; CI 95% (0.7, 6.4), Figure 6A] and adductors [2.5 ± 0.7 cm ²; CI 95% (1.2, 3.9), Figure 6B]. However, hamstrings growth was equivocal across both conditions, yielding negligible between-group effects [0.1 ± 0.6 cm ²; CI 95% (−0.9, 1.4), Figure 6C].

FIGURE 6. Thigh musculature mCSA changes following back squat and barbell hip thrust training, assessed using MRI. Legend: Figure depicts change adjusted for pre-intervention scores for MRI-derived muscle cross-sectional area (mCSA). Left and/or right (A) quadriceps, (B) adductors, and (C) hamstrings. Data include 18 participants in the hip thrust group and 16 participants in the back squat group. Bar graphs contain change scores with individual participant values depicted as dots. (D) A representative pre- and post-intervention MRI image is presented with white polygon tracings of the quadriceps (denoted as Q), adductors (denoted as ADD), and hamstrings (denoted as H).

Strength outcomes

Strength outcomes of SQ relative to HT favored respective group allocation for specific lift 3RM values. Specifically, Squat 3RM favored SQ [14 ± 2 kg; CI 95% (9, 18), Figure 7A], and hip thrust 3RM favored HT [−26 ± 5 kg; CI 95% (−34, −16), Figure 7B]. Results were more equivocal for the deadlift 3RM [0 ± 2 kg; CI 95% (−4, 3), Figure 7C] and wall push [−7 ± 12 N; CI 95% (−32, 17), Figure 7D].

FIGURE 7. Strength outcomes following back squat and barbell hip thrust training. Legend: Figure depicts change adjusted for pre-intervention scores for (A) 3RM barbell back squat values, (B) 3RM barbell hip thrust values, (C) 3RM barbell deadlift values, and (D) wall push as demonstrated in Figure 2. Data include 18 participants in the hip thrust group and 16 participants in the back squat group.

Forecasting training-induced gluteus muscle mCSA changes with sEMG amplitudes

Across-subject correlations

sEMG amplitude's ability to forecast muscle growth across-subjects was generally poor and variable. Mean sEMG amplitudes produce negligible to moderate correlations for lower [ r = 0.18 (−0.30, 0.57)], middle, [ r = −0.03 (−0.32, 0.25)], upper [r = 0.50 (0.03, 0.81)], and medius + minimus [ r = 0.28 (0, 0.53)]. We observed similar results for peak sEMG amplitudes from the lower [r = 0.13 (−0.16, 0.46)], middle [ r = −0.03 (−0.33, 0.21)], upper [r = 0.32 (−0.05, 0.62)], and medius + minimus [r = 0.24 (−0.02, 0.48)].

Across-region correlations

We fit two linear mixed-effects models to assess how differences in sEMG amplitudes across muscles can account for regional growth. Since the response variable was change in muscle size on the log scale, the exponentiated coefficients can be interpreted as the increase in muscle relative to baseline for each additional %MVIC; notably, this effect is multiplicative rather than additive. The first model, which used mean sEMG amplitudes, produced small and variable estimates for both SQ [1.003, PI 95% (0.998, 1.008)] and HT [1.002, PI 95% (0.997, 1.006)] groups. The second model, which used peak sEMG amplitudes, produced even more modest results for both the SQ [1.0003, PI 95% (0.9997, 1.0009)] and HT [1.0002, PI 95% (0.9996, 1.0007)] groups.

Across-exercise variance

Mean sEMG amplitude's ability to capture the group effects was inconsistent for lower [indirect effect = −0.55, CI 95% (−3.87, 0.58)], middle [0.06, CI 95% (−0.82, 1.56)], upper [−2.98, CI 95% (−8.73, −0.38)], and medius + minimus [−0.73, CI 95% (−2.70, 0.14)]. We observed similar results for peak sEMG amplitudes for lower [−0.08, CI 95% (−2.27, 0.59)], middle [0.22, CI 95% (−1.63, 1.89)], upper [−3.04, CI 95% (−8.32, 0.15)], and medius + minimus [−0.86, CI 95% (−2.47, 0)]. These estimates can be compared to the group effects ("total effects") earlier in the Results.

Discussion

To further our understanding of hip extensor exercises and the validity of relying on theory and acute physiological measures for exercise selection, here we acutely (sEMG) and longitudinally (hypertrophy, strength) compared two common hip extension exercises: the back squat and barbell hip thrust. Acutely, HT sEMG amplitudes were generally greater for the HT. However, this did not appear to translate and accurately capture longitudinal adaptations. Across all gluteus muscle hypertrophy outcomes, SQ and HT training yielded modest differences but meaningful growth occurring, except in the gluteus medius and minimus. Thigh hypertrophy outcomes favored SQ in the adductors and quadriceps, with no meaningful growth in either group in the hamstrings. Strength outcomes indicated that hip thrust 3RM changes favored HT, back squat 3RM changes favored SQ, and other strength measures similarly increased in both groups. sEMG amplitudes could not reliably predict hypertrophic outcomes across several analytical approaches. In the following paragraphs, we discuss these results in the context of available evidence and speculate on their potential implications for exercise prescription.

Hypertrophy outcomes

The primary finding of interest was that upper, middle, and lower gluteus maximus muscle hypertrophy was similar after 9 weeks of training with either the squat or hip thrust. This may seem to run counter to the notion that muscle tension in lengthened positions augments growth ( Kassiano et al., 2023b), since the sticking region for the squat occurs in greater hip flexion as compared to the hip thrust. Importantly, much of the previous work on this topic is in muscles being worked in a more isolated fashion ( Maeo et al., 2021; Maeo et al., 2022). Thus, the equivocal findings may suggest that the context in which the muscle is experiencing lengthened loading critically determines subsequent adaptations. Muscle contributions, and not just positions, may need to be jointly considered in determining whether superior hypertrophy outcomes would be achieved. This idea is loosely supported by sEMG and musculoskeletal modeling research, suggesting the gluteus maximus may not be strongly recruited toward the bottom of the squat ( Contreras et al., 2015; Chiu et al., 2017; Kassiano et al., 2023a). This notion would suggest the nervous system may not strongly recruit the gluteus maximus while at its longest length in the squat, and synergist muscle involvement could be precluding the maximum benefits of stretch-augmented hypertrophy.

In addition to motor control governing how the gluteus maximus contributes to and adapts from the squat, there are study-specific considerations. Both exercises may stimulate similar muscle hypertrophy in untrained populations given that RT in general elicits rapid growth early on, potentially creating a ceiling effect on growth rate and thus observed growth. Alternatively stated, skeletal muscle hypertrophy in novice trainees may be less influenced by nuances in exercise selection. Notwithstanding, our results suggest that a 9-week set-equated training program with either the hip thrust, or squat elicits similar gluteal muscle hypertrophy in novice trainees.

Finally, our data show that thigh hypertrophy favored the squat, whereas thigh hypertrophy was minimal in the hip thrust. This is perhaps unsurprising and is consistent with previous literature. The adductors, particularly the adductor magnus, have the largest extension moment contribution at the bottom of a squat. Thus, the nervous system may favor its recruitment for this purpose. In line with this finding, adductor magnus mCSA changes favor a greater squat depth ( Kubo et al., 2019). Hamstring mCSA changes did not occur in either group, in accordance with previous work ( Kubo et al., 2019). Critically, these data imply that the hip thrust exercise primarily targets gluteus muscle hypertrophy while limiting non-gluteal thigh muscle hypertrophy; in other words, the hip thrust appears to be more gluteus maximus-specific.

Strength outcomes

Both groups effectively increased strength outcomes for all exercises tested, with magnitudes in accordance with previous strength literature ( Lacio et al., 2021). However, HT RT better increased hip thrust strength and SQ RT better increased back squat strength, which is to be expected due to training specificity ( Morrissey et al., 1995). Back squat 3RM increased by 17% in the HT group and 44% in the SQ group, while hip thrust strength increased by 63% in HT group and 34% in SQ group. In contrast, deadlift and wall push outcomes increased similarly in both groups. Deadlift increased by 15% in SQ and 16% in HT, and wall push increased by 7.6% in SQ and 10% in HT.

Using acute first bout sEMG to predict hypertrophy

Our secondary aim was to evaluate the ability of sEMG to forecast longitudinal adaptations. In agreement with previous work ( Contreras et al., 2015), gluteus muscle sEMG amplitudes during the hip thrust exercise were greater across all measured gluteal sites. However, these sEMG amplitude differences did not reliably translate to greater hypertrophy, no matter what analytical approach we took. Specifically, i) individuals with greater sEMG amplitudes did not consistently experience greater growth; ii) regions with greater sEMG amplitudes did not consistently experience greater growth; iii) differences in sEMG amplitudes between exercises could not consistently explain differences in growth, in large part since the hypertrophy results were equivocal. This finding implies that acute sEMG readings during a workout bout are not predictive of hypertrophic outcomes, and this viewpoint is supported by a recent review by Vigotsky et al. ( Vigotsky et al., 2022). As indicated by the authors, inconsistent relationships between EMG amplitudes and muscle growth have been previously reported, which may be due to one or several reasons, ranging from biases in the sEMG recordings to assumptions about how adaptations occur ( Vigotsky et al., 2022). Evidently, the reliance on acute sEMG measurements may in fact be an over-reliance, but more work is needed in this realm.

Finally, we also verbally asked participants which exercise they "felt more" in the gluteal muscles after testing both exercises. All participants indicated they felt the hip thrust more in the gluteal region. However, these data were not quantified and, despite these anecdotal sensations and sEMG differences indicating more gluteus muscle excitation during HT, hip thrust RT and squat RT elicited similar applied outcomes. These findings highlight the importance of longitudinal investigations.

Limitations

Our study has a few limitations to consider. First, our participants were young untrained men and women; thus, results cannot necessarily be generalized to other populations including adolescents, older individuals, or trained populations. Additionally, like most training studies, this study was limited in duration. It should also be noted that gluteal hypertrophy was the main outcome, and the MRI coil was placed over this region as subjects were lying prone. Thus, compression may have affected the thigh musculature, and distal measures were not obtained for the thigh. Finally, training volume was equated, and frequency was set at two training days per week. Therefore, results can only be generalized to this protocol.

Although we did not consider female participants' menstrual cycle phase or contraceptive usage, we do not view this as a limitation. In this regard, several reports indicate that contraceptives have no meaningful impact on muscle hypertrophy in younger female participants during periods of resistance training ( Ruzic et al., 2003; Dalgaard et al., 2019; Romance et al., 2019; Dalgaard et al., 2020; Oxfeldt et al., 2020; Riechman and Lee, 2021). Likewise, well-controlled trials indicate that the menstrual cycle phase does not affect strength characteristics ( Romero-Moraleda et al., 2019), and that variations in female hormones during different phases do not affect muscle hypertrophy and strength gains during 12 weeks of resistance training ( Sakamaki-Sunaga et al., 2016).

Future directions

Future research should aim to examine a group that performs both exercises on a volume-equated basis to determine if there are synergetic effects. Comparing these exercises with different volumes/frequencies is also warranted as exercises may have differing volume tolerances. From a mechanistic standpoint, future studies should characterize anabolic signaling between different points on the length-tension curve as well as ascertain where a muscle exists on this curve with more clarity.

Conclusion

Squat and hip thrust RT elicited similar gluteal hypertrophy, whereas quadriceps and adductors hypertrophy was superior with squat training. Further, although strength increases were specific to exercise allocation, both forms of RT elicited similar strength transfer to the deadlift and wall push. Importantly, these results could not be reliably predicted from acute data (sEMG). These current data provide trainees with valuable insight concerning two widely popular hip-specific exercise modalities, and this information can be leveraged for exercise selection based on specific structural or functional goals.

Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Ethics statement

The studies involving humans were approved by the Auburn University Institutional Review Board. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

Author contributions

DP: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Project administration, Writing-original draft, Writing-review and editing. MR: Investigation, Methodology, Supervision, Writing-review and editing. AV: Data curation, Formal Analysis, Methodology, Software, Validation, Writing-review and editing. MM: Investigation, Methodology, Writing-review and editing. EB: Methodology, Writing-review and editing. RU: Investigation, Methodology, Writing-review and editing. CR: Methodology, Writing-review and editing. A-AB: Methodology, Writing-review and editing. MM: Investigation, Methodology, Writing-review and editing. MM: Investigation, Methodology, Writing-review and editing. NK: Investigation, Methodology, Writing-review and editing. SL: Data curation, Formal Analysis, Methodology, Writing-review and editing. AF: Formal Analysis, Investigation, Methodology, Writing-review and editing. CW: Data curation, Formal Analysis, Investigation, Methodology, Resources, Writing-review and editing. WW: Data curation, Methodology, Resources, Software, Writing-review and editing. AB: Data curation, Investigation, Methodology, Resources, Writing-review and editing. RB: Data curation, Methodology, Resources, Software, Writing-review and editing. MH: Conceptualization, Funding acquisition, Writing-review and editing. BC: Conceptualization, Funding acquisition, Writing-review and editing. MR: Conceptualization, Formal Analysis, Funding acquisition, Investigation, Project administration, Supervision, Writing-review and editing.

Funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. Funding for this study was made possible through gift funds (some of which were donated by the International Scientific Research Foundation for Fitness and Nutrition and BC) to the MR laboratory. Other financial sources included indirect cost sharing (generated from various unrelated contracts) from the School of Kinesiology, MM being fully supported through a T32 NIH grant (T32GM141739), and DP being fully supported by a Presidential Graduate Research Fellowship (fund cost-sharing from Auburn University's President's office, the College of Education, and the School of Kinesiology).

Acknowledgments

We thank the participants who volunteered and participated in the study. We also thank Bradley Ruple, Josh Godwin, and C. Brooks Mobley for their assistance and insight throughout the project. We also thank Jeremy Ethier for donating whey protein to the study. Raw data related to the current study outcomes will be provided upon reasonable request by emailing the latter corresponding author (MR) at mdr0024@auburn.edu.

Conflict of interest

BC and MH disclose that they sell exercise-related products and services. However, neither was involved in any aspect of the study beyond assisting with the study design and providing funds to partially cover participant and MRI costs through a gift to the laboratory of MR. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

References

# data[].attributes.attachment_link.title
Frontiers | Hip thrust and back squat training elicit similar gluteus muscle hypertrophy and transfer similarly to the deadlift

# data[].attributes.attachment_link.description
We examined how set-volume equated resistance training using either the back squat (SQ) or hip thrust (HT) affected hypertrophy and various strength outcomes...

# data[].attributes.body
Isometric Hip Abduction and different variations of the hip hitch (pelvic drop) exhibited greatest activation of the glute min.

# data[].attributes.attachment_link.title
Gluteus Maximus Anatomy: Origin, Insertion & Action

# data[].attributes.attachment_link.title
Gluteus Minimus Anatomy: Origin, Insertion & Action

# data[].attributes.body
Hip Hitch (Pelvic Drop) exercises exhibited greatest activation of the Glute Med.

Side lying clams exhibited low-moderate activation of the Glute Med.

# data[].attributes.body
Gluteus Minimus and Gluteus Medius Muscle Activity During Common Rehabilitation Exercises in Healthy Postmenopausal Women

# data[].attributes.body
Gluteus Minimus and Gluteus Medius Muscle Activity During Common Rehabilitation Exercises in Healthy Postmenopausal Women