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Cam Merritt

Cam Merritt is a writer and editor specializing in business, personal finance and home design. He has contributed to USA Today, The Des Moines Register and Better Homes and Gardens"publications. Merritt has a journalism degree from Drake University and is pursuing an MBA from the University of Iowa.

When you watch tennis, or any other sport, you're watching a demonstration of physics, just with more cheering than the typical physics experiment. Central to the action are the three laws of motion described in 1687 by Sir Isaac Newton, the Grand Slam champ of pre-industrial science. In many ways, a tennis match is a test of which player manipulates Newton's laws to greatest effect.

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Newton's first law of motion is commonly called the law of inertia: An object in a state of uniform motion will remain in that motion unless it encounters an external force, and an object at rest will remain at rest unless it is acted upon by an external force. Newton's second law defines the relationship between an object's mass, the force applied to it and the acceleration that results: Force equals mass times acceleration, or F=ma. Newton's third law of motion might be the one that most people are most familiar with, if only because they see it quoted so often: For every action there is an equal and opposite reaction.

In tennis, the most obvious instance of Newton's first law is the path of the ball. When you smack the ball with your racket, it heads off in a certain direction. If you were playing the game in the vacuum of intergalactic space, light years from any gravity-producing body, the ball would continue in that direction more or less indefinitely, because no external forces would be acting on it. On Earth, however, two major forces are at work: Air resistance slows the speed of the ball and gravity pulls the ball toward the ground.

When you thwacked that tennis ball with your racket -- in space or on Earth -- you exerted a force on it. How much force? That's where Newton's second law comes in: Force equals mass times acceleration. In this equation, mass is measured in kilograms and acceleration in a unit called "meters per second per second." Acceleration isn't the same thing as speed; rather, it's the rate at which something is speeding up. If an object is moving at 1 m per second, or "m/s," and it speeds up so that one second later it's moving at 2 m/s, then it sped up 1 m/s in that one second -- 1 m per second per second.

Now back to that tennis ball you hit: A tennis ball has a mass of about 56 g, or 0.056 kg. And let's say that you put enough zing on the ball that one-tenth of a second after you hit it, it reaches 100 mph, or 44.7 m per second. That's a rate of acceleration of 447 m per second per second, or m/s/s. Multiply 0.056 kg times 447 m/s/s and you get 25.032. But 25.032 of what? Force is measured in units called, appropriately enough, Newtons. You hit the ball with 25.032 Newtons of force. Nice serve.

You serve the ball, your opponent returns the serve and you go to return her volley. You plant your foot on the ground and push off. You push in one direction -- at an angle into the ground -- and your body goes in the opposite direction, at an angle away from the ground. The force with which you pushed into the ground is the force with which you're propelled forward. That's action and reaction. You're Newton's third law of motion, in motion.

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How Does Newton's Laws of Motion Interact With Tennis?

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When you watch tennis, or any other sport, you're watching a demonstration of physics, just with more cheering than the typical physics experiment. Central to the action are the three laws of motion described in 1687 by Sir Isaac Newton, the Grand Slam champ of pre-industrial science.

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BALANCE AND STABILITY FOR TENNIS

Sprinting for a drop shot takes a split-second reaction to decide to move forward, the deceleration of your body in the direction that you were going, the power generation needed to accelerate towards the ball and the speed you need to get to the ball. This sequence happens every time you play tennis. This should be second nature to your body. Every time you start or stop your dynamic balance is tested. In the training world we call these changes in direction points of transformation. IF YOU CAN NOT CONTROL YOUR POINTS OF TRANSFORMATION YOU WILL EITHER FALL OR GET INJURED.
We are not talking about standing on one leg and being still. Tennis requires Dynamic Balance. Dynamic Balance is the ability to maintain your center of gravity while moving your legs and arms in many different directions. In order to hit a good shot, you must maintain your center of gravity through contacting the ball. The center of gravity is best described as the point at which your body balances most perfectly.

Research in junior competitive tennis has shown that losing balance while hitting causes 80% of all errors. Usually reasons such as hitting the ball too close or too far away from the body, bad elbow position, too high shoulders, etc. are given; when the real reason is that all these problems are caused by loss of balance. Tennis instructors tell players to maintain their balance - easier said than done. How many of you work on your balance as part of your training regimen?

The 4 Ever Balanced Class is specifically designed improve your dynamic balance by developing neuromuscular control. We target your feet, legs and core by creating unstable situations that will stimulate and strengthen your balance muscles through a wide variety of exercises. Consistently working on these muscles will result in increased flexibility, decreased risk of injury and fewer unforced errors during matches.

One thing to remember when working on balance is the dynamic component of it. It does not do anyone much good to be standing still on one leg. Life and sport are full of movements, so your training regimen must be done while moving. It will be challenging at first but you would be surprised how quickly your foot and lower leg muscles adapt to the challenges you place on them. In no time at all you will be able to move and respond more quickly on one leg or both.

Paul Fezza
NSCA, NATA
Muscle Activation Specialist
Gray Fellow of Applied Functional Science

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BALANCE AND STABILITY FOR TENNIS Sprinting for a drop shot takes a split-second reaction to decide to move forward, the deceleration of your body in the direction that you were going, the power generation needed to accelerate towards the ball and the speed you need to get to the ball.

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Online Tennis Instruction with Florian Meier

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Tennis Serve Tip: Proper Weight Transfer & Balance

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The loading stage entails positioning segments of the body in particular ways which in turn helps produce potential energy. There are two ways to position the athlete's feet: the foot-up technique; and the foot-back technique (Refer Image5). Athletes who compose the tennis serve using the foot-up technique use vertical forces; this ultimately allows the athlete to reach greater heights in comparison to other techniques(Kovacs &Ellenbecker, 2011). In this technique, the rear leg provides a push forwards and upwards, whereas the front leg provides the athlete with a strong stable post which allows rotational momentum. It is important to note that this foot-up technique requires athletes to undergo unconventional training of the lower body for the landing of both feet after the service impact (Kovacs &Ellenbecker, 2011).

On the other hand, the foot-back serving technique requires a greater knee joint extension, which in turn requires lower body training similar to above. Putting this into comparison, the optimal foot-up technique requires a knee extension of 54.1° ± 11.7° and the optimal foot-back technique requires the knee extension of 65.5° ± 12.6° (Kovacs &Ellenbecker, 2011). The advantage of using the foot-back technique is that it allows athletes to produce a wider base of support which in turn permits greater squat depth.

According to Kovacs &Ellenbecker (2011), the velocity of a serve has a correlation with greater muscle force during this third stage, the loading stage. Additionally, the efficiency of a server has a correlation with the internal rotation of the arm which holds the racquet. Although the two foot techniques will impact the outcome of stability and height of the loading stage, it is important to note that the velocity of the ball is not affected through different foot techniques. The lateral rear tilt of the shoulder and pelvis at the end of the loading phase enables the development of angular momentum through the lateral trunk flexion and is an important aspect of all powerful servers [Refer Image 6] (Kovacs &Ellenbecker, 2011).

Image 6: Lateral rear tilt and angles

The fourth phase of the tennis serve, the cocking position (Refer Image 7) relies heavily on efficiently moving through the loading stage (previous stage). Efficiency is produced through the dominant arm when it comes to driving the racquet down while the back of the torso increases the trajectory of the racquet to the ball. The benefit of this position is the fact that it does not need the optimal range of motion, stabilization, positioning throughout the shoulder region, however despite this, the position still allows for greater energy potential (Reid et all., 2008).

Image 7: Cocking and joint angles

During the late preparation stage there are high internal eccentric rotator loads which are being applied (backswing), which later change into stage 5 which is the acceleration phase before the impact with the ball. When the leg drive is used effectively it forces the racquet in a downwardsdirection away from the back of the athlete. This energy is found to help assist in producing racquet velocity throughout the acceleration phase of the tennis serve (Kibler et al., 2007).

Throughout the fourth stage, the cocking position, there is a rise in the vertical ground reaction forces, at the same time there is an increase in muscle activation in the following muscles: vastuslateralis; gastrocnemius; and the vastusmedialis (Chow et al., 2009). The maximum external shoulder rotation is achieved at 0.090 ± 0.014 seconds prior to contacting the tennis ball within professional tennis athletes. At this stage the leg drive is near complete. During the time of the maximum external rotation, the shoulder is abducted at 101º ± 13º, abducted horizontally 7º ± 13º, and rotated externally 172º ± 12º. The elbow at this moment is flexed 104º ± 12º, whilst the wrist is extended at 66º ± 19º (Refer Image 7). The result of this is in a tight and parallel position in between the trunk and the racquet. The motions which are used during this type of degree of external rotation are the trunk extension motion, the scapulothoracic motion and the glenohumeral motion. There is a similarity in the range of a professional athlete who plays baseball as to that of a professional tennis player when it comes to the magnitude of external rotation; in between 178º-175º (Elliot, 2006).

There is a large dependence on the glenohumeral position throughout this stage. In order for the coronal plane and the glenohumeral plane to be anterior to each other, 7º of horizontal adduction from the coronal plane must take place. The infraspinatus/supraspinatus and the glenoid have an increase in the amount of contact pressure between each other with the abducted shoulder which is externally rotated (Knudson, 2007). There is a high risk factor again with injures when it comes to the hyper abduction positioning with the throwing shoulder. When the tossing arm is dropped early as well as an early trunk and hip rotation forward, it is possible that there can be exaggerated horizontal abduction, which can otherwise be known as arm lag - this can place extra loading on the anterior capsular structures and can place the shoulder to posterior impingement.

During the action of maximal external rotation there are angles of 83º and 101º which are quite high and can possibly risk impingement. When there is lower internal and external rotation ratios produced by professional tennis players, it indicates that they have developed selective use of internal rotators relevant to the external rotators (Reid et al., 2008).

Image 8: Muscle groups used

There are two factors which directly affect power production throughout this stage. These are both neuromuscular coordination and strength. 1.68 to 2.12 times a person's body weight is approximately that of what is produced through vertical force production throughout the serve (Kovacs&Ellenbecker, 2011).

It is close the end of the fifth stage of the tennis serve in which peak values are reached for the gastrocnemius, vastusmedialis and the vastuslateralus. The acceleration phase is the phase in which the trunk muscles are getting used the most (Girard et al., 2005). The highest amount of kinetic energy is produced throughout this stage, making it a vital contributor to the power production of the serve.

In order to produce as much acceleration as possible for the tennis serve it is essential that as well as producing power from the trunk muscles, we need to produce acceleration of the racquet before the ball. This is done by a quick lumbar spine rotation reversal for a server which serves with his right hand through hyperextension and right lateral flexion to flexion and left lateral flexion. The corkscrew, as this movement is also known as, allows the transfer of the torque through to the muscle segments (Kovacs&Ellenbecker, 2011). Through constant trunk hyperextensions the serve may cause an athlete stress on the lumbar spine.

At the contact of the tennis ball, the trunk sits at an average tilt of around about 48º; abduction is at 101º; and the leading knee, elbow and wrist are flexed slightly inward in professional tennis athletes. The average shoulder abduction for elite tennis players before contact of the ball is around 100º, which again is quite similar to that of a baseball pitcher who need 100º ± 10º angle in order to produce maximum ball velocity as well as having as little as possible loading on the joints of the shoulders. Optimal contact for a tennis player would therefore be at around 100º ±15º. Elite tennis players at optimal performance are able to produce racquet velocities of around 38 - 47 m/sec [Refer Image 9] (Elliot, 2005).

At the contact of the ball, the ball velocity is produced by flexion of the wrist and internal shoulder rotation. Wrist flexion (15º ±8º), front knee flexion (24º ± 14º), and elbow flexion (20º ± 4º) are very small at contact. Trunk is generally tilted 48º ± 7º above horizontal for professional tennis players (Kovacs&Ellenbecker, 2011).

When the left lateral flexion angle is greater it shows us that more height will be achieved throughout the tennis serve. The left rectus abdomis is also greater in its activity levels at this stage (Kovacs&Ellenbecker, 2011). Tennis players with that of a higher skill are more likely to be subjected to higher asymmetric loads on their lumbar spines as a result of higher lateral flexion.

The muscles which are being used moderately-highly throughout this stage are: the dorsi musculature; deltoid; biceps brachii; serratus anterior; and the posterior rotator cuff. 30% - 35% is the percentage range of the posterior cuff activation because the humerus is slowed down after the point of contact. In order to offset any distraction force and keep glenohumeral congruity [Refer Image 10] (Kovacs&Ellenbecker, 2011).

Image 10: Muscle Groups used

Throughout the whole action of the tennis serve, the left internal oblique is more active than the right internal oblique apart from the deceleration stage (Abrams et al., 2014). The right erector spine becomes very active throughout this part of the serve as it helps to stabilize the trunk when the posture is unbalanced, which is the case in the deceleration stage.

The final stage of the tennis serve finishes on the landing of the lower body (Refer Image 11). This motion generates eccentric forces. The front foot is used as a large horizontal breaking force using the foot up technique, but because the centre of the mass is moved to a forward position, it could possibly hinder players on their serve (Reid et al., 2008).

Image 11: Follow through and landing

In order to increase the amount of torque we need to increase the distance of the muscle to the joint. This is shown through the extension of the arm from the action of the tennis serve. Extending the arm more is valuable as it gives players more acceleration on the head of the racquet and a greater power on the ball (Fleisig et al., 2003). This is due to us extending the length of the lever and the movement arm as mentioned earlier.

The Kinetic chain is a process where the athlete should be required to understand a step by step list of instructions, understanding which mechanisms of their body will be in use throughout the performance of complex tasks required in sports skills; in this case, the tennis serve. It is a routine of movement sequences in which different parts of the body wither activates, mobilize or stabilize in order to generate force, motion production, and protection of muscle tissue damage and increased strain on the athlete's body during activity. If throughout the kinetic chain we are to produce an increased power throughout a certain body part, the object will then have a higher velocity and will have a greater overall kinetic energy (Blazevich, 2013, pp. 102-103).

The main 3 purposes of the kinetic chain are;

Efficiently generating and transferring both force and kinetic energy to distal segment in order to proficiently move an object. This is accomplished by using the principle of 'summation of speed' where both the force and velocity which are produced throughout each different segment are supported and increased by the actions within the proximal segments. A good example of this is the cracking of a whip (Cordo&Nashner, 2008).

The position and the stabilization of the different segments of the body in order to help absorb and control the forces which have been developed at the joint. This is accomplished by making postural movements which are anticipatory which are mixed with the athletic activity pattern in order to maintain a more stable base for activity (Elliot, 2006).

Stabilizing the posture of the body in order to counteract destabilizing effects and any eccentric loads of any athletic movements. This is accomplished by assimilating proximal and distal muscle activation on order to spread the load throughout the whole extremity, controlling eccentric and tension loads and putting joints in a position in which they will be most stable in either the lower or the upper extremity (Zattara et al., 2010) .

Within the serve in tennis there are many different components. The components include; spin and placement and velocity, these all have relations to effectively developing an action throughout the kinetic chain (Nicholls, 2003). The intended result of this is having the optimal placement of the racquet to be at the maximum velocity with the wanted trajectory to be 'up and through' the tennis ball. In order for this to happen, the body of the athlete must first go 'down and back' in to a kinematic position of loading and a kinetic position of cocking, the arms are to next move forward at a rapid pace, and through the ball.

There are two different ways in which you can perform this type of skill: the first is by pushing both the arm and the body through the ball impact; the second contains pulling it through. Both of these types of kinetic patterns have differences which are observable to the naked eye in how the segments are moved and activated. They also differ in the way that they both have different biomechanical results and physiological stresses for the performance of the serve (Putnam 1993).

It is known that within tennis, very rarely do you see two players serve the ball in the exact same way. However the guidelines for every athlete's tennis serve remain the same and this is all based on the principles of the kinetic chain (Refer Image 12).

Image 13: Federer Serving Motion Shots

Roger Federer has one of the most smooth and effortless tennis serves in the world and he has one of the most underrated serves going around. Although Federer does not reach quite the speeds of some of the other professional tennis players, he is much better in many of the other aspects of the serve. Federer's serve is a great model for young people to try and emulate as it is a simpler, more classical and has less extreme elements than other professional tennis athletes (Shiras, 2014). Below is a rundown of the tennis legend's serving technique in accordance to the biomechanical teaching cues above.

The biomechanical principles which are required to achieve the optimum tennis serve can be easily split into 3 phases which are the further split into 8 stages. A confident and well established base of support is crucial to the commencement of the first stage. Throughout the next stage, force summation is used in order to produce the maximum possible momentum using a combination of different body parts. Newton's third law now manifests itself during the ball toss as the player swings for the ball, utilising their trunk in a twist action, bringing the non-dominant arm down. Taking advantage of the knowledge surrounding the kinetic chain, a player will be able to understand how they can create an elastic energy which amplifies the speed of the serve. This speed is also heavily influenced by the proper execution of the cocking stage. This entails the player's elbow flexion to average 115°, wrist extension to average 77° and shoulder abduction to average 113°. The contact point is equally as important. Contact should be made in the 'sweet spot' of the racquet while at the same time the trunk tilt is at an average of 48° and abduction at an average of 101°. The deceleration and follow through stage should be completed accordingly and comfortably. Many injuries occur due to poor technique and heavy landing positions so it is important to generate confidence in these stages. Following the teaching cue stages above which have additionally been connected to Roger Federer's serve, players should produce the optimum tennis serve.

Connections to baseball techniques have been made through the investigation. For example, there is a similarity in the range of a professional athlete who plays baseball as to that of a professional tennis player when it comes to the magnitude of external rotation; in between 178º-175º (Elliot, 2006). Followed by the fact that the average shoulder abduction for elite tennis players before contact of the ball is around 100º, which again is quite similar to that of a baseball pitcher who need 100º ± 10º angle in order to produce maximum ball velocity as well as having as little as possible loading on the joints of the shoulders.

Abrams, G. D., Harris, A. H., Andriacchi, T. P., &Safran, M. R. (2014). Biomechanical analysis of three tennis serve types using a markerlesssystem. British journal of sports medicine, (4), 339-342.

Blazevich, A. J. (2013). Sports biomechanics: the basics: optimising human performance. A&C Black.

Chow JW, Park S, Tillman MD. (2009). Lower trunk kinematics and muscle activity during different types of tennis serves. Sports Med Arthroscopy Rehabilitation Therapy Technol. 1(24).

Cordo PJ, Nashner LM (2008) Properties of postural adjustments associated with rapid arm movements. J Neurophysiol 1982; 47: 287-308.

Kovacs, M., &Ellenbecker, T. (2011). An 8-stage model for evaluating the tennis serve implications for performance enhancement and injury prevention. Sports Health: A Multidisciplinary Approach, 3(6), 504-513.

Martin, C., Bideau, B., Bideau, N., Nicolas, G., Delamarche, P., &Kulpa, R. (2014). Energy Flow Analysis During the Tennis Serve Comparison Between Injured and Noninjured Tennis Players. The American journal of sports medicine, 0363546514547173.

Sandercock, T., & Maas, H. (2009). Force summation between muscles: are muscles independent actuators?. Medicine+ Science in Sports+ Exercise, 41(1), 184.

Shiras, L. (2014). Great Shots: Roger Federer Serve, www.tennis.com, http://www.tennis.com/your-game/2014/01/great-shots-roger-federers-serve

Zattara M, Bouisset S. Posturo (2010) kinetic organization during the early phase of voluntary upper limb movement. J NeurolNeurosurg Psychiatry 1988; 51: 956-965.

Image 1 - 3-Phase, 8-Stage Model of Tennis Serve.

Information used in custom diagram taken from Kovacs &Ellenbecker (2011)

Image taken from Kovacs &Ellenbecker (2011)

Image 3 - Stable base of support

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 4 - Release of Ball

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 5 - Foot Techniques

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 6 - Lateral rear titlts and angles

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 7 - Cocking and joint angles

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 8 - Muscle Groups used

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 10 - Muscle Groups used

Image 11 - Follow through and landing

Custom image and diagram of ATP Junior Player, Mislav Bosnjak

Image 12 - Kinetic Chain of Tennis Serve

Image 13 - Federer Serving Motion Shots

Image taken from Shiras (2014)

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The loading stage entails positioning segments of the body in particular ways which in turn helps produce potential energy. There are two ways to position the athlete's feet: the foot-up technique; and the foot-back technique (Refer Image5).

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Joints and levers in the human body || 3D Animation||Education Biology