# data[].attributes.headline
How do muscles contract and how does hypertrophy occur?

# data[].attributes.subject
How do muscles contract and how does hypertrophy occur?

# data[].attributes.body
According to BCcampus’s online anatomy textbook, skeletal muscle comprises all the muscles attached to tendons and bones whose role is to support and stabilize the skeleton and contract to create movement.

Each individual muscle is a discrete organ, covered and permeated by the Epymesium, a sheath made of connective tissue which prevents the muscle from rubbing against bones or other muscles.

The next subdivision inside the muscle is the Fascicle, a bundle of muscle fibers covered by another layer of connective tissue, the Perimysium. Fascicles can act independently of each other and allow more specific movements in the muscles.

Muscle fibers are the next division. Each fiber is basically a single long cell, about a tenth of a millimeter wide and up to 30 centimeters long. Within each muscle fiber there are several hundred long chains of proteins called Myofibrils. Myofibrils are responsible for contraction and tension in the muscles.

# data[].attributes.body
The source of the power produced by muscle is the Myofibril. According to Lumenlearnings’s article on skeletal muscle, myofibrils are composed of small cylindrical units called sarcomeres.

Each sarcomere is made up of the proteins actin and myosin. These two proteins are arranged in discrete groups which slide past each other to produce contraction.

The mechanism used to pull the myosin and actin filaments closer together is called Crossbridge cycling, where ATP is consumed to slide the proteins along each other.

Meanwhile the sarcomere is held relatively rigid by chemicals like actin, which make up part of its structure.

# data[].attributes.body
All skeletal muscles are always under some tension. According to a Lumenlearning article on the control of muscle tension, the continuous stretching imposed on muscles by the body helps optimize the amount of force they can produce when contracted.

There are two velocities at which contraction can occur- twitch, which are rapid and allow the muscle to begin to relax before it reaches peak force, and tetanic, where twitch movements follow each other rapidly and plateau at peak force.

Muscle tone exists because all muscles are passively contracted at all times, allowing the body to maintain posture.

On a large scale there are two kinds of contraction- isotonic, where muscle is continuously contracted and changes in length (this includes concentric and eccentric movements) and isometric, were force is generated without changing the length of the muscle.

# data[].attributes.body
Hypertrophy is an increase in the mass of an organ due to the growth in size of its individual cells. Muscle growth, or hypertrophy, is accomplished by increasing the size of individual muscle fibers/cells.

According to a University of New Mexico article on hypertrophy, muscle fibers are covered by and permeated by “myosatellite cells”, a kind of adult stem cell which repair muscle fibers after damage or trauma.

This process doesn’t increase the number of muscle fibers in a muscle, instead increasing the size of the contractile proteins inside its Myofibrils.

# data[].attributes.body
Boundless. “Boundless Anatomy and Physiology.” Lumen, Open SUNY Textbooks, courses.lumenlearning.com/boundless-ap/chapter/control-of-muscle-tension/.

OpenStax. Anatomy and Physiology, OpenStax, 6 Mar. 2013, opentextbc.ca/anatomyandphysiology/chapter/10-2-skeletal-muscle/.

“Skeletal Muscle Hypertrophy.” The Science of Breathing, www.unm.edu/~lkravitz/Article folder/hypertrophy.html.

Zatsiorsky, Vladimir M., and William J. Kraemer. Science and Practice of Strength Training. Human Kinetics, 2006.

Images-
Pearson Prentice Hall