There are three types of muscle: smooth, cardiac and skeletal. Smooth muscle lines the organs and artery walls. Cardiac muscle is the heart muscle. Skeletal muscle is the muscle that allows us to move our bodies. Each skeletal muscle is attached to the bones of the skeleton, and when the fibers of a muscle contract (or shorten), they move the bones. This process is accomplished by thousands of tiny cells grouped together that create the muscle itself.
Muscle Function
Skeletal muscle is made up of tendons, which attach the muscle to the bones, and the muscle belly, which houses the mechanisms for contracting the muscle to create movement. The tendons are connective tissue that is formed at the end of the muscle fibers and fused into the bone. When the muscle contracts, it pulls the tendons together, which in turn pull the bones toward each other, which creates the movement. For example, when the biceps muscle of the arm contracts, the tendons attached to the forearm shorten and pull the forearm toward the upper arm. This is how you bend your arm.
Muscle Belly
The muscle belly houses thousands of long cylindrical fibers that run the length of the muscle. These fibers are held together in groups of up to 150 by another layer of connective tissue, called perimysium. These groups or bundles of muscle fibers are then held together by a final layer of connective tissue called the epimysium. This gives the muscle its shape and size.
Muscle Fibers
Each individual muscle fiber contains thousands of small units called sarcomeres. Each sarcomere contains alternating rows of two proteins called actin and myosin. These two proteins actually do the work that makes the muscle contract. The brain sends a signal to move. The myosin reaches out and attaches to and pulls on the actin. This is what makes the muscle contract. When the actin and myosin release, the muscle relaxes back to its original length.
Types of Fibers
There are two types of muscle fibers: fast-twitch and slow-twitch. Fast-twitch fibers are capable of contracting three to five times faster than slow-twitch fibers. Fast-twitch fibers predominate in the calf muscles to help with the push-off and stop-and-go action in sports and running. Slow-twitch fibers contract slowly and allow for longer endurance. These are often found in large numbers in the thigh muscles to help with long runs and prolonged walking or sitting.
Muscle Size
Muscles grow and shrink depending upon their use. If you have ever had an injury to an arm or leg, you might have noticed it seemed smaller after you healed. This is due to the reduction of signals to contract the muscle, which leads to thinning of the muscle fibers. Of course you know that if you use muscles or train them with weights, they will get bigger. Muscle fibers are fragile and will break with overexertion. But afterward, they heal and become thicker and stronger than they were previously to prevent future breakage. This is what causes muscles to become bigger.
References
- "Exercise Physiology: Energy, Nutrition, and Human Performance, 5th Edition;" W.D. McArdle et al.; 2001
- Muscle Structure and Exercise Physiology
