The triceps muscle in the back of the upper arm is an extensor. When it contracts, the elbow straightens and the forearm and hand move away from the shoulder. At the same time, the biceps relaxes so the triceps can pull it back to its original length. How skeletal muscles move a limb Skeletal muscles contract and pull on the bones they attach to when a nerve stimulates them. For example, involuntary movement occurs when a person jerks his or her hand away from a hot object before thinking about doing it.
Skeletal muscles adapt to exercise in special ways, depending on how they are required to work.
For example, muscles grow larger and stronger if a person lifts heavy weights for a short period of time each day. Such exercise causes muscle nuclei to increase production of thick and thin filaments in each exercised muscle fiber. In addition, bone and tendon grow stronger. In this case, muscle fibers increase their ability to produce energy needed to keep up such muscular work. Smooth muscles are found in various organs of the body.