Skeletal muscles are very responsive to use and disuse. Those that are forcefully exercised tend to enlarge. This phenomenon is called muscular hypertrophy. Conversely, a muscle that is not used undergoes atrophy--that is, it decreased in size and strength.

The way a muscle responds to use also depends on the type of exercise. For instance, when a muscle contracts relatively weakly, as during swimming and running, its slow, fatigue-resistant red fibers are most likely to be activated. As a result, these fibers develop more mitochondria, and more extensive capillary network. such changes increase the fibers' abilities to resist fatigue during prolonged exercise, although their sizes and strengths may remain unchanged.

Forceful exercise, such as weightlifting, in which a muscle exerts more than 75% of its maximum tension, uses the muscle's fast, fatigable white fibers. In response, existing muscle fibers develop new filaments of actin and myosin, and as their diameters increase, the entire muscle enlarges. However, no new muscle fibers are produced during hypertrophy.

Since the strength of a contraction is directly related to the diameter of the muscle fibers, an enlarged muscle can produce stronger contractions than before. However, such a change does not increase the muscle's ability to resist fatigue during activities such as running or swimming.

If regular exercise stops, capillary networks shrink and the number of mitochondria within the muscle fibers fall. Actin and myosin filaments diminish, and the entire muscle atrophies. Injured limbs immobilized in casts or accidents or diseases that interfere with motor nerve impulses commonly cause muscle atrophy. A muscle that cannot be exercised may decrease to less than one-half its usual size within a few months.

Muscle fibers whose motor neurons are severed not only decrease in size, but also may fragment and, in time, be replaced by fat or fibrous connective tissue. However, if such a muscle is reinnervated within the first few months following an injury, function may be restored.