The adaptive response of skeletal muscle to increased use.

Skeletal muscle undergoes profound changes in morphological, physiological, and biochemical character when subjected to prolonged periods of increased use. Although increased use may be brought about in a variety of ways, the results show consistent features. In particular, endurance exercise and chronic stimulation differ only in degree: the properties which change in response to exercise are also those which change at an early stage of stimulation; the properties which are resistant to change under exercise conditions change only after prolonged stimulation. There is therefore a hierarchy of stability in the properties of skeletal muscle which is revealed in its response to changing functional demands. The adaptive potential of muscle provides a logical framework for understanding neural influences on the emergence of fiber types during muscle development. It is also relevant to the study of pathological conditions which may involve a sustained departure from normal postural and locomotor patterns of activity.