Muscle mutability. Part 1. General concepts and adaptations to altered patterns of use.

During the past two decades investigators have shown that muscle fiber is highly mutable. A variety of stimuli ranging from patterns of use to the nutritional state of the organism may lead to alterations in fiber structure and function. The purpose of this review is to provide the information necessary to understand muscle fiber mutability (ie, terms and muscle fiber classification schemes) and to critically review the literature about mutability in response to altered patterns of use. The relevance of these studies to physical therapy is discussed, and suggestions for clinical applications are made.