Transitions of muscle fiber phenotypic profiles

Skeletal muscle is a complex, versatile tissue composed of a large variety of functionally diverse fiber types. The overall properties of a muscle lar gely result from a combination of the individual properties of its differen t fiber types and their proportions. Skeletal muscle fiber types, which can be delineated according to various parameters, for example, myofibrillar p rotein isoforms, metabolic enzyme profiles, and structural and contractile properties, are not fixed units but are capable of responding to altered fu nctional demands and a variety of signals by changing their phenotypic prof iles. This brief review summarizes our current understanding of the delinea tion of fiber types, modulations of their phenotypic profiles as induced un der various conditions, and potential mechanisms involved in these transiti ons.