Human Skeletal Muscle Fiber Type Switching Revisited

Skeletal muscles are composed of several fiber types that differ in their structure, molecular composition, metabolic activity, and functional properties. The four major myosin heavy chain (MyHC) isoforms, slow-twitch oxidative (type I), fast-twitch oxidative (type IIa), fast-twitch oxidative glycolytic (type IId/x), and fast-twitch glycolytic (type IIb), are hetero-geneously distributed in the skeletal muscles of mammals, and are useful markers for the muscle fiber types [1]. In humans, MyHC IIb is not detectable in skeletal muscles, but MYH4 , the gene encoding this protein, is present in our genome. Muscle performance is, at least partly, dictated by the composition of muscle fiber types, and precise understanding of the fiber type has attracted much attention from sports scientists. In addition, increased interest in the role of skeletal muscle in metabolic diseases has extended the awareness of muscle fiber types to a wide audience in clinical medicine [2].