Reevaluation of human muscle fast-twitch subtypes: evidence for a continuum

Abstract

Muscle biopsies were removed from the vastus lateralis muscle of four healthy individuals with a unique fiber type distribution. Fibers were divided into slow-twitch (type I) and fast-twitch (types IIA, IIAB, and IIB) based upon the pH lability of myofibrillar ATPase. An unusually high percentage of types IIB and IIAB allowed a quantitative ultrastructural characterization of the subtype populations, especially with regard to type IIAB fibers. Using a cryostat retrieval method, histochemically-identified fibers were investigated by stereological electron microscopy. The volume percent mitochondria was significantly different for all fiber types. Likewise, lipid volume percent was significantly different between type I and type II fibers and between the fast-twitch subtypes IIA and IIB. However, these ultrastructural data revealed considerable overlap between the fiber types for metabolic parameters, as has been described through quantitative histochemistry by others. The fiber type which stained intermediately between types IIA and IIB at a preincubation pH of 4.6 (type IIAB) was also found to be intermediate in its oxidative components. These data suggest a continuum of fast-twitch fiber types which may tranform in response to the amount and type of usage.