ABSTRACT
Myofiber type is defined by a combination of metabolic, contractile, and morphological characteristics. There are many possible combinations of characteristics but fiber type is most simply described as red (slow, oxidative, type I), intermediate (fast, oxidative and glycolytic, type IIa), or white (fast, glycolytic, type IIb). Most muscle proteins have fiber-type-specific isoforms. Fiber-type characteristics are developmentally determined but may be modulated by subsequent neural, endocrine, and mechanical influences. Myosin heavy chain (MHC), an abundant fiber-type marker, undergoes a developmental transition from embryonic to neonatal to adult isoforms. Expression of different proteins with fiber-type-specific isoforms is weakly coordinated in transitional fibers. In the embryo, slow primary fibers are larger than secondary fibers, but fast fibers become larger after birth.
