ABSTRACT
Skeletal muscle is approximately 40 to 50% of total mammalian body weight. Muscle cells or fibers are structurally designed to convert chemical energy into mechanical work and they can undergo a variety of structural adaptations in response to different stresses and stimuli. Skeletal muscle fibers essentially consist of a highly ordered array of proteins, which interact in order to shorten the muscle fiber as necessary. The structure of muscle fibers and their constituent proteins are intimately related to their function. Based on histochemical ATPase staining three main fiber 38groups are readily identified. Type I fibers contract slowly and are resistant to fatigue. They are predominantly responsible for sustained activity and characteristically contain higher levels of intra-mitochondrial oxidative enzymes (succinyl dehydrogenase, NADH-TR). Correspondingly, they have a high lipid and mitochondrial content with a relatively low glycogen content. Type IIb fibers are capable of powerful actions: they contract quickly but have a low resistance to fatigue. They tend to be larger in size than Type I fibers (at least in males) but they have relatively lower lipid and mitochondrial content as well as lower oxidative enzyme levels. They have a higher glycogen content than Type I fibers and are considered glycolytic. Type Ila fibers are essentially a hybrid with both oxidative and glycolytic characteristics. They contain abundant glycogen and mitochondria but less lipid than Type I fibers. They are intermediate between Type I and Type IIb fibers with respect to their speed of contraction and resistance to fatigue.
