Muscle Fiber Types

Type I fibers are adept at utilizing the aerobic oxidative pathway for long periods of time. As shown by electron microscopy, they contain large numbers of mitochondria, which confer a red color to the muscle itself. They react darkly for oxidative enzymes, such as reduced nicotinamide adenosine dinucleotide (NADH) dehydrogenase, succinate dehydrogenase, and cytochrome c oxidase. They have low glycolytic activity, and react weakly for the glycolytic enzyme myophosphorylase. Accordingly, type I fibers have high lipid but low glycogen content. The myofibrillar adenosine triphosphatase (ATPase) in type I fibers is acid stable and alkaline labile; thus they react darkly for ATPase activity after acid preincubation but do not react for ATPase activity after alkaline preincubation. These are slow-twitch fibers, generating low maximum tension. They are, however, fatigue resistant and can maintain activation for longer periods of time. Due to this quality, they are referred to as slow-twitch, fatigue-resistant fibers. They are relatively small compared with type II fibers. Type I fibers activate before type II fibers at low levels of muscle contraction, and their motor unit potentials are those that electromyography assesses.

Type IIa fibers are fast-twitch fibers with both glycolytic and oxidative metabolic activity. They also contain a large number of mitochondria and react somewhat darkly for oxidative enzymes. Unlike Type I fibers, however, they have high glycogen content and stain darkly for glycolytic enzymes. Type IIa myofibrillar ATPase is acid labile and alkaline stable; thus these fibers stain darkly for ATPase activity after alkaline preincubation but not after acid preincubation. As fast-twitch fibers, they are able to generate an intermediate level of maximum tension quickly, but they also fatigue slowly. They are therefore referred to as fast-twitch, fatigue-resistant fibers. They are larger than type I fibers and activate later as sustained contractions are required.

Type IIb fibers lie on the opposite end of the spectrum from type I fibers. They harbor few mitochondria and have a white appearance. As a result, they react faintly for oxidative enzymes. They have high glycolytic activity, however, and react darkly for glycolytic enzymes. They have high glycogen and low lipid content. Their myofibrillar ATPase is acid labile and alkaline stable, similar to type IIa fibers, but they show intermediate reactivity after preincubation at more moderate acidity levels (pH 4.6), unlike type IIa fibers, which have no reactivity under similar conditions. These are fast-twitch fibers but fatigue easily. They are accordingly referred to as fast-twitch, fatigable fibers. They reach high maximal force very quickly but only for short periods of time.

Muscle fiber–type predominance is reflective of certain types of activity typical for a particular muscle. Short bursts of anaerobic activity (such as sprinting) are especially suited to type IIb fast-twitch, fatigable fibers. Longer, aerobic exercise, however, is more suited to type IIa and type I fatigue-resistant fibers. There is evidence to suggest that consistent activity mimicking motor nerve activity of neurons that innervate fatigue-resistant fibers can slow contraction and increase fatigue resistance, suggesting that muscle fiber types can change in response to activity type.

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