Fast-twitch white muscle fiber contraction

Although the human has no muscles that consist entirely of this fiber type, many animals do. Examples are white abdominal muscles of fish and the pectoral muscle of game birds (turkey white meat). These muscles contract rapidly and vigorously (the fast twitch refers to the time to peak tension), but only for short periods. Thus, they are used for activities such as flight in birds and sprinting and weight-lifting in humans. 

EXAMPLE 13.21 Type II muscle fibers are subdivided into type Ila and type Ilb. Type Ila can use both aerobic and anaerobic metabolism to produce ATP whereas type lib fibers use only anaerobic metabolism. In contrast to type I muscle fibers, type lib fibers contract rapidly after stimulation by a nerve impulse. They have evolved for short-lived, powerful contractions by the possession of characteristically active myosin ATPases and a dense packing of contractile filaments. So much of the cytoplasmic space is taken up with filaments that little exists for mitochondria. Similarly, these fibers are associated with a relatively poor blood supply. Type lib fibers are also known as white and fast-twitch muscle fibers and are adapted for short-lived but powerful contractions. The relative paucity of mitochondria and the poor blood supply impose obvious constraints on the generation of ATP during exercise. 

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