Fast twitch fibers

Fast-twitch (FT or Type II) fibers are identified by a quick con- traction time and a low resistance to fatigue. The differences in the speeds of contraction that gives the fibers their names can be explained, in part, by the rates of release of calcium by the sarcoplasmic reticulum (the muscle's storage site for calcium) and by the activity of the enzyme (myosin-ATPase) that breaks down ATP inside the myosin head of the contractile proteins. Both of these characteristics are faster and greater in the FT fibers (Fitts & Widrick, 1996; Harigaya & Schwartz, 1969).

Fast-twitch fibers are further divided into fast-twitch A (FT -A or Type IIA) and fast- twitch B (FT -B or Type lIB) fibers. FT -A fibers have a moderate resistance to fatigue and represent a transition between the two extremes of the ST and FT -B fibers. Structurally, FT -A fibers have a large motor neuron and fiber diameter, a high mitochondrial density, a medium capillary density, and a medium myoglobin content. They are high in creatine phosphate and glycogen and medium in triglyceride stores. They have both a high glycolytic and oxidative enzyme activity. Functionally, they are used for prolonged anaerobic activities with a relatively high force output, such as racing 400 meters.

Fast-twitch B fibers, on the other hand, are very sensitive to fatigue and are used for short anaerobic, high force production activities, such as sprinting, hurdling, jumping, and putting the shot. These fibers are also capable of producing more power than ST fibers. Like the FT -A fibers, FT -B fibers have a large motor neuron and fiber diameter, but a low mitochondrial and capillary density and myoglobin content. They also are high in creatine phosphate and glycogen, but low in triglycerides. They contain many glycolytic enzymes but few oxidative enzymes.