Muscle fiber types.

This review describes muscle fiber types based on their histochemical, ultrastructural, biochemical, and physiologic properties. Reportedly the speed of muscle fiber contraction is directly proportional to relative myosin ATPase activity (at pH 9.4) while fatigability relates to relative oxidative capacity. Type I fibers have low ATPase activity (at pH 9.4), are slow twitch, have high oxidative and low glycolytic capacity, and are relatively resistant to fatigue. Type IIA fibers have high myosin ATPase activity (pH 9.4), are fast twitch, have high oxidative and glycolytic capacity, and are relatively resistant to fatigue. Type IIB fibers have high myosin ATPase activity (pH 9.4), are fast twitch, have low oxidative and high glycolytic capacity, and fatigue rapidly.