Which of the following is characteristic of fast-twitch (glycolytic) muscle fibers?

a. a capacity for sustained effort and numerous mitochondria
b. a capacity for sustained effort and few mitochondria
c. a high concentration of myoglobin and numerous mitochondria
d. a capacity for bursts of intense activity and few mitochondria
e. a capacity for bursts of intense activity and abundant myoglobin


D; Fast-twitch fibers are designed for a rapid response to stimulation. They have reserves of creatine phosphate and glycogen available for glycolysis that provide them with a source of rapidly available ATP. But they are not designed for a sustained response (choices a and b), which requires the continued production of ATP by aerobic respiration. Slow-twitch muscle fibers are so designed and have abundant mitochondria as well as myoglobin to promote diffusion of oxygen through the tissue (choices c and e).

Biology & Microbiology

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