Skeletal muscles continuously produce ATP to provide the energy that enables muscle contractions. Skeletal muscle fibers can be categorized into three types based on differences in their contraction speed and how they produce ATP, as well as physical differences related to these factors. Most human muscles contain all three muscle fiber types, albeit in varying proportions.

Slow-Twitch Muscle Fibers

Slow oxidative, muscle fibers appear red due to large numbers of capillaries and high levels of myoglobin, an oxygen-storing protein. These fibers contain more mitochondria, which produce ATP through oxidative phosphorylation, than fast-twitch muscle fibers.

Slow oxidative muscle fibers use aerobic respiration, involving oxygen and glucose, to produce ATP. In addition to contracting more slowly than fast-twitch fibers, the slow oxidative fibers receive nerve signals more slowly, contract for longer periods, and are more resistant to fatigue. These fibers primarily store energy as fatty substances called triglycerides.

Fast-Twitch Muscle Fibers

Relative to the slow oxidative fibers, the fast-twitch muscle fibers receive nerve signals and contract more quickly, but contract for shorter periods and fatigue more quickly. Fast-twitch muscle fibers primarily store energy as ATP and creatine phosphate.

Fast oxidative, muscle fibers primarily use aerobic respiration to produce ATP. However, they also use anaerobic respiration. Fast glycolytic, muscle fibers primarily use anaerobic respiration, which produces less ATP per cycle than aerobic respiration. Thus, Fast glycolytic fibers tire faster than the other fiber types.

More recently, scientists identified additional muscle fibers with characteristics intermediate between the above three types.