20.6: Classification of Skeletal Muscle Fibers

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.

Skeletal muscles consist of three main fiber types, one slow- and two fast-twitch, each distinct and adapted to a particular function based on their speed and duration of contraction, as well as the source of ATP used to power the muscle.

The first type, slow-twitch oxidative fibers, contract slowly and can sustain repeated muscle contraction over long periods of time for endurance activities, such as marathon running.

They are typically referred to as red fibers, since they are enriched by many capillaries and abundant myoglobin, a pigment that stores and diffuses oxygen reserves from the blood into cells as a form of aerobic energy.

The second type, fast-twitch oxidative fibers, have a rapid speed of contraction and use mostly aerobic energy sources, necessary for sprinting, which requires power and speed. Structurally, they are very similar to their slow counterpart.

The third group, fast-twitch glycolytic fibers, contract rapidly and with greater force, but fatigue quickly due to their short-lived glycogen reserves for fuel, making them suitable for intense bursts, like power-lifting.

These large white fibers have less blood supply and myoglobin content, and are not dependent on oxygen and nutrients diffusing from the blood.

Overall, the proportional distribution of muscle fiber types is determined largely by genetics, but can also be regulated by increasing hormone levels within the blood and altering the kind of training placed upon muscles.