20.6: Classification of Skeletal Muscle Fibers
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 energy source of ATP used to power the muscle.
The first to be recruited are Type 1 slow-twitch oxidative fibers, which contract slowly and are able to 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, Type lla 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, Type IIb 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 powerlifting. 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.