Fibertype

A fibertype, or muscle fiber type, is a classification used to describe skeletal muscle fibers according to their contractile speed, metabolic pathways, and fatigue resistance. In humans, the dominant classification is typically into slow-twitch and fast-twitch fibers, with further subdivisions. Fiber type composition varies between muscles and individuals and is influenced by genetics and training history.

Type I fibers are slow-twitch, oxidative, rich in mitochondria and myoglobin; they contract slowly and resist fatigue, supporting endurance activities. Type II fibers are fast-twitch and are subdivided: IIa are fast-twitch oxidative-glycolytic, combining relatively high fatigue resistance with rapid force production; IIx (and in some species IIb) are fast-twitch glycolytic, capable of very fast contractions but prone to rapid fatigue.

Plasticity: fiber-type composition can adapt to activity: endurance training tends to increase the proportion and oxidative capacity of Type I and IIa fibers, while strength or sprint training can enhance the size and performance of fast-twitch fibers; a shift from IIx to IIa is commonly observed with training. Aging can reduce Type II fibers and overall size; genetic factors set baseline proportions.

Assessment: fiber types are typically identified by histochemical staining for myosin ATPase activity at specific pH levels or by immunohistochemistry using fiber-type–specific myosin heavy chain antibodies; single-fiber analyses and molecular methods can be used for detailed profiling.

Clinical and performance relevance: understanding fiber type composition informs athletic training, rehabilitation, and studies of metabolic health, muscle disease, and aging. The concept of fiber type remains a core element of skeletal muscle physiology.