TypIIx

Type IIx fibers are a class of fast-twitch skeletal muscle fibers found in mammals, including humans. They contract rapidly and generate high force but fatigue quickly. In humans, IIx is a fast-twitch subtype alongside IIa; IIx fibers typically have greater glycolytic capacity and lower oxidative capacity than IIa fibers.

Biochemically, Type IIx fibers show high myosin ATPase activity and abundant enzymes for glycolysis, enabling rapid ATP production without oxygen. They have relatively low mitochondrial density and low myoglobin, limiting their endurance under aerobic conditions.

Functionally, IIx fibers are recruited for short, high-intensity actions such as sprints, jumps, and heavy lifts. They produce high peak power and fast shortening velocity but accumulate lactate quickly, contributing to fatigue.

Plasticity and aging, training effects: The proportion of IIx fibers can change with training and age. Endurance-type training can shift some IIx fibers toward a more oxidative Type IIa phenotype, while high-intensity resistance training can help preserve or augment IIx content in certain muscles. Aging generally reduces the number of fast-twitch fibers and may alter their distribution, often decreasing IIx proportion.

Distribution and variability: The exact share of Type IIx fibers varies widely among individuals and across muscles, influenced by genetics, activity history, and athletic specialization. In non-human species, IIx fibers may be classified differently or correspond to what some literature labels IIb.