IIX

Type IIx fibers, or IIx, are a category of skeletal muscle fibers found in mammals, including humans. They sit between slow-twitch Type I and fast Type IIa in the traditional three-fiber model. IIx fibers are fast-contracting and capable of generating high peak force. They differ from slow fibers by their faster shortening velocity and their reliance on anaerobic metabolism. They have high myosin ATPase activity and high levels of glycolytic enzymes, but relatively low mitochondrial density, capillary supply, and oxidative capacity, making them more fatigue-prone during prolonged activity. Their color is typically pale.

Within humans, IIx fibers occur in many limb muscles but in varying proportions, often comprising a minority of total fibers. The exact distribution is influenced by genetics, training history, age, and muscle. Athletes specializing in short, high-intensity efforts (sprint/power) generally have a greater proportion of IIx than endurance athletes, though the absolute percentages vary among individuals.

Training and age can modulate IIx content. Endurance-type activity tends to decrease the relative amount of IIx as some fibers transition toward the Type IIa phenotype; disuse or aging may be associated with a relative increase in IIx. High-intensity resistance or sprint training can help preserve or enhance fast-twitch capabilities in some individuals.

In practice, IIx fibers enable explosive, high-power contractions over short durations but are less capable of sustained activity, contributing to rapid fatigue under aerobic conditions.