Fasttwitch

Fasttwitch refers to muscle fibers optimized for rapid, high-force contractions. In humans, fast-twitch fibers are primarily Type II, subdivided into Type IIa (fast oxidative-glycolytic) and Type IIx (fast glycolytic). Type IIb is present in some animals but not typically in healthy adult humans.

These fibers contract quickly and produce greater force per motor unit than slow-twitch (Type I) fibers, but they fatigue more rapidly due to a greater reliance on anaerobic metabolism, lower mitochondrial density, and reduced capillary supply. Type IIa fibers can use both aerobic and anaerobic pathways, providing a balance between speed and endurance, while Type IIx fibers rely more on glycolysis for rapid energy production and power.

Fast-twitch fibers are recruited for high-intensity, short-duration activities such as sprinting, jumping, and heavy lifting. Their distribution varies among individuals due to genetics, training history, and aging. Training influences their properties: resistance or strength training increases fiber size and force output, while endurance training can enhance oxidative capacity and, in some cases, shift IIx toward IIa. Aging and disuse tend to reduce the size and proportion of fast-twitch fibers more than slow-twitch fibers.

Measurement of fiber type is typically done via muscle biopsy with histochemical staining for myosin ATPase or via immunohistochemistry, though performance outcomes also reflect neural activation and overall training status. Understanding fasttwitch fibers helps explain differences in athletic performance and how training can modify strength, speed, and fatigue resistance.