Muscle Fiber Types: Slow-Twitch vs Fast-Twitch

The types and how the mix differs

Human muscle fibers are generally classified by the myosin they contain, from slow to fast contractile speeds: type I (slow-twitch) fibers, and the fast-twitch types IIa and IIx. A 2021 review notes that hybrid fibers that carry more than one type of myosin may make up a significant share, which makes the mix hard to measure¹.

The mix varies widely between people. In untrained people the share of type I fibers in the thigh muscle is typically around 50%, with a range of 5% to 90%, and a 2012 review of the genetics put the inherited component of fiber-type proportion at 40 to 50%, with the rest set by environment². Some runners start closer to the short end and some closer to the long end.

What training changes

Training changes how well the fibers work far more reliably than it changes which type they are. Across training studies in general, not runners alone (5,973 participants in total, from 353 articles on mitochondria and 131 on capillaries), endurance training raised mitochondrial content by 23% (plus or minus 5) and capillaries per fiber by 15% (plus or minus 3), and the proportion of type I fibers remained unaltered by training on average. People with lower initial fitness made larger percentage gains³.

Whether fibers shift between slow and fast types is less settled. A 2021 review says current evidence suggests fibers can shift between hybrid and pure forms and between slow and fast types, and that longitudinal data have not clearly converged on how¹. Do not count on turning a fast-twitch fiber into a slow one, and do not assume it cannot happen.

What it means for training

You do not need to know your mix to train well. Research measures fiber type from a muscle biopsy, and no test you can run at home comes close. Train for the event you are preparing for, and let your own race results and how you respond to a block show where you are strong. The part of the muscle that trains reliably is the aerobic machinery in the fibers you have, which is the work the guides on the aerobic system and training specificity cover. For strength work, see strength training for runners.