Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
Skeletal muscle contains different fibre types. Slow-twitch (Type I) fibres are fatigue-resistant and aerobic, suited to endurance — marathon runners have a high proportion. Fast-twitch (Type II) fibres produce greater force and power but fatigue faster, suited to sprinting and lifting — sprinters and powerlifters have a high proportion. Type IIa fibres are an intermediate, somewhat trainable type. While the overall fibre ratio is largely genetic, training shifts fibre characteristics and recruits them differently. This partly explains why some athletes naturally excel at endurance and others at power.
- Type I (slow): endurance, fatigue-resistant
- Type II (fast): power, fatigue quickly
- Ratio largely genetic but characteristics trainable
- Explains endurance vs power athlete differences
In practice
Muscle fibre type composition is largely genetic, but training can shift some fibres along the fast-slow spectrum over time — sprinters and powerlifters tend to have a higher proportion of fast-twitch (Type II) fibres suited to explosive, powerful efforts, while marathon runners typically have more slow-twitch (Type I) fibres suited to sustained, fatigue-resistant aerobic work.
FAQ
Can you change your muscle fibre type through training?
Fibre type is largely genetically determined, but training can shift some fibres along the fast-to-slow spectrum (particularly intermediate Type IIa fibres), though complete conversion between fibre types is limited.
Why do sprinters have more fast-twitch fibres than marathoners?
Partly genetic predisposition that draws people toward sports suited to their natural fibre composition, and partly training-induced adaptation that reinforces whichever fibre type is used most.