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.
Key points
- Type I (slow): endurance, fatigue-resistant
- Type II (fast): power, fatigue quickly
- Ratio largely genetic but characteristics trainable
- Explains endurance vs power athlete differences