Type II muscle fibreFast-twitch fibre · Type II muscle fiber

Type II fibres contract fast and produce high force, drawing more heavily on anaerobic energy pathways. They divide into type IIa, which retain reasonable fatigue resistance, and type IIx, the fastest and quickest to tire.

It matters because these are the fibres that dominate sprinting, jumping and maximal lifting, and they have the greater growth potential — a significant part of why heavy training builds visible size. They are also the fibres lost preferentially with age, which is why power declines faster than endurance in older adults.

In practice they are recruited last, under either heavy load or high fatigue, in line with the size principle of motor-unit recruitment. That is why both heavy sets and lighter sets taken close to failure can grow them, and why easy submaximal work largely does not.