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

Type II fibres are the fast twitch half of your muscle: fibres that contract quickly and produce force rapidly, at the cost of fatiguing sooner than their slow twitch counterparts. They subdivide further, with type IIa sitting between the extremes and type IIx being the fastest and least fatigue resistant.

They matter because they are the fibres that sprinting, jumping and heavy lifting depend on, and because they are also the fibres with the greatest capacity for growth. A muscle that looks noticeably bigger has usually added a disproportionate amount of fast twitch cross-sectional area.

They are recruited last, which is the single most useful thing to know about them. The nervous system calls on motor units in order of size, so the fast twitch units join in only when the effort is heavy or when lighter sets have fatigued the slower units enough to need help. That is why a set has to be genuinely hard to train them, and why proximity to failure matters more than the weight on the bar.

Your mix is largely inherited. Elite sprinters and elite distance runners have measurably different fibre compositions, and no amount of training turns one into the other. Training does shift the properties within a type, particularly between IIx and the more fatigue resistant IIa.

They are also the fibres lost first with age. Sarcopenia preferentially strips fast twitch area, which is part of why older people lose power faster than they lose strength, and why the ability to catch a stumble fades before the ability to carry shopping.

That gives resistance training a specific role in later life: it is one of the few things that recruits and maintains these fibres at all, and power work, moving a moderate load quickly under supervision, targets them more directly than slow grinding does.

In practice, train close to failure, include some heavier and faster work, and keep doing it as you get older.