Motor unit

A motor unit is the smallest functional element of voluntary movement: one motor neuron plus all the fibres it innervates. Units vary enormously in size — those controlling the eye may drive a handful of fibres, while a large leg muscle unit may drive over a thousand.

It matters because of the size principle: the nervous system recruits small, fatigue-resistant units first and larger, more powerful ones only as demand rises. This is why intent and effort matter in training — a light load lifted casually never recruits the high-threshold units at all, while the same load taken near failure eventually does.

In practice, much of a beginner's rapid early strength gain is improved motor-unit recruitment and coordination rather than new muscle. That is also why strength can be regained quickly after a layoff, before size returns.