Motor unit

A motor unit is one motor neuron plus every muscle fibre it controls. It is the smallest thing your nervous system can switch on: you cannot contract half a motor unit, so all its fibres fire together or none of them do.

Their size varies enormously with the job. A muscle that moves the eye may have a neuron controlling a handful of fibres, giving exquisitely fine control; a large muscle in the thigh may have neurons controlling a thousand or more, trading precision for force.

They are recruited in order, smallest first. This is the size principle, and it is the single most useful piece of neuromuscular physiology a lifter can know. Light effort calls on small, fatigue resistant units. As force demand rises, or as the small units tire during a set, progressively larger and faster units join in.

That principle is why proximity to failure matters more than load. A set of 20 taken to the edge eventually recruits the same large units as a heavy set of 5, which is why light and heavy training produce similar muscle growth when both are taken close to failure, and why a comfortable set of 20 trains very little.

It also explains why strength arrives before size. Early gains come substantially from the nervous system learning to recruit more units, to fire them faster, and to coordinate them better, which is real strength with no new tissue involved.

Ageing thins them out. Motor neurons are lost with age and the fibres they controlled are either orphaned or adopted by surviving units, which coarsens control and costs power. Resistance training slows the loss and is one of the few interventions that does.

In practice: take sets close enough to failure to matter, and remember that the last few uncomfortable reps are the ones doing the recruiting.