Movements require specific combinations of force in different muscles. This includes finely graded-low force contractions for detailed movements yet high-force when needed. This is achieved by a combination of: different motor unit types and the size principle of motoneuron recruitment

Motor unit types

Motor units are a group of muscle fibres all innervated by a single motorneuron. Different motor units are more suited to different fucntions:

Motor unit	Slow	Fast fatigue resistant	Fast fatiguable
Muscle fibre type	I	IIa	IIb
Twitch speed	Slow	Intermediate	Fast
Power	Strong	Intermediate	Weak
Energy generation	oxidative	oxidative	glycolytic
Colour	Red (much myoglobin)	Intermediate	White (little myoglobin)
Fatiguability	Low	Intermediate	High

Motoneuron recruitment

Muscle twitches fuse into a tetanus at relatively low motoneuron firing rates, so force control really depends on gradual recruitment of motor units. The ‘size principle‘ of motor unit recruitment means low-force motor units are recruited first so increase in power is as gradual as possible initially. This is because smaller diameter motoneurons with a low firing threshold innervate motor units with less fibres. Through developmental plasticity, likely as these motor units are used most often, these fibres to become slow twitch and low force (and vice versa). This also eliminates the need for active brain control of motoneurons.