Recruitment of more units and increased firing rate, following the size principle
Expansion
A motor unit is one motor neurone and all the fibres it supplies. The innervation ratio determines precision: about 1:10 in extraocular muscles and 1:2000 in gastrocnemius.
Force is graded two ways:
- Recruitment: activating more motor units, following the size principle. Small motor neurones supplying type I fibres have a lower threshold and fire first, with large neurones supplying type II fibres added as force increases
- Rate coding: increasing firing frequency, so twitches summate and eventually fuse into tetanus
Unlike cardiac muscle, skeletal muscle can be tetanised, because its refractory period is short relative to the duration of contraction.
At low forces, asynchronous firing of different units produces smooth movement rather than tremor.
Two clinical applications: the size principle explains why type II fibres atrophy first in disuse and ageing, since they are recruited least; and electrical stimulation reverses the normal order, recruiting large fibres first, which is why it fatigues muscle quickly in rehabilitation settings.