Motoneurone populations and the gradation of muscle force

Muscle force may be graded by two mechanisms (often used in parallel): (a) by activating a larger or smaller number of units (recruitment gradation), (b) by changing the spike-frequency for already recruited units (rate gradation). Recruitment gradation typically takes place such that, within a given muscle, weak and slow units are more easily activated than stronger and faster ones (size principle). However, task-related modifications of this pattern are also known. The gain of recruitment gradation will be influenced by changes of the intra-pool distribution of synaptic ‘driving’ currents, and the gain of rate gradation might be altered by synaptic modification of neuronal discharge characteristics. Furthermore, the firing characteristics of motoneurone pools will also be influenced by the presence of the recurrent Renshaw inhibition. The force of a maximum voluntary contraction (MVC) may come close to, and for some muscles incidentally attain, the maximum evocable force.