Does reflex inhibition of motor units follow the "size principle"?

The "size principle" is known to dictate the sequence of recruitment of motor neurons during voluntary or reflex activation of muscles. The present study sought to determine whether the size principle also determined the sequence of reflexly-elicited inhibition of motor neurones. It was found that all masseter motor units were about equally inhibited by a mildly-noxious stimulus to the lip, providing that they were tested at the same level of pre-stimulus excitability, i.e., firing frequency. At higher prestimulus firing frequencies, more intense stimuli were required to inhibit motor unit activity. Thus it is the firing frequency of individual motor neurons, rather than their size, which determines their susceptibility to inhibition.