Reversal of recruitment order of single motor units produced by cutaneous stimulation during voluntary muscle contraction in man.

Abstract

THE strength of a voluntary muscle contraction is determined by the number of motor units active and their discharge frequency1. As voluntary effort is increased, increasing numbers of motor units become active in a repeatable sequence2. Although some exceptions have been reported3–8 most of the experimental evidence in animals and man supports the view that this sequence is fixed and invariable9–13. We show here, however, that the order of motor unit recruitment can in fact be changed simply by stimulating cutaneous afferents. Of a sample of 41 motor units (eight experiments, five subjects) all but two units had their recruitment threshold changed by cutaneous stimulation. Examples in which such an effect was sufficient to reverse the normal order of recruitment of two units recorded simultaneously have been found in every subject tested (12 pairs of units, eight experiments, five subjects).