Potentiation of "late" responses evoked in muscles during effort.

A study has been made of the effect of voluntary contraction on the `late' responses which can be recorded in muscles after indirect stimulation. Two late responses have been described of which the first was shown to consist mainly of a potentiated H-reflex. The potentiation of this wave was greatest in the abductor pollicis brevis muscle and depended largely on `direct' descending pathways to motoneurones. In addition, a later evoked muscle response appeared during effort; the nature of this wave was uncertain. Simple formulae have been devised to express the excitability of motoneurones to inputs from muscle nerves during effort.