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.