Responses to force perturbations preceding voluntary human arm movements.

Brief force perturbations were applied 30-120 ms prior to onset of step-tracking forearm movements by normal humans. The perturbations altered the first agonist burst of the movement-related triphasic EMG pattern. Perturbations opposing the movement resulted in an increase in the magnitude of the late part of the first agonist burst, the early part being unchanged. Conversely, in movements which would be assisted by the perturbation, EMG magnitude decreased during the late part of the burst. No reflex EMG responses were elicited during the period following the perturbation and preceding onset of the first agonist burst.