The shortening reaction of forearm muscles: the influence of central set

Objective: The EMG of the forearm muscles shortened by an imposed wrist joi nt displacement has been studied at different levels and distribution of ba ckground muscle activity and with different instructions to the subjects, i n order to test the hypothesis that the recorded EMG response (shortening r eaction, ShoRe) could be deliberate in origin. Methods: Ten normal subjects were examined. A torque motor induced 50 degre es wrist extension or flexion at 500 degrees /s. The subjects were relaxed or exerted a 10% maximal voluntary contraction. They were instructed either not to intervene, or to oppose the displacement, or else to assist it. Sev eral trials were repeated at different initial angles. Results: We found st short-latency reflex (SR) in the stretched muscle, be it flexor or extensor, and a later inconstant ShoRe in the antagonist. ShoR e latency was compatible with that of a reaction time (RT), and was not inf luenced by the initial wrist angle. When subjects assisted the movement, th e EMG burst in the shortening muscle was in every respect a RT; when they o pposed the movement, the ShoRe disappeared. There was a strict temporal rel ationship between SR duration and ShoRe latency. Conclusions: We suggest that the brain would deliberately trigger the ShoRe on recognizing the displacement direction. The occurrence of such activity in the shortened muscle makes the SR to abruptly stop. The temporal relati onship between the duration of the SR and onset of the ShoRe can be an expr ession of the inhibition on the SR burst by the cortical drive to the antag onist muscle being shortened, possibly through the action of spinal inhibit ory interneurones. The ShoRe would complete the movement momentarily braked by the SR and redistribute the muscle tone across antagonists, appropriate for the new muscle length. (C) 2001 Elsevier Science ireland Ltd. All righ ts reserved.