Voluntary inhibition of the myotatic reflex and premotor response to joint angle displacement

EMGs from the deltoid muscle of the nonpreferred arm were obtained from 18 subjects (9 males, 9 females). The EMGs resulted from a stretch reflex (reflex latency) from the fall of the relaxed arm from an electromagnet and from the resultant EMGs of a voluntary response to stop the preceived falling arm (premotor response). Ten normal trials were followed by 10 inhibition trials. By conscious relaxation of the arm and inhibition of the deltoid muscle, all subjects on most trials were able to completely eliminate the voluntary premotor responses. Elimination of the myotatic reflex proved much more difficult with two subjects completely inhibiting it on all trials and ten subjects on at least one trial; however when the reflex was not inhibited, the inhibited condition resulted in significantly reduced reflex duration and an increased reflex latency. No sex difference was found in the ability to inhibit the EMG responses.