Effects of postural change of the shoulder on EMG reaction time of triceps brachii.

Postural dependence of electromyographic reaction time (EMG-RT) of triceps brachii in normal subjects was examined at neutral (N) and facilitating (F) positions. EMG-RT was faster at F than at N; moreover, the slower one's EMG-RT, the larger the difference of EMG-RTs. F position of arms causes the shifting of EMG-RT through two biasing systems. The action of one system arises from either arm and acts on both arms, as the facilitation and its intensity depend on one's EMG-RT. The other system arises from one arm, and facilitates the ipsilateral arm but inhibits the contralateral arm; its intensity is independent of one's EMG-RT. The amount of the shift is determined by the added contribution of those two actions.