Postural forearm changes induced by predictable in time or voluntary triggered unloading in man.

Human subjects sitting in a chair were asked to maintain their right forearm in a horizontal position in half supination. The forearm was loaded with a constant weight of one kilogram. Vertical force at the wrist level, angular position of the elbow and EMG activity of biceps, brachio-radialis and triceps muscles were recorded. Unloading was tested under four different conditions, the first two having been used in a previous study (Hugon et al. 1982): Voluntary unloading by the subject's other hand. An "anticipatory" deactivation of the load bearing forearm flexors is observed preventing the elbow rotation of that arm. Unpredictable passive unloading. This results in an upward forearm rotation which provokes the classical "unloading reflex". Two new conditions were tested in the present paradigm: Imposed unloading predictable in time (tone signal preceding unloading by a fixed interval). Unloading being actively triggered when the subject presses a key. Under the two latter conditions, no anticipatory deactivation of the flexor supporting muscles preceding the onset of unloading as in situation A was observed. During the first 120 ms after the onset of unloading, the forearm rotation was the same as in situation B (unpredictable passive unloading). Thereafter, the rotation was smaller in some subjects, apparently due to an ameliorated reflex action. It is concluded that temporal information concerning the precise time of the unloading or the triggering of the load release by a voluntary movement (key press) was not by itself able to induce the anticipatory deactivation of the forearm flexors that was seen with a coordinated voluntary release of the load by the contralateral arm.