Anticipatory postural changes induced by active unloading and comparison with passive unloading in man.

Normal human subjects, sitting in a chair, were required to maintain stable elbow flexion against loads of 0.5 kg or 1.0 kg. Unloading was affected either 'passively' by the experimenter, or 'actively' with the subject's own contralateral arm. Elbow angle, force exerted by the load, and electromyographic activity (EMG) of biceps and triceps muscles of both arms were recorded and averaged. 'Passive' unloading was followed by a reduction of biceps EMG activity, starting 50--80 ms after weight lift, and by an upward deflection of the forearm. With 'active' unloading, however, a reduction of the biceps EMG activity slightly preceded the onset of unloading (0--30 ms). This reduction of the 'actively' unloading arm occurred at about the same time as the activity of the contralateral unloading arm. In this experiment, the unloaded forearm maintained an almost stable position. Thus, the anticipatory adjustment of elbow posture, observed when unloading was performed by the subject, appears to optimize limb stability during the mechanical perturbation.