THE ROLE OF MOTOR ACTION IN ANTICIPATORY POSTURAL ADJUSTMENTS STUDIEDWITH SELF-INDUCED AND EXTERNALLY TRIGGERED PERTURBATIONS

This study investigated the relation between the magnitude of a motor action triggering a postural perturbation and the magnitude of anticip atory postural adjustments. Subjects stood on a force platform and hel d, in extended arms, a balloon with a 2.2-kg load suspended on a rigid cord. In different series, unloadings were induced by fast bilateral shoulder abduction movements, by popping the balloon with a tack taped to the subject's right middle finger, or by the experimenter popping the balloon. Anticipatory postural adjustments were seen during all se lf-initiated unloadings as changes in the level of activation of postu ral muscles and in displacements of the center of pressure. However, a bsolute values of these changes were significantly smaller in the seri es with balloon popping as compared to the series with shoulder abduct ions. Such reactions were absent when the unloading was triggered by t he experimenter. We conclude that a self-triggered perturbation is alw ays associated with anticipatory postural adjustments, while the magni tude of the adjustments may be scaled with respect to the magnitude of a motor action used to induce the perturbation.