Functional postural responses after perturbations in multiple directions in a standing man: a principle of decoupled control

The objective of this study was to assess Functional postural responses by analyzing the net joint torques (NJT) in the ankles and the hips resulting from perturbations delivered in multiple directions to subjects standing qu ietly. A total of eight subjects were standing on two force platforms while an apparatus randomly delivered controlled perturbations at the level of t he pelvis in eight directions: anterio-posterior (AP), medio-lateral (ML), and four combinations of these principal directions. Perturbations were rep eated five times in each direction for six conditions (i.e., three differen t perturbation strengths and three different feet orientations). The compar ison of the averaged ankle sum NJT (AP) responses showed that the time cour ses of the responses elicited by a perturbation acting only in the AP direc tion were identical to those elicited by a combination of two corresponding AP and ML perturbations. In contrast the observed averaged ankle NJT (ML) responses did not follow the same similarity. The comparison of the average d ankle and hip sum NJT (ML) responses revealed that the time courses of th e responses elicited by a perturbation acting only in the ML direction were identical to those elicited by a combination of two corresponding AP and M L perturbations. These findings were invariable of the experimental conditi ons and were consistent among all the eight subjects. Thereby, we conclude that the ankle sum NJT (AP) and the ankle and hip sum NJT (ML) are the glob al variables being controlled. This shows that CNS controls the recovery fr om the multiple direction perturbations of moderate strength by decoupling the AP-ML postural space into two orthogonal directions (AP and ML). (C) 20 01 Elsevier Science Ltd. All rights reserved.