Human standing posture control system depending on adopted strategies.

Control of the standing posture of humans involves at least two distinct modes of operation to restore the body balance in the sagittal plane: the ankle strategy and the hip strategy. The objective of the study was to estimate the contribution of vestibular, visual and somatosensory feedbacks to these distinct strategies. The body dynamics was described as the motion of two linked rigid segments that represented the legs and the rest of the body. The posture controller received the inclination angles of the two body segments as inputs and regulated the moments around the ankle and hip joints. The controller had four feedback paths that were characterised by transfer functions connecting the two inputs and the two outputs. To evoke the distinct strategies, the floor conditions were varied by narrowing the support surface under the feet. A continuous pseudo-random external disturbing force was applied to the waist and the thigh independently. The inclination angles of the body segments and the ground reaction force were measured, and the transfer functions of the controller were estimated with the maximum-likelihood system identification procedure. Six healthy male adult subjects participated in the experiment. When the hip strategy became evident under the narrow support surface conditions, the transfer function relating the leg inclination angle and the ankle joint moment decreased its DC gain (16%), whereas the other three transfer functions increased the gains (20-140%) (ANOVA, p < 0.05). Based on a criterion for simplicity in the modification of the posture controller, these changes suggest a new hypothesis that, when posture control becomes difficult, the central nervous system selectively activates the somatosensory feedback paths from the hip joint angle to the moments around the ankle and hip joints.