Sensory-Related Changes in Two-Segment Dynamics on a Sway-Referenced Support Surface

In its simplest form, the human postural control system can be described as a closed-loop control system consisting of a plant (body segments and musculotendon actuators) and feedback. Previous efforts to understand the contributions of plant and feedback employed techniques to "open the loop" which is problematic with the study of posture because the plant is unstable without feedback. In the present experiment, a closed-loop system identification method was used to "open the loop" without removal of sensory feedback. Subjects stood on a movable platform facing a visual scene, both of which were capable of rotation about an axis coaxial with the subject's ankles. The visual stimulus (present all trials) consisted of a 10-frequency sum-of-sines while movement of the support surface consisted of the following conditions: 1. Stationary; 2. Sway-referenced to the subject's body sway; 3. 10-frequency sum-of-sines; 4. Combined sway-referenced and sum-of-sines. Closed-loop frequency response functions were calculated for visual stimulus to EMG and visual stimulus to body sway angle. The open loop frequency response function for the plant was determined by dividing the frequency response functions, mathematically canceling the effects of feedback. With respect to the visual stimulus, gains for the leg segment showed no differences between the four platform conditions. Phase for the stationary condition was lower at the higher stimulus driving frequencies than for any of the moving platform conditions. In contrast, trunk segment gains were lower for the sway-referenced conditions at lower stimulus frequencies than for the stationary and sum-of-sines conditions. Phase showed a slight lead of the legs over the trunk for the sway-referenced conditions. The phase relationship between the trunk and leg segments, typically in-phase below ~1 Hz and anti-phase above ~1 Hz, showed a gradual transition at a lower frequency for the sway-referenced conditions than for the stationary or sum-of-sines conditions. Complex coherence showed a "legs-leading" coordinative relationship at the phase mode transition for the two sway-referenced conditions. Differences in the frequency response functions demonstrate that the plant changes with platform condition requiring different postural control strategies to maintain stability