Reciprocal angular acceleration of the ankle and hip joints during quiet standing in humans

Human quiet standing is often modeled as a single inverted pendulum rotatin g around the ankle joint, under the assumption that movement around the hip joint is quite small. However, several recent studies have shown that move ment around the hip joint can play a significant role in the efficient main tenance of the center of body mass (COM) above the support area. The aim of this study was to investigate how coordination between the hip and ankle j oints is controlled during human quiet standing. Subjects stood quietly for 30 s with their eyes either opened (EO) or closed (EC), and we measured su btle angular displacements around the ankle (theta (a)) and hip (theta (h)) joints using three highly sensitive CCD laser displacement sensors. Reliab le data were obtained for both angular displacement and angular velocity (t he first derivative of the angular displacement). Further, measurement erro r was not predominant, even among the angular acceleration data, which were obtained by taking the second derivative of the angular displacement. The angular displacement, velocity, and acceleration of the hip were found to b e significantly greater (P<0.001) than those of the ankle, confirming that hip-joint motion cannot be ignored, even during quiet standing. We also fou nd that a consistent reciprocal relationship exists between the angular acc elerations of the hip and ankle joints, namely positive or negative angular acceleration of ankle joint is compensated for by oppositely directed angu lar acceleration of the hip joint. Principal component analysis revealed th at this relationship can be expressed as: <theta>(h) = gamma theta (a) with gamma=-3.15+/-1.24 and y=-3.12+/-1.46 (mean +/-SD) for EO and EC, respecti vely, where theta is the angular acceleration. There was no significant dif ference in the values of gamma for EO and EC, and these values were in agre ement with the theoretical value calculated assuming the acceleration of CO M was zero. On the other hand, such a consistent relationship was never obs erved for angular displacement itself. These results suggest that the angul ar motions around the hip and ankle joints are not to keep the COM at a con stant position, but rather to minimize acceleration of the COM.