Correlated hip motions during quiet standing

Kinematic measurements of two simultaneous coordinates from postural sway during quiet standing were performed employing multiple ultrasonic transducers. The use of accurate acoustic devices was required for the detection of the small random noise displacements. The trajectory in the anteroposterior - mediolateral plane of human chest was measured and compared with the trajectory in anteroposterior direction from the upper and lower body. The latter was statistically analyzed and appeared to be strongly anti-correlated. The anti-correlations represent strong evidence for the dominance of hip strategy during an unperturbed one minute stance. That the hip strategy, normally observed for large amplitude motions, also appears in the small amplitude of a quite stance, indicates the utility of such noise measurements for exploring the biomechanics of human balance.