The postural stability during quiet stance and during walking was investigated in 22 elderly and 20 young subjects. A motion analysis system was used to simultaneously record movements of 14 markers on the body while a force plate recorded movement of the centre of pressure (COP) during stance (but not during walking). The movements of the body during stance could be well described (> 90 % of the variance explained) as a simple inverted pendulum moving about the ankles in the anterior-posterior and medial-lateral directions. This model was applicable to both young and elderly subjects and also predicted the records of COP movement well (r > 0.90). When account was taken of the ground reaction forces the prediction was further improved. The greater COP movements commonly observed in the elderly are shown to be due to increased pendulum sway in the medial-lateral direction, compared to young subjects. The inverted pendulum model also gave an adequate description of the deviations from the mean path ("sway") during walking which are larger than those during stance. The static measurement that best predicts sway during walking is medial-lateral movements of the COP when standing on a compliant surface with the eyes closed. The relationship between muscle strength and COP displacement was examined in a larger group of elderly subjects (N = 56). Maximum voluntary force per cross-sectional area was found not to be correlated with COP movements during quiet stance. This suggests that muscle weakness and increased sway in the elderly have separate physiological causes. A method was developed for inducing a trip-like perturbation of gait as subjects walked on a treadmill. Muscle activation patterns and body kinematics were recorded in 9 young subjects to establish the normative response to such a perturbation with a view to investigating these responses in the elderly
