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Postural asymmetry in humans is generally associated with different pathologies. However, its specific influence on undisturbed upright stance is poorly understood. To evaluate its separate effects on each support, the centre of pressure (CP) displacements were recorded through two force platforms. In a second step, the complex resultant centre of pressure trajectories (CP(Res)) were computed and decomposed into two elementary components: the horizontal displacements of the centre of gravity (CG(h)) and the difference in the plane of support between the vertical projection of CG(h) and CP(Res) (CP-CG(v)). These motions were then processed through a frequency analysis and modelled as fractional Brownian motion to gain some additional insight into their spatio-temporal organisation. Ten healthy adults were tested in three conditions consisting of various weight distributions. The quality of the mechanism involved in the control of the unloaded support CP motions appears to decrease as the asymmetry becomes more pronounced. To be precise, larger increases of the CP displacements are observed for the unloaded support compared to the loaded one. As a result, the CP(Res) motions are themselves augmented in the ML direction, inducing in turn larger CG(h) and CP-CG(v) motions. Postural asymmetry thus constitutes an important constraint on the control of upright undisturbed stance by generating changes in the control of both supports and by reducing the efficiency of the hip load/unload mechanisms. On the other hand, by inducing larger body sways, postural asymmetry necessitates higher energy expenditure and the setting of particular control mechanisms.

Abstract: Distributing asymmetrically the body weight (BW) has been previously described through static parameters such as the mean position of the resultant centre of pressure (CPRes) along the medio-lateral axis or mean BW distribution on the two legs. However, neither the respective dynamic contribution of each leg in the CPRes displacements nor the role of an ankle mechanism and a load/unload hip mechanism was established. The main goal of this study is to investigate whether asymmetric postural control can be better assessed through such information. To this aim, 14 healthy adults were required to stand with or without weight-bearing asymmetry. The recorded CPRes trajectories were re-computed by substituting for each time the average value of the left or the right foot plantar CP trajectory. The contribution of ankle and hip mechanisms, consisting in modifying either the pressure distribution under the feet or the loading–unloading of the BW on each leg, respectively, was also assessed through an identical substitution principle. Distributing asymmetrically the BW reinforces the contribution of the CP displacements under the loaded foot in the generated CPRes movements, especially along the antero-posterior axis. The predominant role played by the ankle mechanisms, observed along the antero-posterior axis, is significantly decreased by the BW asymmetry. The reduced correlations with the traditional parameters used for characterizing BW asymmetry along the medio-lateral axis let suggest that this novel approach could be useful to differently assess the effects on postural control of a BW asymmetrically distributed in normal and impaired individuals. [Copyright &y& Elsevier]