Dynamical assessment of weight-bearing asymmetry during upright quiet stance in humans

Distributing asymmetrically the body weight (BW) has been previously described through static parameters such as the mean position of the resultant centre of pressure (CP(Res)) along the medio-lateral axis or mean BW distribution on the two legs. However, neither the respective dynamic contribution of each leg in the CP(Res) 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 CP(Res) 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 CP(Res) 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.