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.

[1]  A. Shumway-cook,et al.  Postural sway biofeedback: its effect on reestablishing stance stability in hemiplegic patients. , 1988, Archives of physical medicine and rehabilitation.

[2]  D. Pérennou,et al.  Contribution of Each Lower Limb to Upright Standing in Stroke Patients , 2008, Stroke.

[3]  P. Rougier Relative contribution of the pressure variations under the feet and body weight distribution over both legs in the control of upright stance. , 2007, Journal of biomechanics.

[4]  P Rougier,et al.  Influence of an asymmetrical body weight distribution on the control of undisturbed upright stance. , 2005, Journal of biomechanics.

[5]  Jérôme Froger,et al.  Posturography in Patients With Stroke: Estimating the Percentage of Body Weight on Each Foot From a Single Force Platform , 2008, Stroke.

[6]  David A. Winter,et al.  Medial-lateral and anterior-posterior motor responses associated with centre of pressure changes in quiet standing , 1993 .

[7]  Y. Brenière Why we walk the way we do. , 1996, Journal of motor behavior.

[8]  J. Duysens,et al.  Selective activation of human soleus or gastrocnemius in reflex responses during walking and running , 2004, Experimental Brain Research.

[9]  D A Winter,et al.  Sampling duration effects on centre of pressure summary measures. , 2001, Gait & posture.

[10]  P. Rougier,et al.  How spreading the forefeet apart influences upright standing control. , 2008, Motor control.

[11]  R L Kirby,et al.  The influence of foot position on standing balance. , 1987, Journal of biomechanics.

[12]  G. Mochizuki,et al.  Synchronization of motor units in human soleus muscle during standing postural tasks. , 2005, Journal of neurophysiology.

[13]  J. Duysens,et al.  A review of standing balance recovery from stroke. , 2005, Gait & posture.

[14]  P. Rougier,et al.  Quiet postural control of patients with total hip arthroplasty following joint arthritis. , 2008, Motor control.

[15]  P. Rougier,et al.  Analyse biomécanique de la contribution relative de chacun des appuis dans le contrôle de la station debout non-perturbée , 2003 .

[16]  D. Winter,et al.  Unified theory regarding A/P and M/L balance in quiet stance. , 1996, Journal of neurophysiology.