The relation between postural stability and weight distribution in healthy subjects.

Knowledge of the effects of leg-loading asymmetry on postural control and control asymmetry during quiet upright standing in healthy young and middle-aged subjects is necessary before these relationships in patients with lateralized disorders can be assessed and understood. A posturographic procedure was developed, using a dual-plate force platform, during which 10 younger and 10 middle-aged healthy individuals were required to adopt various degrees of (a) symmetrical weight distributions (0, 5, 10, 20 and 30% of extra body weight loaded onto either leg). Postural control and control asymmetry were quantified by centre of pressure (CP) fluctuations in the lateral (LAT) and anterior-posterior (AP) directions under both feet together and individually. Subsequently, the relationship between weight distribution on one hand and postural control and control asymmetry on the other hand, was calculated. Results demonstrated that with increasing weight-bearing asymmetry (WBA), the overall control of postural sway velocity increased mainly in the LAT direction, where a first-order polynomial function fitted best. The asymmetry of control of postural sway velocity increased with increasing weight-bearing asymmetry in favour of the more loaded leg in LAT and AP directions. A first-order polynomial was used for both AP and LAT direction. Effects of weight-bearing asymmetry on postural control and control asymmetry are due to changes in the biomechanical constraints of upright standing. It was suggested that through increasing weight-bearing asymmetry the postural instability increased by reducing the efficiency of hip load/unload mechanisms and increasing the (compensatory) ankle moments.