Regulation of lateral position of body centre of mass in standing balance

On average, normal adults stand with a nearly symmetric distribution of body weight between the legs, although there is a wide range of individual differences. In the static case, the symmetric distribution of weight implies a position for the centre of body mass (CM) that would maximise stability in the face of random, laterally directed, disequilibrating forces. It therefore seems reasonable to hypothesise that people regulate CM position. In a new experiment with 204 normal adult subjects, we examined the change in weight distribution on raising an arm out to the side. Compared to standing symmetrically, arms by the side, holding one arm out resulted in a statistically reliable change in weight distribution. However, the change was significantly less than would be predicted from the uncompensated postural effect on CM of one arm being raised. This finding supports the hypothesis of CM position regulation. We suggest that in the training of standing balance, the traditional emphasis on an even distribution of body weight would be complemented by attending to the effects of changes to the postural configuration of the body.