Could a motor action that has no direct relation to expected perturbation be associated with anticipatory postural adjustments in humans?

Anticipatory postural adjustments (APAs) in standing subjects were studied using different motor actions. We tested the hypothesis that a direct relation between the forthcoming perturbation and motor action is needed to trigger robust anticipatory postural adjustments. The same postural perturbation of a standard load release was induced by either a shoulder abduction, utilizing a motor action with a direct relation, or a biting or blowing action that had an indirect relation with the forthcoming perturbation. Electromyogram activity of trunk and leg muscles as well as the displacement of the center of pressure was recorded. Reduced APAs were observed in experimental trials using motor actions with an indirect relation to a perturbation as compared to the control series with perturbations induced by shoulder abduction. These results suggest that a direct relation between motor action and expected perturbation is important in the generation of anticipatory postural adjustments. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.