Anticipatory postural adjustments during self-initiated perturbations of different magnitude triggered by a standard motor action.

The purpose of the study was to define whether anticipatory postural adjustments scale with the magnitude of a self-triggered postural perturbation when a standard motor action triggers the perturbation. Standing subjects generated vertical forces of different magnitude with their extended arms against a bar connected through a rigid cord to the floor. They released the bar with a standard bilateral shoulder abduction. Anticipatory postural adjustments were seen as changes in the level and/or timing of the background activation of postural muscles. Muscles of the dorsal part of the legs and of the trunk demonstrated an anticipatory decrease in the level of activation, commonly leading to its complete disappearance. There was no relation between the magnitude of the unloading and the timing of the changes in the background activity of these muscles. Muscles of the frontal part of the legs and of the trunk demonstrated an anticipatory increase in their activity whose timing and amplitude correlated positively with the magnitude of the perturbation. We conclude that anticipatory postural adjustments can be scaled with respect to the magnitude of a self-triggered perturbation.