Interactions between automatic postural adjustments and anticipatory postural patterns accompanying voluntary movement.

In order to examine interactions between centrally initiated postural activity preceding voluntary arm movements and compensatory postural activity, we studied patterns of postural muscle activity preceding a vigorous bilateral reach and grasp task or triggered by support surface motion. The reaching task required movement onset to be coincident with a predictable stimulus, and in some trials a brief backward platform perturbation was timed to occur before, during, or after the reach onset. Centrally initiated anticipatory postural activity was subject-specific and was very often absent when perturbation-induced postural activity was elicited just prior to movement onset. Likewise, compensatory postural activity patterns elicited by the platform perturbation did not occur when they would have coincided with the anticipatory postural activity. These data support the idea that the central neural processes which determine the specific activation pattern of the supporting limb musculature are influenced by both the intended dynamic outcome and the current dynamic status of the body.