Equilibrium perturbing forces associated to any voluntary upper-limb movement are strong enough to displace the body centre of mass. In this condition Anticipatory Postural Adjustments (APAs), developing in muscles other than the prime mover, are essential for maintaining the equilibrium stability. Here we test the hypothesis that APAs preceding an upper-limb target reaching could also be crucial in controlling movement accuracy.

Standing subjects (10) flexed their shoulder to reach the centre of a target positioned in front of them. The reaching task was also performed while wearing and after doffing prismatic lenses (shifting the eye field rightward). EMGs from different upper- and lower-limb muscles and mechanical actions to ground were recorded.

We found that: i) before wearing prisms, subjects performed very accurate reaching movements, ii) after wearing prisms, apparent pointing errors occurred, iii) these pointing errors were progressively compensated within each session, and iv) changes in APA amplitude were associated to amplitude of pointing error.

These results demonstrate a strict coupling between voluntary movement and APAs pattern, suggesting that, besides stabilizing the whole body equilibrium, APAs also guarantee movement accuracy. Thus, recalling Bernstein's "the movement responds as a whole to changes in each small part" we should consider APAs and primary movement as a whole, since they represent two subsequent aspects of the same voluntary motor act.