The precision of a voluntary movement relies on its associated anticipatory postural adjustments

Equilibrium perturbing forces associated to a voluntary upper-limb movement (e.g. pointing) can be strong enough to displace the whole-body centre-of-mass. In this condition, Anticipatory Postural Adjustments (APAs), developing in muscles other than the prime mover, are usually considered essential in maintaining the whole body equilibrium stable. Here we test the hypothesis that APAs preceding an upper-limb pointing movement could also be crucial in ensuring movement precision: if so, a perturbation of such a control could results in motor dysmetria. Standing subjects (10) were asked to flex their shoulder to point-and-touch the centre of a cross target positioned in front of them. The pointing task was also performed while donning and after doffing goggles equipped with prismatic lenses (20 dioptres, shifting right the eye field). During each session (BEFORE, DURING and AFTER prisms) 25 pointing trials were collected. EMGs from different upper- and lower-limb muscles and mechanical actions to the ground were recorded. Our results show that: i) pointing movements are associated to APAs in EMG and force traces, ii) prisms introduce an apparent pointing error and iii) this error is compensated by pointing practice (prisms adaptation). Moreover, iv) a positive correlation between APA amplitude and magnitude of the pointing error is apparent when matching BEFORE to DURING sessions, while contraction of the shoulder prime mover is virtually unchanged; an increased APAs/prime mover amplitude ratio is thus associated to out-of-target pointing movements. Finally, v) when movement precision is recovered after adaptation to prisms, APAs/prime mover intensity ratio recovers to values observed before wearing goggles. By demonstrating a linkage between APAs and movement precision, these results suggest an APAs role in refining the prime movement accuracy and stress Bernstein's original statement according to which a movement “responds as a whole to changes in each small part”.