Target size modifies anticipatory postural adjustments and subsequent elementary arm pointing

We herein studied whether arm-pointing movements from an upright posture (i.e. Belenkii's paradigm) toward various targets demanding a low degree of precision could influence associated anticipatory postural adjustments (APAs) and subsequent arm movements. Six subjects were asked to use their right arm to point (without finger touch) to targets of 2, 4 and 8 cm in diameter (respectively, D2, D4 and D8). APAs were measured by recording the electromyographic activity of the right anterior deltoid and biceps femoris, as well as the kinematics of the right arm. Longer APA durations and lower values for the ratio between acceleration duration and total duration of the focal movement were observed for D4 compared to D2 and D8, whereas precision was constant across all three targets. Thus, the medium target seemed to engender an optimum motor strategy for accomplishing the accuracy and velocity requirements of the task. These results emphasize that subjects build perceptual representations of their environment as well as representations of the actions to be produced. We suggest that, even in this simple movement traditionally studied from a biomechanical perspective, APAs function not only to compensate for perturbations to equilibrium, but also reflect a cognitive representation of the precision requirements of the task.