Specificity of postural sway to the demands of a precision task.

We examined a precision aiming task in which a handheld laser pointer was controlled by the postural system. The task was performed in two orientations of the body's coronal plane to the target. In the parallel orientation medio-lateral (ML) sway had to be minimized, in the perpendicular orientation antero-posterior (AP) sway had to be minimized. In the parallel orientation ML sway decreased and AP sway increased with target distance and size. The pattern reversed in the perpendicular orientation. Nonlinear measures found independence of the two directions of sway and differences in their deterministic structure. Apparently a postural organization for upright standing and aiming (as in archery) entails two independent postural subsystems with different but reciprocally related dynamics. Furthermore, it seems as if some amount of postural variability is needed to ensure stability in quiet standing; if postural activity is reduced in one direction it is compensated for in the other direction.