How attentional focus on body sway affects postural control during quiet standing.

The purpose of this study was to investigate how attentional focus on body sway affects postural control during quiet standing. To address this issue, sixteen young healthy adults were asked to stand upright as immobile as possible on a force platform in both Control and Attention conditions. In the latter condition, participants were instructed to deliberately focus their attention on their body sways and to increase their active intervention into postural control. The critical analysis was focused on elementary motions computed from the centre of pressure (CoP) trajectories: (1) the vertical projection of the centre of gravity (CoG(v)) and (2) the difference between CoP and CoG(v) (CoP-CoG(v)). The former is recognised as an index of performance in this postural task, whilst the latter constitutes a fair expression of the ankle joint stiffness and is linked to the level of neuromuscular activity of the lower limb muscles required for controlling posture. A frequency-domain analysis showed increased amplitudes and frequencies of CoP-CoG(v) motions in the Attention relative to the Control condition, whereas non-significant changes were observed for the CoG(v) motions. Altogether, the present findings suggest that attentional focus on body sway, induced by the instructions, promoted the use of less automatic control process and hampered the efficiency for controlling posture during quiet standing.