Short-term effects in undisturbed postural control induced by visual feedback training

INTRODUCTION The visual feedback (VFB) technique is recognised as an appropriate tool for re-establishing postural stability following various motor dysfunctions. Indeed, VFB training programs are used to improve postural balance after a number of diseases such as hemiplaegia (Shumway-Cooke et al.; 1988). These studies have shown that, with daily and weekly training sessions, a decrease in amplitude of postural oscillations can be observed. Most of the time, these training protocols are programmed for rather long periods. Only a few studies have been focused on the short term VFB effect on postural control for healthy individuals (Takeya et al., 1976; Clarke et al., 1990). The former report observes a slight augmentation of stability after five sessions of three trials (30s) with 20-30s rest periods whereas the latter observes a deterioration in performance after three training sessions of four trials (1mn) with short rest periods. Classically, postural investigations quantify the postural control by analysing the centre of pressure (CP) trajectory, which is appropriated through a force platform. The study of this global signal does not include the two elementary tasks involved in maintaining postural balance: the limitation of variations of resulting moments in ankles and the restriction of the centre of gravity (CG) motions in a reduced zone. Consequently it seems coherent to dissociate these CP trajectories into two elementary components: the vertical projection of CG (CGv) and the difference between the former and the latter: CP-CGv (Rougier et Caron; 2000). Biomecanically, the CGv variable represents the net postural performance (Winter et al., 1998) whereas the proportionality existing between CP-CGv amplitudes and CG horizontal accelerations (Caron et al., 2000). Even decomposed, these different motions are controlled. To this aim, recourse to mathematical models such as fractional Brownian motion (fBm), constitutes a very sensitive analysis method. This study is thus aimed at establishing the short-term effects of a training protocol during which healthy subjects repeatedly perform this VFB task. With this aim in mind, a comparison between pretraining and post-training with closed eyes is made to observe modifications in postural control.