[How mirror feedback improves undisturbed upright stance control].

Objective: To evaluate, in normal and healthy adults, the postural effects induced by a protocol of body sway mirror feedback.

Materials And Methods: Two experimental conditions, one consisting in staring eyes open (EO) and one consisting in mirror feedback (MFB), are randomly proposed. Displacements of the centre of pressure (CP), issued from a force platform, are advantageously decomposed along medio-lateral ML and antero-posterior AP directions into two elementary components: the horizontal motions of the centre of gravity (CG(h)) and the difference CP-CG(v). The former is recognised as the main controlled variable in upright stance whilst several clues can be assessed by the latter: the overall muscular activity, the ankle stiffness and the horizontal accelerations communicated to the CG. Elementary motions CG(h) and CP-CG(v) are then processed in the frequential domain and characterized through RMS and MPF parameters.

Results: For CP-CG(v) motions the MFB protocol induces larger MPF along the AP direction and thus an increased ankle stiffness. In addition, the RMS are slightly decreased, indicating that less motor units are called into play. On the other hand, no real effect is observed in the ML direction. A general trend is that the CG(h) amplitudes are diminished by the MFB protocol, especially in the ML direction where a statistical effect is noticed.

Conclusion: The specificity of the effects induced by the MFB technique makes it an appropriate tool for re-establishing, especially on the AP direction, the balance functions, in particular those involving exaggerated CP-CG(v) motions.