RECALIBRATION OF SOMESTHETIC PLANTAR INFORMATION IN THE CONTROL OF UNDISTURBED UPRIGHT STANCE MAINTENANCE

Abstract

To assess the effects of changes in somesthetic plantar information on upright quiet stance, a rotary plantar massage was applied under the feet of healthy subjects for ten minutes. The controlling variable, the centre of pressure (CP) displacements, were recorded, before and after massage, through a force platform and decomposed into two elementary motions: the vertical projection of the centre of gravity (CG(v)) and the difference between the latter and the CP (CP-CG(v)) along medio-lateral ML and antero-posterior AP directions. These motions were processed through frequency analysis and modelled as fractional Brownian motion. For CP-CG(v) motions, the frequency analysis shows that massage under the plantar soles induces a decrease of the amplitudes along the ML direction suggesting reduced overall muscular activity (abductor-adductor muscles of the hip according to Winter et al.). A general trend is that the CG(v) amplitudes are also diminished after massage especially in the ML direction, indicating a better distribution of the body weight on the two supports. On the other hand, the effects tend to vanish after about 8 minutes. Conversely, when the massage was given under the toes, no particular effect on any elementary motion was observed, suggesting that the plantar mechanoreceptors under the toes necessitate stronger stimulation to respond significantly and/or that the greater sensitivity obtained was not used by the CNS. Overall, this data emphasises the fact that a recalibration of somesthetic cues may occur when enhanced afferent information is fed to the postural system.