[What compensatory motor strategies do patients with multiple sclerosis develop for balance control?].

INTRODUCTION

One of the main features of multiple sclerosis (MS) is the deterioration of motor pathway axons, and in some cases, sensory system axons. Consequently, experimental sensori-motor testing with the undisturbed upright stance paradigm might be useful. It can be hypothesized that the postural strategies could be differently affected depending on the degree of dysfunction of both sensory and motor tracts.

METHODS

Twelve and fifteen patients, classified from sensory clinical tests as ataxo-spastic (SEP-AS) or only spastic (SEP-S), respectively were included in this study and compared to 12 healthy adults matched for age. The postural strategies were assessed from the centre of pressure trajectories (CP), measured from the force platform on which the subjects were instructed to stand upright eyes open for a trial lasting 51.2 s. biomechanical modelling was applied to these trajectories to compute the movements of the centre of gravity (CG) and consequently, the vertical difference between the CP and then the CP-CG, two elementary movements known to characterize postural performance movements for CGv and horizontal acceleration communicated to the CG for the CP-CG movements, and consequently overall neuro-muscular activity. To estimate the relative contribution of each of these elementary movements, an analysis based on frequency parameters (RMS and MF) was conducted.

RESULTS

Both SEP-AS and SEP-S groups demonstrate larger CG and CP-CG movements than the age paired individuals. However, some statistically significant differences has to be emphasised between the two MS subgroups but only for the CP-CG component: the RMS of these movements are largely increased for the SEP-AS group, as compared to the SEP-S one. Biomechanically, this feature expresses the necessity for these very patients to produce exaggerated horizontal forces, and thus an increased energy expenditure, to handle the CG movements. The lack of effect observed for the CG movements underlines the capacity for the SEP-AS group to set appropriate control mechanisms for counteracting these less favourable initial conditions.

CONCLUSION

By demonstrating specific trends in the postural organisation aimed at controlling undisturbed upright stance maintenance, this study can be of interest for the practitioner by legitimating this experimental paradigm as a simple and non invasive way to diagnose appropriately the sensori-motor deficiency.