Selective hypergravity stimulation : Its effects on the human balance and gait functions. A model to assess, in normal gravity conditions, some aspects of the perturbations induced on human body by microgravity conditions

Abstract

To assess on Earth some reactions of the muscle mechanoreceptors to transitions from normogravity to microgravity, we studied the effects of transitions from hypergravity to normogravity. Hypergravity was selectively applied to the extensor muscles by increasing their activity during half an hour by means of 2 rubber extensible springs stretched from shoulders to feet. Immediate effects and post effects of such a stimulation were measured on quantifying dynamic balance (angular or linear displacement) and gait functions (spatio-temporal parameters and inferior limb length variations). The main results are : (1) a post effect on the balance function, appearing 3 minutes after the end of the selective hypergravity stimulus and improving the efficiency of balance function compared with the basal one, (2) a post effect on the gait function, appearing immediately after the end of the selective stimulation. It concerns the measures which quantify the gait phases during which flexor muscles are active (swing phases). It decreases the efficiency of the gait function compared with the basal one. It disappears 3 minutes after the end of the selective hypergravity stimulation. According to these results, if the effects on the muscle mechanoreceptors of the transitions from normogravity to microgravity looks like those of transitions from hypergravity to normogravity, post effects could be a mechanism of the motor perturbations at the beginning of the orbital flights.