Changes of posture during transient perturbations in microgravity.

The control of goal-directed arm movement and of body stability before, during, and 3 d after a 7-d spaceflight has been investigated. The findings show that the anticipatory and compensatory activities of the postural muscles were highly reproducible during the first days of the space mission. The sequence of these activities, studied in two situations--in which the platform either was fixed or could rotate about near the rotation axis of the ankle--was similar to a ground-based situation. The trajectory of various body segments demonstrates that a 7-d exposure to microgravity did not result in major changes in posture. Furthermore, vision seemed to play an important role in the control of standing posture at the beginning of the flight. Postural perturbations, elicited by unexpected displacements of the foot support, involved leg muscle reflexes whose amplitudes were greatly reduced compared to those on earth.