Body orientation and center of mass control in microgravity

The control of the body orientation and the center of mass position with respect to the feet was investigated under normo- and microgravity (space flight Altair), during erect posture and at the end of a forward or backward upper trunk movement. It was observed that during erect posture, the trunk orientation with respect to the vertical was inclined some 6 degrees forward in both subjects under microgravity, whereas it was vertical or slightly backward oriented under normogravity. Under microgravity, on the contrary, the initial position CM changed either backwards or forwards. This result suggests that the inclined trunk posture might be due to misevaluating the verticality under microgravity and that different control mechanisms are involved in orienting the trunk and placing the CM. It was also noted that the final position of the CM at the end of the movement did not differ markedly between microgravity and normogravity. This result suggests that the kinematic synergies which stabilize the CM during upper trunk movements may result from an automatic central control which is independent from the gravity constraints.