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The present paper focuses on the organization of posture and movement under normal and microgravity conditions. Two reference values subserving the control of erect posture and the performance of movements are analyzed. The first is 'geometrical' in nature and corresponds to the orientation of a body segment with respect to the external world. The second reference value, which involves the mass and inertia of the body segments, is the position of the centre of mass with respect to the foot support area. The reorganization of these parameters which occurs under microgravity is discussed in the framework of a hierarchical model of posture. Suggestions are made for training procedures which could be used to prevent loss of balance from occurring in astronauts on landing after long space flights., (Copyright 1998 Elsevier Science B.V.)

The ability to voluntarily stabilize the head in space during lateral rhythmic oscillations of the trunk has been investigated during parabolic flights. Five healthy young subjects, who gave informed consent, were examined. The movements were performed with eyes open or eyes closed, either during phases of microgravity or phases of normal gravity. The main result to emerge from this study is that the head may be stabilized in space about the roll axis under microgravity conditions with, as well as without vision, despite the reduction of the vestibular afferent and the muscle proprioceptive inputs. Moreover, the absence of head stabilization about the yaw axis confirms that the degrees of freedom of the neck can be independently controlled, as it was previously shown. These results seem to indicate that voluntary head stabilization does not depend crucially upon static vestibular afferents. Head stabilization in space may be in fact organized on the basis of either dynamic vestibular afferents or a postural body scheme.