Is the regulation of the Center of mass maintained during leg movement under microgravity conditions ? Mouchnino, L., M. Cincera, J.C. Fabre, C. Assaiante, B. Amblard, A. Pedotti, J. Massion. Laboratory of Neurobiology and Movements, CNRS, 31 chemin Joseph Aiguier, 13402 Marseille cedex 20, FRANCE, Faculty of Sport Sciences, 163 av. de Luminy 13288 Marseille cedex 9, FRANCE, Centro di Bioingegneria, Politecnico, via Gozzadini 7, Milano, ITALY.
APStracts 3:0037N, 1996.
1. Investigations on stance regulation have already suggested that the body's center of mass is the variable controlled by the central nervous system to maintain equilibrium. The aim of this study was to determine how the center of mass of the body is regulated when leg movements are made under different gravito-inertial force conditions. 2. Kinematic and electromyographic (EMG) recordings were made during both straight-and-level flight (earth-normal gravity condition, nG) and periods of weightlessness in parabolic flight (microgravity condition, G). The standing subjects were restrained to the floor (kept from floating away in G) and were instructed to raise one leg laterally to an angle of 45 degrees as fast as possible. 3. Two modes of center of mass (CM) control were identified during leg movement in nG: a "shift mode" and a "stabilization mode". The "shift mode" served to transfer the CM towards the supporting side prior to the leg raising and it preceded the phase of single limb support. The "stabilization mode" took place after the CM shift was completed and was aimed at stabilizing the CM during raising of the leg. In this phase, the movement of the raising leg is counterbalanced by a lateral inclination of the trunk in the opposite direction. As a consequence, CM position did not change with respect to the position reached before the leg raising and its projection on the ground remained within the support area delineated by the stance foot. 4. Under G, the CM position did not change before the leg raising. Moreover, Gastrocnemius Medialis activity observed in the moving leg under nG, preceding the initiation of the body weight transfer towards the supporting leg, was greatly reduced. While the leg is raising, the simultaneous and opposite lateral trunk movement was still present in G. 5. Results suggest that the body weight transfer corresponding to the "shift mode", might depend on the gravity constraints, while the "stabilization mode" which remains unchanged in G might be a motor stereotype which does not depend on the gravity conditions.

Received 1 September 1995; accepted in final form 19 January 1996.
APS Manuscript Number J578-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 14 February 96