DOES THE CENTER-OF-MASS REMAIN STABLE DURING COMPLEX HUMAN POSTURAL EQUILIBRIUM TASKS IN WEIGHTLESSNESS

In normal gravity conditions the execution of voluntary movement invol ves the displacement of body segments as well as the maintenance of a stable reference value for equilibrium control. It has been suggested that centre of mass (CM) projection within the supporting base (BS) is the stabilised reference for voluntary action, and is conserved in we ightlessness. The purpose of this study was to determine if the CM is stabilised during whole body reaching movements executed in weightless ness. The reaching task was conducted by two cosmonauts aboard the Rus sian orbital station MIR, during the France-Russian mission ALTAIR 199 3. Movements of reflective markers were recorded using a videocamera, successive images being reconstructed by computer every 40ms. The posi tion of the CM, ankle joint torques and shank and thigh angles were co mputed for each subject pre- in- and post-flight using a 7-link mathem atical model. Results showed that both cosmonauts adopted a backward l eaning posture prior to reaching movements. Inflight, the CM was displ aced throughout values in the horizontal axis three times those of pre -flight measures. In addition, ankle dorsi flexor torques inflight inc reased to values double those of pre- and post-flight tests. This stud y concluded that CM displacements do not remain stable during complex postural equilibrium tasks executed in weightlessness. Furthermore, in the absence of gravity, subjects changed their strategy for producing ankle torque during spaceflight from a forward to a backward leaning posture. (C) 1998 Elsevier Science Ltd. All rights reserved.