Adaptation of center of mass control under microgravity in a whole-body lifting task

Human balance in stance is usually defined as the preservation of the verti cal projection of the center of mass (COM) on the support area formed by th e feet. Under microgravity conditions, the control of equilibrium seems to be no longer required. However, several reports indicate preservation of CO M control in tasks such as arm or leg raising, tiptoe standing, or trunk be nding. It is still unclear whether COM control is also maintained in comple x multijoint movements during short term exposure to microgravity. In the c urrent study, the dynamics of equilibrium control were studied in four subj ects performing two series of seven whole-body lifting movements under micr ogravity during parabolic flights. The aims of the study were to examine wh ether the trajectory of horizontal COM motion during lifting movements chan ges in short-term exposure to microgravity and whether there is any sign of recovery after several lifting movements. It was found that, compared with control movements under normal gravity, the horizontal position of the COM was shifted backward during the entire lifting movement in all subjects. I n the second series of lifting movements under microgravity, a partial reco very of the COM trajectory toward the normal gravity situation was found. U nder microgravity, angles of the ankle, knee, hip, and lumbar joints differ ed significantly from the angles found under normal gravity. Recovery of jo int angular trajectories in the second series of lifting movements mainly o ccurred for those angles that could contribute to a reduction of the backwa rd COM shift. It is to be pointed out that COM control under microgravity i s not redundant but functional. Persisting COM control under microgravity m ay be required for pure mechanical reasons, since rotational movements of t he body are dependent on adequate control of the COM position with respect to external forces. It is shown that, from a mechanical perspective, subjec ts can benefit from a backward displacement of the COM in the downward as w ell as the upward phase of the lifting movement under microgravity.