Axial synergies under microgravity conditions.

Fast forward and backward upper trunk movements were analyzed and compared under microgravity and under preflight and postflight conditions. The kinematic analysis showed that the upper trunk movements were accompanied by hip and knee movements in the opposite direction both under microgravity and under normal gravitational conditions. This suggests that the center of mass position with respect to the feet is still regulated under microgravity when the feet are attached to the floor. The EMG analysis during backward movements shows that under preflight conditions a set of muscles (ErSp, BF, Sol) in the back of the body are activated early on. Under microgravity, the early Sol activation was replaced by an early TA activation, which was still present at the first postflight recording and was then replaced by the early Sol activation observed under preflight conditions. This finding shows that the EMG pattern underlying the axial synergies is flexible and that adaptive changes take place both under microgravity and after return to Earth.