LONG TERM EXPOSURE TO WEIGHTLESSNESS REVEALS COMBINED EFFECTS OF LIMB MECHANICS AND GRAVITATIONAL LOADING ON REACHING

On Earth, gravity shapes arm movement kinematics: vertical movements typically exhibit directional asymmetries in velocity profiles or curvature, whereas the horizontal movements addressed in most studies to date do not. Current theories attribute asymmetries in vertical hand movements to an optimization process in the motor system that accounts for gravitational forces to minimize energy expenditure. However, because previous works focused mainly on single joint movements, the role of limb mechanics remains unclear. We therefore investigated the combined effects of gravity and limb biomechanics on bi-articular arm-movement kinematics in eleven astronauts performing sequences of visually guided reaching movements towards targets aligned along vertical and horizontal axes before, during and after long-duration spaceflight missions aboard the ISS. In weightlessness, velocity profiles of vertical movements became more symmetrical across upward and downward directions, consistent with previous findings. Unexpectedly, horizontal hand movements exhibited directional asymmetries between forward and backward motions, which varied with the distance from the body and which were amplified in 0g. The interaction between movement direction and gravity level demonstrates that limb biomechanics play a clear role in shaping movement kinematics. These effects can be understood within a movement optimization framework. We designed an optimal control model incorporating arm dynamics and gravity, with a cost-function combining muscular effort and motion smoothness. Simulations reproduced directional asymmetries across motion axes and gravity conditions, and their dependence on joint configuration, closely matching the astronauts’ data. These results support the hypothesis that hand movement trajectories are shaped by internal representations of gravitational and biomechanical constraints.