Hypergravity is more challenging than microgravity for the human sensorimotor system

While recent findings demonstrated the importance of contextual estimates about gravity for optimal motor control, it remains unclear how gravitational changes are taken into account by the central nervous system to perform complex motor skills. Here, we investigated the effect of microgravity and hypergravity on the neuromuscular control of whole-body reaching movements compared to normogravity. Standing participants (n=9) had to reach toward visual targets during parabolic flights, which allowed us to test the influence of gravity level on sensorimotor planning and control processes. Also, to specifically test the efficiency of online motor control mechanisms, unexpected mechanical perturbations were used. Whole-body kinematics and muscular activity were adjusted in microgravity, allowing arm reaching to be as accurate as in normogravity. In contrast, systematic undershooting was observed in hypergravity, where main parameters of whole-body kinematics remained unchanged and muscle activations insufficiently adjusted to keep the same accuracy as in normogravity. Conversely, muscular synergies exhibited during whole-body reaching were found similar in the various gravitational contexts, as were local muscular adjustments in response to unexpected mechanical perturbations. This suggests that online feedback control remains functional across very distinct gravitational environments. Overall, our findings demonstrates that hypergravity creates challenges that the human sensorimotor system is unable to solve rapidly, contrary to microgravity.