Feedback-driven adaptation of gravity-related sensorimotor control to an upside-down posture

The ability to move is a vital and essential feature of human existence. We are experts at producing a variety of movements and have refined our control through evolution. As gravity is a major feature of our everyday environment, we have learned to take advantage of it by optimizing its effects to minimize the cost of our actions. This can be illustrated by systematic differences in the temporal organization of our movements according to their direction. Studying motor control in the face of various gravitational level modifications (hypergravity, hypogravity, or weightlessness), the scientific literature has shown that movement kinematics are rapidly adapted to new gravitational conditions. Hitherto, most researchers varied gravitational intensity to probe its neural integration into sensorimotor control. Here, we investigated the effects of a reversal of gravitational direction in the egocentric reference frame. Our results reveal a major effect of body-orientation reversal on motor control. This effect then progressively disappears, such that arm kinematics reached values that were close to the known baseline optimal ones. These results reveal that the effects of a simple reversal of body orientation cannot be fully anticipated to produce arm movements. Furthermore, comparing the evolution of varied parameters that were collected at different time points during each movement, our results reveal that adaptation first occurred during the late movement phases (i.e., around the time to peak velocity and time to peak deceleration), where online feedback contributions are typically larger, while not ruling out trial-to-trial updates of predictive control.NEW & NOTEWORTHY This study explores how the human nervous system adapts motor control when gravity's direction is reversed within the egocentric reference frame. Our results reveal a progressive, feedback-driven adaptation of arm movement kinematics that compensates for the altered gravitational effect. This demonstrates the plasticity of motor control strategies in atypical gravity orientations, providing new insights into the neurophysiological mechanisms underlying sensorimotor optimization and internal gravity representation.