Contraction-dependent intracortical inhibition supports gravity-efficient motor control

Efficient control of voluntary movements along the gravity axis requires adapted shifts in muscular contraction modes. In daily life, rising the arm up involves shortening (i.e., concentric) contractions of arm flexors, while the reverse movement can rely on lengthening (i.e., eccentric) contractions of the same muscles with the help of gravity force. Although this muscular-control mode is universal, the neuromuscular mechanisms that subserve the control of such gravity-oriented movements remain unknown. In this study, we designed two neurophysiological experiments that allowed tracking modulations of cortical, spinal, and muscular outputs of arm flexors while healthy humans carried out vertical pointing movements. In conditions where upward and downward movements revealed kinematic features reminiscent of optimal motor commands (i.e., directional asymmetries), we report fine contraction-dependent modulations of the corticospinal output. The overall corticospinal excitability dropped during lengthening contractions (downward movements) compared with shortening contractions (upward movements). Specifically, we did not observe any change in spinal motoneuron responsiveness from cervicomedullary stimulations but a specific increase in intracortical inhibition during lengthening vs. shortening contractions. We discuss these fine contraction-dependent modulations of the supraspinal motor output in the light of feedforward mechanisms that may support gravity-tuned motor control. Generally, these results shed a new perspective on the neural policy that optimize movement control along the gravity axis.