Distinct Proximal and Distal Corticospinal Signals Embed Limb Dynamics Through the Modulation of Stiffness

Abstract

How the neural motor system recruits muscles to support the arm against gravity is a matter of active debate. It is unknown how the neural motor system compensates for the changing gravity-related joint moments either when holding a steady-state posture or during movement between postural steady states, e.g., during reaching. Here we used single-pulse transcranial magnetic stimulation to compare the roles that the human primary motor cortex plays in the muscle recruitment to compensate for gravity. We hypothesized that the motor cortex contributes to muscle recruitment to both maintain posture and to compensate for changes in gravitational passive joint moments during movement. To test this hypothesis, we used visual targets in virtual reality to instruct five postures and three movements with or against gravity. We then measured the amplitude and gain of motor evoked potentials in multiple muscles of the arm at several phases of the reaching motion and during posture maintenance. Stimulation below the resting motor threshold, calibrated to the biceps muscle, caused motor evoked potentials in all muscles during all postural and reaching tasks. The amplitude of motor evoked potentials was proportional to the motoneuronal excitability measured as muscle activity. The coefficient of proportionality was positively correlated with the postural component of muscle moment during posture and movement. Altogether our results support the hypothesis. The observed contribution of the motor cortex to the recruitment of multiple antagonistic muscles suggests a whole-limb strategy for overcoming passive gravity-related moments with both active muscle moments and muscle co-activation that modulates limb impedance. New & NoteworthyMaintaining static posture and producing motion appear to be contradictory tasks for the nervous system. In contrast to this seeming dichotomy, our results show that the motorcortical control signals play the same role in both tasks when it is framed in biomechanical terms, i.e., muscle contractions needed to compensate for gravity.