Rapid feedback responses when planning, performing and terminating goal-directed reaching movements

The healthy nervous system transitions seamlessly between actions with different goals. A tennis player, for example, waits in a ready position before lunging sideways to return a serve. This simple action involves disengaging from posture to perform the movement and then stabilizing again after contact to prepare for the next return. These transitions occur seamlessly despite evidence that the neural control supporting these goals may be distinct. We know little about how neural control changes when transitioning between these distinct goals. Here, we examined how healthy participants corrected their movements when disturbed by mechanical perturbations (n = 40; 20 females) while planning, performing, and terminating a movement (Experiment 1). Hand displacement and muscle activity were measured to quantify feedback responses and assess changes in neural control. We found that participants were displaced less and generated larger muscle responses when disturbed during movement compared to when holding posture prior to movement or after reengaging posture control in the goal target. Additionally, peak displacements were smaller, and muscle responses were larger when reengaging posture control in the goal target compared to holding posture during the planning phase before movement. The contribution of each muscle varied across the phases of movement and was shaped by the level of activity when the disturbance occurred. Pre-existing muscle activity impacted the responsiveness of individual muscles. Therefore, we added a background load to observe how it altered the amplitude and scaling of corrective responses. Participants countered a background load that excited the elbow extensor muscles (Experiment 2). They also encountered the same perturbations as Experiment 1. When the background load was applied, it reduced peak displacements and increased the amplitude of muscle responses, particularly when the loaded muscle was stretched by the perturbation. Indeed, the background load increased sensitivity to proprioceptive information, especially when the loaded muscle was stretched by the perturbation. Overall, our findings demonstrate that the nervous system alters its neural control when engaging in movement from posture (and vice versa). The nervous system is more sensitive to sensory feedback when moving and the least sensitive to the same perturbations when maintaining posture before movement.