Stopping Muscle Contractions and Relaxations during Action Inhibition Involves Global and Targeted Control Dependent on Muscle State.

The mechanisms underpinning the stopping of muscle contractions and relaxations during action inhibition remain unclear. Central stop commands may be targeted and act on task-active muscles only, or instead be global, acting on task-passive muscles as well. We addressed this question in three stop signal task experiments with human participants (n = 54; 18 male, 36 female). While maintaining baseline force levels (10% MVC) in both index fingers, Go signals required participants to increase or decrease this force in the task-active finger (Task-active Contract vs Task-active Relax) while keeping activity in the task-passive muscle constant. On 30% of trials, delayed stop signals instructed participants to stop the task-active responses. Stop-related activity was detected in task-active muscles at the single trial level, using electromyography (EMG), and used to determine whether stop-related activity was also present in task-passive muscles. We found that stop commands act on both task-active and task-passive muscles, suggesting global control. This global control was furthermore muscle state specific, by decreasing muscle activity when stopping contractions and increasing muscle activity when stopping relaxations. However, stopping muscle contractions involved more sustained suppression of muscle activity in task-active than task-passive muscles, suggesting additional targeted control. This was not the case when stopping muscle relaxations, which only showed evidence of global control. Our results may explain how complex, real-world actions are inhibited. Global stop commands that are sensitive to muscle state may rapidly adjust muscle activity across the body, with additional control targeted to contracting, task-active muscles.