Early rise and persistent inhibition of electromyography during failed stopping

Reactively canceling movements is a vital feature of the motor system to ensure safety. This behavior can be studied in the laboratory using the stop signal task. There remains ambiguity about whether a "point-of-no-return" exists, after which a response cannot be aborted. A separate question concerns whether motor system inhibition associated with attempted stopping persists when stopping is unsuccessful. We address these two questions using electromyography (EMG) in two stop signal task experiments. Experiment 1 (n = 24) involved simple right and left index finger responses in separate task blocks. Experiment 2 (n = 28) involved a response choice between the right index and pinky fingers. To evaluate the approximate point-of-no-return, we measured EMG in responding fingers during the 100 ms preceding the stop signal and observed significantly greater EMG amplitudes during failed than successful stop trials in both experiments. Thus, EMG differentiated failed from successful stopping prior to the stop signal, regardless of whether there was a response choice. To address whether motor inhibition persists after failed stopping, we assessed EMG peak-to-offset durations and slopes (i.e., the rate of EMG decline) for go, failed stop, and successful stop (partial response EMG) trials. EMG peak-to-offset was shorter and steeper in failed stop trials compared to go and successful stop partial response EMG trials, suggesting motor inhibition persists even when failing to stop. These findings indicate EMG is sensitive to a point at which participants can no longer successfully stop an ongoing movement and suggest the peak-to-offset time of response-related EMG activity during failed stopping reflects stopping-related inhibition.