Context-specific early recruitment of small motor units in the shoulder muscle reflects a reach movement plan.

Abstract

Understanding how motor plans are transformed into appropriate patterns of muscle activity is a central question in motor control. While muscle activity during the delay period has not been reported using conventional EMG approaches, we isolated motor unit activity using a high-density surface EMG signal from the anterior deltoid muscle to test whether heterogeneity in motor units could reveal early preparatory activity. Consistent with our previous work (Rungta et al., 2021), we observed early selective recruitment of small-amplitude size motor units during the delay period for hand movements like early recruitment of small-amplitude motor units in neck muscles of non-human primates performing delayed saccade tasks. This early activity was spatially specific and increased with time and resembled an accumulation to threshold model that correlated with movement onset time. Such early recruitment of ramping motor units was observed at the single trial level as well. In contrast, no such recruitment of large amplitude size motor units, called non-rampers, was observed during the delay period. Instead, non-rampers became spatially specific and predicted movement onset time after the delay period. Interestingly, spatially specific delay period activity was only observed for hand movements but was absent for isometric force-driven cursor movements. Nonetheless, muscle activity was correlated with the time it took to initiate movements in both task conditions for non-rampers. Overall, our results reveal a novel heterogeneity in the EMG activity which allows the expression of early motor preparation via small amplitude size motor units which are differentially activated during initiation of movements.