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 delayed period has not been reported using conventional EMG approaches, we isolated motor unit activity using high-density surface EMG signal in the anterior deltoid muscle of the shoulder to test whether heterogeneity in motor units could reveal early preparatory activity. Consistent with previous work (Rungta et al., 2021) which showed early recruitment of small-amplitude motor units during the delay period in neck muscles of non-human primates performing delayed saccade tasks, we observed similar early selective recruitment of small- amplitude motor units. The ramping activity was spatially specific and could explain movement onset times as revealed by an accumulation to threshold model. Early recruitment of ramping motor units was also observed at a single trial level. In contrast, no such recruitment of large-amplitude motor units, called non-rampers, was observed during the delay period. Instead, they became spatially specific and predicted movement onset time after the delayed period. Interestingly, spatially specific delay period activity was only observed for delayed hand movements but was absent for delayed isometric force-driven cursor-based 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 in small motor units which are differentially activated during the initiating hand and cursor- based movements.
Significance statement We studied the spatial and temporal aspects of response preparation in the anterior deltoid muscle using high-density surface EMG. Our results showed that early spatially specific ramping activity that predicted reaction times could be accessed from muscle activity but was absent during isometric force-driven cursor-based movements. Such ramping activity could be quantified using an accumulator framework across trials, as well as within single trials but was not observed in isometric reach tasks involving cursor movements.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Abstract and text updated