Motor planning, initiation and execution of shoulder abduction against gravity: Evidence from startReact

Stroke affects the sensorimotor control of the upper extremity, limiting functional movements which require coordination between the shoulder and distal joints. StartReact has been used to investigate motor planning, independent of motor initiation and execution, but it is unclear if observed deficits in motor planning at the shoulder are due to task requirements or stroke-related impairments. Our aim was to decouple these factors by studying motor planning of shoulder movement in participants who have not had a stroke. Participants performed unilateral shoulder abduction movements from two initial positions in response to auditory cues. Muscle activity was recorded from the sternocleidomastoid (SCM) muscles as an indicator of startReact. Middle deltoid (DELT) activity and shoulder abduction position were also recorded. We hypothesized that unilateral shoulder abduction could be triggered by a loud acoustic stimulus (LAS) used to study startReact. We further hypothesized that these LAS-triggered movements would be initiated at shorter onset latencies but otherwise would not differ in movement timing or kinematics compared to volitional movements as evidence of motor planning. We found a significantly higher probability of early SCM activity with LAS delivery, consistent with evidence of motor planning for unilateral shoulder abduction from both initial positions. Shoulder abduction onset, movement duration, and DELT onset were shorter and peak velocity was greater with LAS-triggered movement compared to voluntary movement for both initial positions. Differences in the displacement between LAS-triggered and voluntary movements depended on task training. Overall, our findings further knowledge of shoulder sensorimotor control and highlight challenges with using startReact to investigate stroke-related movement impairments.