Spatiotemporal distribution of location and object effects in the electromyographic activity of upper extremity muscles during reach-to-grasp

In reaching to grasp an object, proximal muscles that act on the shoulder and elbow classically have been viewed as transporting the hand to the intended location, while distal muscles that act on the fingers simultaneously shape the hand to grasp the object. Prior studies of electromyographic (EMG) activity in upper extremity muscles therefore have focused, by and large, either on proximal muscle activity during reaching to different locations or on distal muscle activity as the subject grasps various objects. Here, we examined the EMG activity of muscles from the shoulder to the hand, as monkeys reached and grasped in a task that dissociated location and object. We quantified the extent to which variation in the EMG activity of each muscle depended on location, on object, and on their interaction—all as a function of time. Although EMG variation depended on both location and object beginning early in the movement, an early phase of substantial location effects in muscles from proximal to distal was foll...