Individuated finger movements--those in which one or more fingers are moved relatively independently of the movement or posture of other body parts--are produced in part by the action of the extrinsic finger muscles. Flexion/extension movements of the fingers are particularly dependent on these extrinsic muscles, most of which are multitendoned. How can contraction of multitendoned muscles move one digit without producing equivalent motion in other digits? This question was addressed by recording EMG activity from muscles of the forearm as trained rhesus monkeys performed flexion and extension individuated movements of each digit of the hand and of the wrist. Recordings showed that during movements of different fingers, a given muscle could act as an agonist, antagonist, or stabilizer of the digits it serves. Furthermore, during a given finger movement, several different muscles typically were active. A three-level connection model was developed that computed the relative motion of the digits during each finger movement based on the changes in EMG activity in the recorded muscles. The model showed that EMG activity changes in the extrinsic finger muscles, and the thenar muscles, could account for most of the motion of both the instructed digit and noninstructed digits. These results indicate that individuated finger movements were produced not by independent sets of muscles acting on each digit, but by the activity of several muscles, many of which act on more than one digit, combined such that the net effect was movement of one digit more than others.
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