MUSCULAR PRODUCTION OF INDIVIDUATED FINGER MOVEMENTS - THE ROLES OF EXTRINSIC FINGER MUSCLES

Individuated finger movements-those in which one or more fingers are m oved relatively independently of the movement or posture of other body parts-are produced in part by the action of the extrinsic finger musc les, Flexion/extension movements of the fingers are particularly depen dent on these extrinsic muscles, most of which are multitendoned. How can contraction of multitendoned muscles move one digit without produc ing equivalent motion in other digits? This question was addressed by recording EMG activity from muscles of the forearm as trained rhesus m onkeys performed flexion and extension individuated movements of each digit of the hand and of the wrist. Recordings showed that during move ments of different fingers, a given muscle could act as an agonist, an tagonist, or stabilizer of the digits it serves. Furthermore, during a given finger movement, several different muscles typically were activ e. A three-level connection model was developed that computed the rela tive motion of the digits during each finger movement based on the cha nges in EMG activity in the recorded muscles. The model showed that EM G activity changes in the extrinsic finger muscles, and the thenar mus cles, could account for most of the motion of both the instructed digi t and noninstructed digits. These results indicate that individuated f inger movements were produced not by independent sets of muscles actin g 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 mov ement of one digit more than others.