COORDINATION OF INDEX FINGER MOVEMENTS

The purpose of this investigation was to describe the patterns of coor dination among the joint motions of the index finger, and among the EM Gs of index finger muscles. Index finger movements involving all three joints were varied in speed and direction. Joint motions were recorde d along with fine-wire EMG from all the muscles that insert into the i ndex finger. We observed nearly linear relationships for angular posit ion between the two interphalangeal (IP) joints, and between the metac arpophalangeal (MP) and proximal IP (PIP) joints regardless of movemen t, speed and direction. The activities of the extrinsic flexors were o f similar magnitude and were highly correlated when they acted as agon ists but were poorly correlated when they acted as antagonists to the movement. Extrinsic extensor muscles behaved in this way also. The act ivation patterns of the intrinsic musculature correlated weakly except for extension movements voluntarily limited to the IP joints. We conc lude that the highly coordinated action of the extrinsic flexors durin g flexion contribute importantly to the linked motions of the IP joint s in part because these muscles span two or all the three index finger joints. Hence, interjoint movement patterns appear not to arise solel y from restraints imposed by passive tissues, especially for fast flex ion movements. The weakly correlated intrinsic muscle activity does no t uncouple the flexion motions at the PIP and DIP joints because these muscles exert extensor torques at both IP joints. However, the action s of the intrinsic muscles are necessary for stabilizing the MP joint in flexion postures during IP motion and in producing motions voluntar ily limited to the MP joint.