Coordination of index finger movements.

The purpose of this investigation was to describe the patterns of coordination among the joint motions of the index finger, and among the EMGs of index finger muscles. Index finger movements involving all three joints were varied in speed and direction. Joint motions were recorded along with fine-wire EMG from all the muscles that insert into the index finger. We observed nearly linear relationships for angular position between the two interphalangeal (IP) joints, and between the metacarpophalangeal (MP) and proximal IP (PIP) joints regardless of movement, speed and direction. The activities of the extrinsic flexors were of similar magnitude and were highly correlated when they acted as agonists but were poorly correlated when they acted as antagonists to the movement. Extrinsic extensor muscles behaved in this way also. The activation patterns of the intrinsic musculature correlated weakly except for extension movements voluntarily limited to the IP joints. We conclude that the highly coordinated action of the extrinsic flexors during flexion contribute importantly to the linked motions of the IP joints in part because these muscles span two or all the three index finger joints. Hence, interjoint movement patterns appear not to arise solely from restraints imposed by passive tissues, especially for fast flexion movements. The weakly correlated intrinsic muscle activity does not uncouple the flexion motions at the PIP and DIP joints because these muscles exert extensor torques at both IP joints. However, the actions of the intrinsic muscles are necessary for stabilizing the MP joint in flexion postures during IP motion and in producing motions voluntarily limited to the MP joint.