Relations between surface EMG of extrinsic flexors and individual finger forces support the notion of muscle compartments.

The goal of this experiment was to investigate the relationship between individual fingertip forces and the surface EMG of multi-digit muscles. The surface EMG of the hand extrinsic flexors (flexor digitorum profundis and superficialis) was recorded in eight subjects during multi-digit force production tasks. In one session, subjects pressed with all four fingers (IMRL, I = index, M = middle, R = ring, and L = little finger) with the total force ranging from 10% and 90% of their maximum force (MVC). Results showed a close linear relationship between an integrated EMG index and force. In another session, subjects produced constant total force of either 10% or 30% of their IMRL MVC, with different finger combinations such that the degree of involvement of each finger was manipulated (15 finger combinations were tested). The EMG level of the flexors depended greatly on the finger combination (P < 0.001). Multi-variable regression made it possible to describe the flexor EMG as a linear function of individual fingertip forces. These results suggest that: (1) hand extrinsic flexors muscles are arranged in functional compartments serving individual fingers, and (2) each compartment has a force/EMG relationship that is close to being linear.