A biomechanical simulation of the effect of the extrinsic flexor muscles on finger joint flexion

The mechanisms responsible for producing flexion of the metacarpophalangeal (MCP) joints of the fingers remain a matter of debate. Especially, the role of the extrinsic finger flexor muscles has been questioned. Using computer simulation techniques, we sought to determine if it is possible for the extrinsic flexor muscles to produce rotation at all three finger joints and to initiate rotation at the MCP joint. A planar open-link chain with three revolute joints and four links was created to model the index finger. The tendons from the extrinsic flexor muscles were simulated as ropes passing through pulleys. Passive joint stiffness and damping values were obtained from system identification experiments involving the input of angular perturbations to the joint of interest and the measurement of the resulting resistance torque. Simulation output revealed that in the absence of passive joint stiffness and damping, shortening of the extrinsic flexor tendons actually resulted in slight extension of the MCP joint. However, with the inclusion of stiffness and damping, tendon shortening produced simultaneous rotation of all three joints. These results suggest that the extrinsic flexor muscles may play a primary role in controlling MCP flexion.