The effect of finger extensor mechanism on the flexor force during isometric tasks

The role of the intrinsic finger flexor muscles was investigated during fin ger flexion tasks. A suspension system was used to measure isometric finger forces when the point of force application varied along fingers in a dista l-proximal direction. Two biomechanical models, with consideration of exten sor mechanism Extensor Mechanism Model (EMM) and without consideration of e xtensor mechanism Flexor Model (FM), were used to calculate forces of extri nsic and intrinsic finger flexors. When the point of force application was at the distal phalanx, the extrinsic flexor muscles flexor digitorum profun dus, FDP, and flexor digitorum superficialis, FDS, accounted for over 80% o f the summed force of all flexors, and therefore were the major contributor s to the joint flexion at the distal interphalangeal (DIP), proximal interp halangeal (PIP), and metacarpophalangeal (MCP) joints. When the point of fo rce application was at the DIP joint, the FDS accounted for more than 70% o f the total force of all flexors, and was the major contributor to the PIP and MCP joint flexion. When the force of application was at the PIP joint, the intrinsic muscle group was the major contributor for MCP flexion, accou nting for more than 70% of the combined force of all flexors. The results s uggest that the effects of the extensor mechanism on the flexors are relati vely small when the location of force application is distal to the PIP join t. When the external force is applied proximally to the PIP joint, the exte nsor mechanism has large influence on force production of all flexors. The current study provides an experimental protocol and biomechanical models th at allow estimation of the effects of extensor mechanism on both the extrin sic and intrinsic flexors in various loading conditions, as well as differe ntiating the contribution of the intrinsic and extrinsic finger flexors dur ing isometric flexion. (C) 2001 Elsevier Science Ltd. All rights reserved.