Lumbrical function: interaction of lumbrical contraction with the elasticity of the extrinsic finger muscles and its effect on metacarpophalangeal equilibrium.

Since conflicting statements have been made in the literature regarding the influence of lumbrical contraction on the metacarpophalangeal (MP) joint, a study was undertaken to determine the length-tension curve for the index flexor profundus. Four fresh cadaver hands were used. Measurements for the flexor superficialis and for the common extensor were determined in there. The measurements were correlated with measured displacement of these tendons in six finger positions. Isolated lumbrical contraction was then mechanically simulated, acting against spring homologues of the index extrinsic muscles. The finger moved from the rest position toward the intrinsic position with loads of less than 5 N. This demonstrated that in addition to its effect on interphalangeal joint extension, the lumbrical acting alone can cause flexion of the metacarpophalangeal joint.