How the lumbrical muscle contributes to placing the fingertip in space: a three dimensional cadaveric study to assess fingertip trajectory and metacarpophalangeal joint balancing

Three-dimensional fingertip trajectory was examined under different force levels of the lumbrical muscle to clarify the function of the lumbrical muscle in free index finger motion. The metacarpophalangeal joint balancing effect of the lumbrical muscle in the thumb-up position was also examined. The motions of the finger bones were recorded during simulated contraction of flexor digitorum profundus when different forces (0.000–1.960 N) were applied to the lumbrical muscle in cadaveric specimens. The greater the force with which the lumbrical muscle was pulled, the larger the arc formed by the fingertip, and the greater the rebalancing influence on the metacarpophalangeal joint. This result indicates that the lumbrical muscle functions simultaneously to enlarge the fingertip trajectory and to balance the metacarpophalangeal joint against gravity in the axial plane. A 0.980 N force was ideal for maximal finger movement. The lumbrical muscle rebalanced the metacarpophalangeal joint against gravity in the thumb-up position with a force ⩾0.980 N.