Lumbrical muscle function as revealed by a new and physiological approach.

The lumbrical muscle is clearly one of several possible extensors of the interphalangeal joints. With an origin on the flexor digitorum profundus tendon it is credited with unloading the elastic tension across the interphalangeal joints and thereby facilitating their extension. Its role at the metacarpophalangeal joint is not a matter of universal agreement. Attempts to simulate its action with weights over pulleys have not clarified this role, since true simulation would require the development of a means of applying force along the course of the lumbrical without pre-determining which end would move. Such a system is herein described; it uses a Bowden cable, which is commonly used to activate the brakes of a bicycle. After constructing length-tension curves of the profundus muscle in four fresh cadavers prior to the onset of rigor mortis, the interaction of realistic lumbrical loads with profundus elastic tension was studied. By contraction a lumbrical muscle adds a small but significant flexor force at the metacarpophalangeal joint, and thereby it is also capable of contributing to radial deviation and possibly rotation. As it runs from a flexor tendon to an extensor tendon and is endowed with a great many muscle spindles, the lumbrical could play a part in the control of finger movement by monitoring the rate of hand closing during grasp.