Biomechanical considerations of lumbricalis behavior in the human finger

This paper combines anatomic and electromyographic information in a mechanical analysis of the lumbricalis muscle's special significance in the human hand. Mathematical expressions are developed to include the passive and active tensions in the muscles controlling a hypothetical finger with a metacarpophalangeal joint and only one interphalangeal joint. Average anatomic data about tendon locations relative to the joint centers of rotation at each joint are employed along with expressions for the stiffness of each of the four muscles presumed to be controlling these joints so that the angle at each joint is defined in terms of the active and passive muscle tensions. These mathematical relationships are then used to explain the consistent electromyographic activity of the lumbricalis and emphasize the contributions made by its passive properties in normal hand motions. The conclusions drawn from the analysis of this two-joint finger model are valid for the normal finger because the two interphalangeal joints in the normal finger are coupled so they do not normally perform independent motions.