An anatomical study of finger movements has been reviewed. It has been shown that the function of a muscle with respect to a certain joint cannot be inferred from the position of the muscle with respect to that one joint alone. In the fingers, especially, the tendons bridge more than one joint; a number of joints must be analyzed before it can be determined which part of the system can be investigated as a separate unit. For the finger one such unit is represented by the two interphalangeal joints; the other, by the biarticular system composed of the metacarpophalangeal and the proximal interphalangeal joints. For such a polyarticular system the conditions of equilibrium must be defined.

The interphalangeal joints comprise one biarticular system in which the middle phalanx—the interealated bone—tends to assume a position of flexion, provided the extensor apparatus and the flexor profundus muscle are active. By this movement into flexion the middle phalanx constantly loads the central tendon of the extensor mechanism. The coordinated pattern of movement of the interphalangeal joints has been analyzed on the basis of this relationship to the extensor apparatus.

The metacarpophalangeal and the proximal interphalangeal joints compose another biarticular system in which the interealated bone is the proximal phalanx. If this bone were just under the control of the extensor apparatus and one or both flexors only, it would tend to go into extension (extension of the metacarpophalangeal joint; flexion of the proximal interphalangeal joint). Normally, however, the interossei and lumbricales counterbalance this effect and bring the system into equilibrium.

An analysis has been made as to why abnormal hyperextension of the proximal interphalangeal joint interferes with the normal patterns of finger movement. It has been shown that this change in the habitual position of the interealated bone seriously affects the normal function of the finger.