A concept of finger motion and control, based on electromyographic observations in 868 muscles in 256 normal, adult subjects, is presented. This concept is a logical extension of the hypothesis previously reported10.
Hand control depends on active contractile forces during both lengthening and shortening of the muscles, on the viscoelastic properties of all of the tissues of the hand and forearm, especially the muscles, and on the anatomical configuration of related structures. The control mechanism can be described in terms of the contributions of the several structures during the process of reaching or holding the four common, terminal positions of the fingers.
1. The fully extended finger: To reach or hold the position of full extension of all joints, contraction of both the extrinsic and intrinsic muscles is required. The extensor digitorum and lumbricalis actively contract of provide the extending force. The lumbricalis is particularly effective in interphalangeal extension since it pulls the flexor digitorum profundus tendon distally, relieving the interphalangeal joints of the passive flexing force produced by the viscoelastic properties of the profundus.
2. The clawed finger: To reach or hold the position of interphalangeal flexion and metacarpophalangeal extension, contraction of only the extrinsic muscles is required. The flexor digitorum profundus (and to a lesser extent the flexor digitorum superficialis) and the extensor digitorum are the active muscles.
3. The fully flexed finger: To reach or hold the position of full flexion of all joints, contraction of only the extrinsic muscles is required. The active muscles are the flexor digitorum profundus (and to a lesser extent the flexor digitorum superficialis) and the extensor digitorum. The actively contracting muscles are therefore the same here as in the clawed position. The ability of the finger to use these same muscles to reach the fully flexed position depends upon: the lengthening contraction of the extensor digitorum as the finger is fully flexed permitting controlled metacarpophalangeal flexion and the increased tension in the oblique track, primarily the result of stretching of the bi-articular interossei.
4. Metacarpophalangeal flexion with the finger straight: To reach or hold the position of interphalangeal extension and metacarpophalangeal flexion, contraction of the intrinsic muscles is required, and to a lesser extent contraction of the extrinsic extensor. The motion is performed by contraction of the lumbricalis and the interossei. Of the flexion-extension motions, only those reaching or holding this position are associated with action of the interossei as a group. Activity of the extensor digitorum, a frequent accompaniment of this motion, assists interphalangeal extension and prevents hyperflexion of the metacarpophalangeal joint.
The lumbricalis emerges as an extremely consistent performer in all individuals and fingers. The interossei have certain group characteristics, but vary moderately from finger to finger and from subject to subject. This paper has described these variations and presented the supporting data; lumbricoid and pure metacarpophalangeal flexor variations of interosseus behavior are noted. The flexor digitorum profundus consistently overshadows the superficialis, except in the index finger where the superficialis sometimes predominates. The extensor digitorum and the extensors proprii appear to have identical functions. The muscles of the hypothenar eminence do not contribute significantly to flexion-extension motions of the small finger, but the opponens digiti minimi may participate as a depressor of the fifth metacarpal during closing motions.