The Functions of the Muscles of the Thumb, the Index, and Long Fingers

SYNCHRONOUS RECORDING OF MOTIONS AND ACTION POTENTIALS OF MUSCLES

¹ From the Orthopaedic Hospital, Los Angeles

A single-process, synchronized system for simultaneous recording of both motion of body parts and the action potentials of several muscles producing the motion is described. The method owes its advantages of simplicity and efficiency to the dual-lens, rotating prism-shutter motion-picture camera. This instrument operates over a range of five to 11,000 frames per second and superimposes on film moving at constant speed, sequential photographs of the moving parts and continuous oscillograms. Fine control of recording speed makes possible closely grouped series of action potentials approaching in effect an integrated form of the record. Thus a few frames may be projected or printed as charts showing, in a very short space, complete movements and the pertinent electrical activity.

Motions of the thumb, index and long fingers were studied, recording the electrical activity of six to eight muscles of each. Isolated as well as functional movements were analyzed.

Some observations were made concerning intrinsic muscle behavior. The activity of the lumbricals during interphalangeal extension was apparent, but lack of discernible difference in the actions of the index finger after local anesthetic block of the motor nerve of the first lumbrical cast serious doubt on the functional significance of this muscle. Differences in behavior of the first dorsal interosseus compared with that of the remaining interossei were observed. Owing to the difference in its insertion, which is confined to the proximal phalanx, strong electrical stimulation of this muscle produced metacarpophalangeal-joint abduction and flexion, but no evidence of interphalangeal extension whatsoever. Other interossei demonstrated action potentials on interphalangeal extension in some situations.

Special studies of the components of the flexor digitorum sublimis muscle were also done. The components of this muscle's superficial layer, those for the ring and long fingers, as well as the three usually discrete components of the deep layer, the trigastric muscle, containing components to the index and little fingers together with a proximal belly common to both of these digits, were individually penetrated with electrodes and electrically stimulated, then records were made.

The advantage of the system used is that all necessary data—motion-picture records of movement and synchronized displays of the electrical activity of several participating muscles—are simultaneously acquired and recorded in a single process. Since the dual-lens, rotating prism-shutter moving-picture camera may be operated at very slow speeds, the original photographic data serve as excellent chart material, after enlargement of the photographs, as well as material for repetitive study as projected moving-picture films.

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