ON THE INTERPRETATION OF THE ELECTROMYOGRAM FROM VOLUNTARY AND REFLEX CONTRACTIONS

1. The electromyogram from voluntary contractions of the muscles of the forearm is made up of two types of waves, the large or primary waves, called Type A waves by Wachholder, occurring at frequencies varying from 5 to 65 per second, and small secondary waves, Wachholder's Type B waves, occurring at frequencies between 180 and 250 per second. Besides these two types, we may also distinguish on the large waves irregular projections—Nebenzacken. 2. The primary waves increase in frequency as the intensity of the contraction increases. Their amplitude also increases with the contraction, but only to a certain stage, beyond which the amplitude becomes smaller again. At a maximum contraction the amplitude is smaller than it is just before the maximum is reached. 3. The secondary waves usually do not appear in the electromyograms unless a tight string is used. Then they may be seen distributed fairly evenly throughout the curve. Their presence is most conspicuous, and they can be most easily counted, however, in the troughs between the large waves. They increase in amplitude as the contraction increases, but their amplitude is never very great compared with that of the primary waves. This is contrary to Wachholder's belief that at the height of the contraction their amplitude may become just as great as that of the primary waves. 4. The secondary waves must be distinguished from the irregular projections or Nebenzacken in the large waves. These can be studied best with a loose string and the secondary waves with a tight string. The presence of the Nebenzacken appears to depend on the low frequency of the primary waves. They are conspicuously present with frequency near 40 per second, and then gradually disappear, until at 60 per second they are entirely absent. 5. An attempt was made to determine the origin of these various types of waves, in order to see what light they might throw on the nature of the impulses arriving at the muscle during voluntary contraction. Records obtained from the same muscles of the forearm during the reflex grasping response of the human infant showed very regular, large waves with a 60 to 75 per second frequency. Because of the fact that the grasping reflex response is present long before voluntary activity begins, the results suggest that the large waves must have their origin somewhere within the reflex arc. 6. It was then shown how, on the basis of the results of the recent work of Adrian and Zottermann, the origin of the large waves could be explained almost in every detail in terms of the impulses arriving at the muscle from the proprioceptive end-organs. The end-organs discharge at approximately the same frequency, and also increase in frequency as the stretch applied to the muscle is increased. 7. The irregular projections (NebenZacken) superposed on the large waves may indicate that the proprioceptive impulses are out of phase, and their absence at frequencies near 60 per second may mean that the impulses are all in phase. 8. It is demonstrated that the frequency at which the large waves from the muscles come into phase seems to depend on the length of the arc rather than on an inherent rhythm of the anterior horn cells as suggested by Piper and others. The shorter the arc the higher the frequency. Thus in the masseter muscle, with a short arc, the frequency is high, near 100 per second, while in the tibialis anticus, with its longer reflex arc, the frequency is low, near 40 per second. 9. Our data regarding the origin of the secondary waves are very meagre, but the results we have so far obtained suggest that these waves have their origin in the mechanisms involved in the production of voluntary activity. 10. If the theory developed in this paper regarding the origin of the primary and secondary waves is correct, then every voluntary activity must be dependent to a large extent on impulses from the proprioceptive end-organs as Hoffmann has suggested. We wish to acknowledge our indebtedness to the Department of Obstetrics and Pediatrics of the Johns Hopkins Hospital for their co-operation in this work.