Electromyographic Study of the Extensor Apparatus of the Forearm A. A. TRAVILL’ Department of Anatomy, Queen’s University, Kingston, Ontario, Canada ABSTRACT An electromyographic study of the extensor apparatus of the forearm, using a multichannel electromyograph and bipolar needle electrodes, was carried out on 12 subjects. Recordings were obtained concurrently from medial, lateral and long heads of the triceps brachii and anconeus. The investigation confirms that the three heads contract independently during extension of the forearm, that the medial head and the anconeus are always active, and that the lateral head and, t o a lesser extent, the long head act as reinforcing auxillaries. The anconeus was found to be active also during resisted but not free loaded pronation and supination, contrary to commonly held opinion. During the past decade almost all of the major muscle groups of the upper limb have been studied electromyographically. However, the extensor apparatus of the elbow (triceps brachii and anconeus) has received only scant attention from electromyographers. Presumably, this neglect has been due to the apparent ease with which one can study these muscles by the classical methods of inspection and palpation. Undoubtedly the major function of all heads of the triceps is forearm extension, but the functioning of the individual heads cannot be stated with such certitude. What knowledge we do have about the action of each head appears to be based on the writings of Duchenne a century ago. Duchenne (1867) stated, and most modern authors reaffirm, that the isolated action of both the medial and lateral heads is identical and that the long head extends the forearm with much less force than the other two. He was also the first to suggest that the anconeus brought about abduction of the ulna during pronation in addition to being a n extensor. However, this opinion has not received universal acceptance. AS part of a continuing electromyographic study of the upper limb musculature and in order to establish more precisely the inter-relationships between the three heads of the triceps brachii and the anconeus the present study was undertaken. MATERIAL AND METHODS Twelve medical students (ten male and two female) volunteered to be the subjects for this study. Recordings were made on a special six channel Stanley Cox electromyograph which has been described elsewhere (Basmajian, ’62). The action potentials were picked up from the muscles with bipolar concentric needle electrodes that had been thrust into the middles of the bellies of the three heads of the triceps and the anconeus through previously anesthetized areas of skin. Each subject was examined sitting upright in a n electrostatically shielded room. Recordings were obtained during simple extension of the forearm while the arm was in each of the following positions: ( a ) full flexion at the shoulder, ( b ) hanging vertically half-way between flexion and extension, ( c ) in as full a position of extension at the shoulder as possible, and finally ( d ) in a position of 90” abduction from the chest. In each of the above positions two series of recordings were made during slow extension of the elbow first with no resistance offered, and second against the resistance of the observer’s grip. Finally, with the elbow flexed at 90” so that the forearm was lying horizontally at rest on a table top, slow pronation and slow supination both freely and against resistance were performed. The records obtained were analyzed and tabulated according to the system previously described (Travill and Basmajian, ’61). This method of analysis allows the observed action potentials to be categorized as “very slight” or “negligible,” “slight,” “moderate,” “ m a r k e d and finally “very 1 Present address: Department of Anatomy, Medical School, The Creighton University, Omaha 2, Nebraska. 373 3 74 A. A. TRAVILL marked.” The absence of activity was categorized as “nil” activity. 12) exhibited “slight” activity in the lateral head of the triceps brachii. I n the minority of subjects the muscle was elecOBSERVATIONS trically silent in all positions of the shoulMedial head of triceps brachii. The me- der. However, when extension was redial head was almost universally active dur- sisted, all subjects exhibited activity which ing extension of the forearm both during was either “moderate” or “marked.” Again the free movement and during extension there was no pattern of change in activity against resistance (fig. 1). Its activity associated with change of shoulder posiwas either “slight” or “moderate” when the tion. On comparing the activity of the latmovement was unresisted. When the move- eral head with the medial head during free ment was resisted the range of activity extension it was found that the former exwas greater, from “slight” to “marked.” hibited greater activity in a minority (6 of Though the activity of the medial head 48) movements, while equal activity was varied slightly with the different shoulder exhibited 14 times and less activity in the positions, there was no generally appreci- remaining majority ( 2 6 of 48). During able change and no distinct pattern of resisted movements the activity was equal in both muscles on every occasion. change was discernible. Long head. Regardless of the position Lateral head. During free extension of the elbow a majority of the subjects ( 9 of of the shoulder, the majority of unresisted A 1 2 3 4 Tm Fig. 1 Typical electromyograms illustrating A, unresisted extension of the forearm and B, resisted extension of the forearm. Channel 1, triceps brachii medial head; 2, lateral head; 3, long head; 4, anconeus. (Time marker: 0.01 sec. intervals.) ELECTROMYOGRAPHY O F FOREARM EXTENSORS movements (33 of 48) elicited “negligible” or “nil” activity from the long head. When activity was present it was “slight,” and again no pattern of change in activity could be discerned. During forced extension, however, action potentials were always recorded in all subjects from the long head. This activity ranged from “slight” to “marked and was always greater than during unresisted extension. It was only when the shoulder was abducted and resistance was offered to extension that one could observe comparable activity both in the long head and the medial head; at all other times the medial head was the more active of the two. Similarly the activity of the lateral head was greater than that of the long head whatever the position of the shoulder might be. 375 Anconeus. In eight of nine subjects tested, the anconeus was active during all movements of elbow extension whether free or resisted. The remaining subject exhibited “slight” activity and this was only during resisted extension. During unresisted pronation only one of the nine subjects examined showed electromyographic activity in anconeus. However, during resisted pronation all but two subjects showed “slight” or “moderate” activity. Similarly, during free supination only one subject exhibited “slight” activity while during resisted supination all but one subject exhibited activity ranging from “negligible” to “moderate” (fig. 2). The recordings showing the comparative activity of the anconeus and the individual heads of the triceps reveal no precise A 4 Tm 8 1 2 3 4 Fig. 2 Electromyograms showing activity during A, resisted pronation; B, resisted supination. Channel 1, triceps brachii medial head; 2, lateral head; 3, long head; 4, anconeus. (Time marker: 0.01 sec. intervals.) 376 A. A. TRAVILL firms essentially the classical view that the greatest activity in the anconeus occurs during extension. Also the anconeus acts during both pronation and supination whenever these actions are resisted, notwithstanding Da Hora’s (’59) electromyoDISCUSSION graphic findings that the anconeus is The present electromyographic study active during all forms of pronation and has resolved some of the outstanding prob- supination whether free or resisted. Filems relating to the function of the fore- nally, it would seem that discrepancies in arm extensor apparatus. One may state the detailed understanding of the funewith a certain confidence that it is the tion of the anconeus during pronation can medial head rather than the combined only be removed by precisely controlled medial and lateral heads as Duchenne electromyographic investigation of the suggested, that is the prime mover in muscle in all positions of the elbow and extension. As the medial head is the exten- during different types of movement. sor counterpart of the brachialis, described ACKNOWLEDGMENT as the “workhorse” of the flexors (Basmajian and Latif, ’57), it is only natural that The author wishes to acknowledge his it should be the chief extensor of the elbow indebtedness to Dr. J. V. Basmajian for joint. permission to carry out this project in his The present study confirms Duchenne’s laboratory and for his encouragement durviews that of the two superficial heads of ing all stages of the investigation. the triceps the long head was the less LITEEATUEE CITED powerful in extension. This lesser power of the long head is probably due to the Basmajian, J. V. 1962 Muscles Alive: Their Functions Revealed by Electromyography. The lack of fixation of the scapular origin and Williams and Wilkins Co., Baltimore, pp. the necessity of adducting of shoulder with 23-26. the forearm either flexed or extended. Too Basmajian, J. V., and A. Latif 1957 Integrated strong a contribution from the long head actions and functions of the chief flexors of the elbow: a detailed electromyographic analwould tend to give extension during adducysis. J. Bone Joint Surg., 3 9 A : 1106-1118. tion of the arm. Da Hora, B. 1959 0 “musculos anconeus.” ConThe current view of the action of the tribuiclo ao estudo da sua arquitetura e was anconeus appears to be correctly stated funq6es. Thesis, University of Recife, Recife, Brazil, p. 120. by Gardner, Gray and O’Rahilly (’60) who G. B. 1867 Physiology of Motion. write “It is believed to assist the triceps in Duchenne, Translated and edited by E. B. Kaplan (’59). extending the forearm. It may abduct the W. B. Saunders Co., Philadelphia and London, ulna during pronation.” This view seems pp. 94-96. to be based, in part, on the electromyo- Gardner, E., D. J. Gray and R. O’Rahilly 1960 Anatomy: A Regional Study of Human Strucgraphic work of Ray, Johnson and Jameture. Ibid., p. 173. son (’51) who noted that action potentials Ray, R. D., R. J. Johnson, and R. M. Jameson obtained from the anconeus during exten1951 Rotation of the Forearm: An Experimental Study of Pronation and Supination. J. Bone sion of the elbow were insignificant. HowJoint Surg., 33A: 993-996 ever, they were able to obtain significant Travill. A.. and .T. V. Basmaiian 1961 Electropotentials from the muscle throughout the myobaphy of -the Supinaiors of the Forearm. &hole of pronation. The present work conAnat. Rec., 139: 557-560. pattern of interrelated activity. However, during free extension of the elbow the anconeus was more active than the long head in the majority of recordings (26 of 36).