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Electromyographic study of the extensor apparatus of the forearm.

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Electromyographic Study of the Extensor Apparatus
of the Forearm
Department of Anatomy, Queen’s University, Kingston, Ontario, Canada
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.
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.
3 74
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
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.)
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.
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
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.)
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
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
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
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
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).
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electromyography, stud, forearm, apparatus, extensor
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