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Electromyography of brachial muscles in Pan troglodytes and Pongo pygmaeus.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 61:75-83 (1983)
Electromyography of Brachial Muscles in Pan troglodytes and
Pongo pygmaeus
RUSSELL H. TU'ITLE, MARGARET J. VELTE, AND JOHN V.
BASMAJIAN
Department of Anthropology, the University of Chicago, Chicago, Illinois
60637 (R.H. T,M.J. V), and Rehabilitation Centre, Chedoke Hospitals,
McMaster University School of Medicine, Hamilton, Ontario, Canada U N
3L6 (J. VB.)
KEY WORDS Electromyography, Arm muscles, Chimpanzees,
Orangutan, Knuckle-walking, Suspensory behavior
ABSTRACT
Electromyographic recordings were taken from all heads of
the triceps brachii and biceps brachii muscles and from the anconeus, brachialis, and brachioradialis muscles in a chimpanzee and an orangutan as they
stood still and walked quadrupedally on horizontal and inclined surfaces,
engaged in suspensory behavior, reached overhead, and manipulated a variety
of foods and artifacts.
Like the gorilla (Tuttle and Basmajian, 1974a1, the chimpanzee and orangutan possess special close-packed positioning mechanisms that allow the bulky
muscles that cross their elbow joints to remain silent during quiet pendant
suspension.
We found no major myological features that would dramatically separate the
arms of knuckle-walking African apes from those of the orangutan. With a few
exceptions, which could as well be attributed to individual variation as to
interspecific differences, the brachial muscles acted similarly during quadrupedal positional behaviors, irrespective of whether the hands of the subjects
were knuckled (African apes), fisted (chimpanzee and orangutan), or placed in
modified palmigrade postures (orangutan).
Evolutionary transformations, from brachial and elbow complexes like those
of Pongo to ones like Pan, or vice versa, would probably be achieved quite
readily as the species changed its substrate preferences and positional habits.
We report here results of electromyographic (EMG) studies on the brachial muscles of a common chimpanzee (Pan
troglodytes) and a Sumatran orangutan
(Pongo pygmaeus). The experiments were
conducted to determine which muscles are
principally and otherwise active during
quadrupedal walking, suspensory behavior,
and manipulation and in order to infer the
extent to which muscle activity and closepacked positions (MacConaill and Basmajian, 1969, pp. 25-26), or other osseoligamentous relations are implicated to maintain the
elbow joint during particular positional behaviors. This study complements our report
on the brachial muscles of Pan gorilla (Tuttle
and Basmajian, 1974a).
0 1983 ALAN R. LISS, INC.
METHOD
Eight EMG recording sessions were conducted during a 14.6-month span on the right
arm of a captive-born male chimpanzee which
was between 4.25 and 5.4 years old and
weighed 17.9-29.1 kg. Six EMG experiments
were performed over the same period on the
right arm of a captive-born female orangutan
which was between 6.25 and 7.4 years old
and weighed 23.0-30.2 kg.
Indwelling fine-wire bipolar electrodes
were used according to procedures that have
been described in previous papers (Tuttle and
Basmajian, 1974a,b). The electrodes were
Received October 8, 1982; accepted December 15,1982.
76
R.H. TUTTLE, M.J. VELTE, AND J.V. BASMAJIAN
placed centrally in the bellies of the muscles.
The subjects could move freely in the testing
area and had the opportunity to climb on a
trapeze and reach for foods suspended on
strings from the ceiling, attached high on the
walls, and held overhead by a n investigator
(Fig. 1; Tuttle and Basmajian, 1978; Tuttle
et al., 1979).
The number of experiments during which
each muscle was recorded is listed in Table
1. The quality of the records is good to excellent.
RESULTS
Triceps brachii
Prominent activity was exhibited by the
long head of the triceps brachii muscle early
in the experiments, when the subjects were
still groggy. Maximum EMGs were registered during elbow extensions when the animals attempted to rise to quadrupedal stance
from prone and crouched postures.
Before full recovery from anesthesia,
marked and moderate potentials characterized the long head of the triceps brachii muscle during the load-bearing phases of
knuckle-walking (chimpanzee), fist-walking
(both subjects), and modified palmigrade progression (orangutan). Marked potentials also
occurred during load-bearing phases in a
short bout of crutch-walking by the orangutan.
Once the subjects had recovered fully,
slight and negligible EMG potentials characterized the long head of the triceps brachii
muscle during quadrupedal progression on
the floor. The EMG activity commenced a t
midswing, as the elbow extended to place the
hand forward, and continued through the
load-bearing segment of the stance phase.
This is true of chimpanzee knuckle-walking
and fist-walking by both subjects. High EMG
activity occurred during load bearing as the
chimpanzee walked quadrupedally up and
down a ramp (with a slope of 17.8") and descended from its summit (51 cm high) directly to the floor with weight borne by his
extended forelimb. Negligible and slight potentials accompanied the chimpanzee's stationary quadrupedal postures on the floor.
There were no detectable differences in the
activities of the long head of the triceps brachii muscle when he stood with his hand
fisted versus knuckled.
The long head of the triceps brachii muscle
acted a t low levels during most manipulatory behaviors in which the elbow was ex-
tending. For instance, slight potentials
occurred as the orangutan rubbed a towel on
the floor and a platform; negligible potentials were recorded as she picked up candies
from the floor; and negligible and slight potentials were produced as she slowly lifted a
large, light plastic ball overhead.
In the chimpanzee, there were variable levels of activity in the long head of the triceps
brachii muscle as he reached overhead for
the food sill and trapeze. Nil or negligible
EMGs accompanied the initial elbow extension of a reach and then EMG activity increased as the goal was approached. In the
orangutan, prominent EMGs occurred as she
extended her elbow while reaching overhead
for the trapeze. However, when she simply
raised her extended forelimb a t the shoulder
joint, low potentials were recorded.
In the orangutan, the muscle was silent as
she hoisted herself and hung bimanually and
unimanually on the trapeze (Fig. 2). Nil EMG
activity also characterized the long head of
the triceps brachii muscle during the suspensory behavior of the chimpanzee except during one experiment in which some negligible
potentials occurred intermittently during a
bout of pendant suspension.
Overall the patterns of activity in the medial head of the triceps brachii muscle closely
resemble those of the long head in both subjects.
High potentials were common as groggy
subjects rose to quadrupedal positions and
during load-bearing phases of knuckle-walking, fist-walking, and crutch-walking early
in the experiments. Later, only negligible and
slight potentials were recorded from the medial head of the triceps brachii muscle as the
subjects progressed quadrupedally on the
floor. Once, high EMGs occurred during
stance phases as the orangutan pressed a
large ball between her volarflexed hand and
forearm, and walked on her elbow instead of
her hand.
Early in the experiments, the chimpanzee
exhibited sustained moderate or marked
EMGs during quadrupedal and tripedal
stances. But later in the sessions, counterpart stances were accompanied by slight potentials. In the orangutan, tripedal and
quadrupedal stances elicited slight or moderate EMGs in the medial head of the triceps
brachii muscle.
Similar EMG levels occurred when the
seated orangutan leaned forward and bore
weight on her fully extended forelimb. Once,
EMG OF BRACHIAL MM. IN CHIMPANZEE AND ORANGUTAN
TABLE 1. Number of experiments in which each muscle was recorded
Pan
troglodytes
M. triceps brachii, caput longum
M. triceps brachii, caput mediale
M. triceps brachii, caput laterale
M. anconeus
M. brachialis
M. biceps brachii, caput longum
M. biceps brachii, caput breve
M. brachioradialis
Fig. 1. Chimpanzee (a) knuckle-walking down ramp
(note flexion set of elbows) and unimanually hoisting
himself on a narrow sill (b) and a trapeze (c). Orang-
Pongo
pygmaeus
4
2
5
5
1
1
5
4
4
5
5
5
utan (d) fist-walking and unimanually hoisting herself
on the trapeze (e,f).
77
R.H. TUTTLE, M.J. VELTE, A N D J.V. BASMAJIAN
78
a
,,,..,,,,.,I
.,..,..,,,,,,.
Fig. 2. EMG recording of brachial muscles in an orangutan while she hoisted herself unimanually until her
elbow was approximately orthogonal (first 5 seconds)
and then sustained the posture (seconds 5-14) as she
cautiously groped for fruit in a pan that was attached
near the ceiling. Her hindlimbs hung pendant and her
.
.,..
/,,.
,,.
. .,.. . , .
/,
right hand was semiprone on the trapeze bar (as in
Fig. 10. Thereafter (seconds 14-15) she descended to
the floor. Symbols: (a)M. triceps brachii, caput longum,
6)M. brachialis, (c) M. biceps brachii, caput longum,
(d) M. brachioradialis, (el biceps brachii, caput breve.
t, time in seconds.
the crouched chimpanzee leaned forward and subjects propped themselves from prone to
propped himself with the fully extended fore- sitting and quadrupedal postures, and durlimb, thereby producing sustained moderate ing the load-bearing phases of knuckle-walkEMG potentials in the medial head of his ing (chimpanzee) and fist-walking (both
triceps brachii muscle. A burst of moderate subjects) early in the experiments. Later,
EMGs occurred as he dropped from a bipedal negligible and slight potentials characterposture onto his forelimbs.
ized quiescent quadrupedal and tripedal
As the subjects reached overhead for the stances, and the second half of swing and the
trapeze, initially low EMGs were produced stance phases of quadrupedal progression.
by the medial head of the triceps brachii There were no differences in EMG patterns
muscle. Then, EMG potentials increased to of the lateral head of the triceps brachii musmoderate (both subjects) or marked (chim- cle between knuckle-walking and fist-walkpanzee) as the goal was approximated. Very ing (chimpanzee) or between fist-walking and
high potentials were produced when the modified palmigrade progression (orangutan)
chimpanzee vigorously shook the trapeze bar on the floor. High EMGs occurred in the latby alternately pronating and supinating his eral head of the triceps brachii muscle as the
chimpanzee eccentrically loaded his exhand with the forearm extended (Fig. 3).
The medial head of the triceps brachii mus- tended forelimb while stepping off a platform
cle was silent during all bouts of hoisting and (51 cm high) onto the floor.
pendant suspension by both subjects.
In the chimpanzee, negligible and slight
The lateral head of the triceps brachii mus- potentials occurred as he reached for the tracle exhibited marked potentials as the groggy peze. In the orangutan marked EMGs were
EMG OF BRACHIAL MM. IN CHIMPANZEE AND ORANGUTAN
79
Anconeus
The anconeus muscle was highly active
early in the experiments as the knucklewalking and fist-walking chimpanzee bore
weight on his forelimb. Later low EMGs
characterized the stance phases of his quadrupedal locomotion on the floor and ramp.
Unlike the triceps brachii muscle, the anconeus muscle was not active during the latter
part of swing phase when the subject was
fully alert. It exhibited slight EMGs as the
chimpanzee stood quietly quadrupedally and
tripedally and as he eccentrically loaded his
extended forelimb while descending from the
summit of the ramp onto the floor. His anconeus muscle was negligibly active as he progressed quadrupedally while sliding his
knuckles on the floor.
Marked EMGs were recorded from the
chimpanzee’s anconeus muscle during concurrent elbow extension and supination of
the hand as he rapidly shook the trapeze
(Fig. 3). His anconeus muscle was silent as
he reached for the trapeze.
In the orangutan, marked potentials were
exhibited by the anconeus muscle during elbow extension when she rubbed a towel on
the floor. Negligible and slight EMGs occurred as she rose from sitting to quadrupedal postures, stood quadrupedally, and
reached overhead for the trapeze and food
sill.
We do not have data on the anconeus muscle during suspensory behavior of the subjects.
Fig. 3. EMG recording of brachial muscles in a bipedal
chimpanzee while he shook the trapeze bar by alternately pronating and supinating his hand. The M.
biceps brachii, caput breve (a), and the M.anconeus
(d), acted during supination. The M.triceps brachii,
caput mediale (c), acted continuously during the bout
and the M.brachialis 6)was virtually silent. See text
for further discussion.
exhibited as she reached overhead and
rubbed her fingertips from right to left along
the food sill.
The lateral head of the triceps brachii muscle was silent during all hoists and pendant
suspension by the orangutan. We do not have
comparable data for the alert chimpanzee.
Brachialis
The brachialis muscle acted as a flexor of
the elbow Chumeroulnar) joint in both subjects.
While the subjects were groggy, the brachialis muscle exhibited marked potentials
at the initiation of swing phase (i.e.,just prior
to elevation of the hand from the floor) in
quadrupedal locomotion. Thereafter, EMGs
dropped to slight, negligible, and occasionally nil (chimpanzee only) at prerelease of the
hand during knuckle-walking (chimpanzee),
fist-walking (both apes), and modified palmigrade progression (orangutan). The brachialis muscle was silent during all
quadrupedal, tripedal, and quiet sitting postures.
The chimpanzee’s brachialis muscle exhibited high EMGs when he stood bipedally on
the platform and pulled on the trapeze with
80
R.H. TUTTLE, M.J. VELTE, AND J.V. BASMAJIAN
his semiprone hand. Further, as he hoisted
himself on the trapeze, moderate and marked
potentials occurred. His hand was invariably
placed prone, semiprone, or supine on the
bar. Marked potentials accompanied hoisting
by the orangutan, who held the bar with her
hand either semiprone or supine (Fig. 2).
In the orangutan, the brachialis muscle
was silent during bimanual and unimanual
pendant suspension. This was also generally
true of the chimpanzee, except for occasional
short bursts of negligible or slight potentials.
He was rarely fully quiescent while hanging
from the trapeze.
Other than the trapeze bar, the chimpanzee was much less inclined to manipulate
objects than the orangutan was. Moderate
EMGs occurred in the brachialis muscle as
he grabbed the large plastic ball and clutched
it against his ventrum. Negligible and slight
EMGs were exhibited as the seated chimpanzee pulled on his toes and medial sole with
his hand semiprone.
Marked EMGs occurred in the brachialis
muscle when the orangutan yanked a towel
from an investigator’s hand and waved it
about her head. When she brought food to
mouth rapidly, marked EMGs were exhibited by her brachialis muscle. Slower feeding, which was more common, was
accompanied by slight and moderate potentials. Extended bursts of moderate and
marked potentials were recorded from her
brachialis muscle as she manually pressed
orange slices against her anterior teeth,
pressed the plastic ball against her mouth
and head with her hand semiprone, and
grabbed and clung to a n investigator’s lower
limb with her hand semiprone.
Biceps brachii
In each subject, the two heads of the biceps
brachii muscle usually produced very similar
EMG patterns. The long and short heads
were recorded concurrently during four experiments with the chimpanzee and three
experiments with the orangutan. Therefore,
we will discuss the heads separately only
when they acted uniquely (see below under
Simultaneity). In both subjects, the biceps
brachii muscle was recruited during behaviors that required powerful flexion of the elbow joint wherein it would augment the
actions of the brachialis and brachioradialis
muscles.
In the chimpanzee the most marked EMGs
occurred as he bimanually hoisted his full
weight on the trapeze with his hands prone,
semiprone, or supine. Slight and moderate
EMGs occurred when he leaned his weight
on the trapeze with the elbow flexed and
with his feet or toes on a substrate. On the
contrary, when the orangutan slowly hoisted
herself bimanually with supinated hands and
pedally grasped the bar early in the elevation, the biceps brachii muscle exhibited only
negligible and slight potentials. However,
when she released her left hand during the
ascent and further elevated herself to reach
foods suspended above the trapeze, the biceps
brachii muscle became prominently active.
Similar levels of EMG were recorded irrespective or whether one of her feet touched
the bar or was free of it (Fig. 2).
In both subjects, no EMG occurred in the
biceps brachii muscle as they hung pendant
from the trapeze with their elbow joints fully
extended.
When the chimpanzee knuckle-walked or
fist-walked, the biceps brachii muscle typically exhibited short bursts of negligible and
occasionally slight potentials a t the beginning of swing phase as the fingers left the
substrate. Once, when he shuMed across the
room and slid his fist on the floor, moderate
EMGs occurred.
The orangutan’s biceps brachii muscle also
exhibited brief bursts of negligible and slight
potentials at the initiation of swing phases
during fist-walking and crutch-walking. Toward the ends of recording sessions, it was
occasionally silent during progression on the
floor.
In both subjects, the biceps brachii muscle
was silent during quiescent tripedal, quadrupedal, and sitting postures.
Negligible EMGs were evinced in the biceps brachii muscle as the chimpanzee
brought objects to mouth and during other
nonvigorous manipulatory activities. However, moderate potentials occurred when he
pressed the large plastic ball between his
supinated hand and shoulder. High EMGs
were also exhibited in the biceps brachii
muscle as he clung to an investigator with
his hand semiprone.
The organutan’s biceps brachii muscle produced marked EMGs when she rapidly flexed
her elbow while flipping a towel overhead
and as she hugged objects against her head
and trunk, e.g., when pressing the plastic
ball between her supinated hand and forehead. Slight, negligible, and nil EMGs variously accompanied slow and unresisted
EMG OF BRACHIAL MM. IN CHIMPANZEE AND ORANGUTAN
elbow flexion as the orangutan retrieved objects from the floor and brought her hand to
her face and upper trunk.
Brachioradialis
Like the biceps brachii muscle, the brachioradialis muscle acted mainly during
powerful flexion of the elbow joint in both
subjects.
Marked potentials were exhibited by the
brachioradialis muscle as the chimpanzee
hoisted himself on the narrow food sill (with
a cliff-hanging grip of his fingertips) and trapeze (Fig. lb,c) and as he rotated about the
partially flexed forelimb while hanging beneath the trapeze. Moderate potentials occurred when he stood bipedally and tugged
on the bar with his hand semiprone. The
orangutan’s brachioradialis muscle also
evinced high EMGs as she hoisted herself
bimanually and unimanually on the trapeze
(Fig. 2). It was silent when she hung pendant. During quiescent suspension from the
trapeze, the chimpanzee’s brachioradialis
muscle was usually silent, but occasionally
it exhibited brief bursts of negligible and
slight potentials while the trapeze was in
motion.
Short bursts of moderate and marked
EMGs occurred a t prerelease of the hand into
swing phase as the groggy chimpanzee knuckle-walked. When he was fully coordinated,
negligible, nil, and occasionally slight potentials characterized the brachioradialis muscle during prerelease of the hand during
quadrupedal progression. The orangutan’s
brachioradialis muscle was generally silent
during quadrupedal locomotion, though
bursts of slight potentials occasionally accompanied prerelease of the hand during fistwalking early in the recording sessions. In
both subjects, it was silent during quiet
quadrupedal and tripedal stances.
During most manipulatory activities, the
brachioradialis muscle was silent or active a t
low levels in both subjects. For instance, it
was silent as the chimpanzee flexed his elbow and supinated his hand to bring food to
his mouth. And it evinced slight EMGs as he
smacked the plastic ball with his hand semiprone and prone. Although the orangutan’s
brachioradialis muscle was silent during
quiet feeding off the floor and moving the
ball on the floor, negligible EMGs were exhibited as she flexed her elbow joint and supinated her hand to press the ball against
her mouth and as she squeezed orange slices
81
between her anterior teeth and the medial
side of her wrist. During one bout of pushing
the ball against her mouth with the hand
fully supinated, bursts of marked potentials
occurred in her brachioradialis muscle.
Simultaneity
In both subjects, quadrupedal progression
was accompanied by simultaneous activity
in the three heads of the triceps brachii from
midswing through the load-bearing segment
of the stance phase. The anconeus muscle
acted simultaneously with the triceps brachii
muscle during a more limited (viz, the loadbearing) segment of the stance phase.
The brachial flexor muscles showed much
less regularity in their activities during
quadrupedal progression. The activity was
evinced as brief bursts of EMG potentials
during prerelease of the supporting hand, and
any combination of the brachialis, biceps brachii, and brachioradialis muscles might act
concurrently. The most common pairing was
the biceps brachii muscle with the brachialis
muscle, though each of them also acted in
the absence of activity by the other. The brachioradialis muscle seldom acted alone. Instead, it augmented the actions of one or both
of the other brachial flexor muscles during
pre-release of the hand.
The chimpanzee’s brachialis produced low
EMGs concurrently with marked EMGs in
his triceps brachii muscle when he climbed
onto the platform with weight passing eccentrically over his partially flexed elbow
joint. The same two muscles also acted synchronously a s he slid his hand along the floor
and retained his elbow partially flexed during a bout of quadrupedal progression.
Typically, the brachialis, biceps brachii,
and brachioradialis muscles were concurrently active as the chimpanzee hoisted himself and maneuvered on the trapeze. Once,
early in a n experiment when the chimpanzee
hoisted himself on the trapeze, the three
heads of the triceps brachii muscle evinced
negligible EMGs while his brachialis muscle
was active a t marked levels. During all subsequent hoists, his triceps brachii muscle was
silent. During several bouts of pendant suspension, the chimpanzee’s long head of triceps brachii and brachialis muscles acted
simultaneously a t negligible levels. The brachioradialis muscle also acted briefly with
the triceps brachii muscle as the chimpanzee
reached overhead for the food sill and flexed
his elbow to retrieve morsels of food.
82
R.H. TUTTLE, M.J. VELTE, AND J.V. BASMAJIAN
When the bipedal chimpanzee repeatedly
supinated his hand to shake the trapeze bar
rapidly, there were synchronous brief bursts
of moderate potentials in the anconeus and
short head of the biceps brachii muscles with
more sustained high EMGs in the medial
head of the triceps brachii muscle (Fig. 3). He
maintained his elbow at a 150" angle as he
rattled the bar. Apparently the extensors
acted to stabilize the humeroulnar joint while
the biceps brachii muscle supinated the forearm a t the radioulnar joints.
The orangutan's brachial flexors displayed
a pattern during bimanual hoisting that is
somewhat different from that of the chimpanzee. The brachialis muscle consistently
acted a t high levels; the brachioradialis muscle only acted at high levels during the final
segment of the hoist; and the biceps brachii
muscle was never markedly active during
bimanual hoisting. All three muscles exhibited high potentials during unimanual hoisting (Fig. 2).
In the orangutan, the long head of the triceps brachii muscle acted simultaneously
with the short head of the biceps brachii
muscle. As she dropped from the trapeze,
while retaining her elbow joint flexed, she
then rapidly extended it to snatch a piece of
food from the floor. In several instances, the
long head of her triceps brachii muscle and
her biceps brachii muscle acted concurrently
as she held the perforate, plastic, soccer-sized
ball with her elbow flexed and abducted her
shoulder to raise it overhead. Sometimes the
short head of the biceps brachii and the brachialis muscles joined the consistently active
long head of the biceps brachii muscle when
she flexed her elbow while maneuvering the
ball, which contained coveted transparent
packets of candies, clearly seen through the
perforations, overhead. In addition to serving
as the prime flexor of the humeroulnar joint
in these situations, the long head of the biceps brachii muscle may have acted to stabilize the abducted glenohumeral joint.
DISCUSSION
The activity patterns of brachial muscles
in the chimpanzee are generally similar to
those of the gorilla (Tuttle and Basmajian,
1974a1, though some differences also appear.
In both species, quadrupedal stances and locomotion on the floor were accompanied by
low levels of activity in the elbow extensors,
even though the chimpanzee had permanent
carrying angles (confirmed by passive manipulation under anesthesia) of the elbow
joints and the gorilla did not. The chimpan-
zee characteristically walked quadrupedally
with his elbows partially flexed, while the
gorilla extended her forearms into alignment
with her arms.
During horizontal knuckle-walking, the
patterns of activity in the brachial extensor
muscles are the same in the two African apes.
In both species, the triceps brachii muscle
acts modestly to extend the hand forward
and to maintain the forelimb in extension
while the load passes over the knuckled
hand. The anconeus muscle briefly augments
the triceps brachii muscle during the loadbearing segment of the stance phase of the
knuckle-walking cycle.
Increased loads on the knuckled supporting limb, e.g., during descents down the ramp
and off its top, were accompanied by higher
EMGs in the triceps brachii muscles of both
species than when they knuckle-walked horizontally. However, during quiescent quadrupedal and tripedal stances and, to a lesser
extent, during knuckle-walking, the anconeus muscle was much more often silent in
the gorilla (Tuttle and Basmajian, 1974a).
The chimpanzee also differed from the gorilla
in the relative activity of the long head of the
triceps brachii muscle during quadrupedal
ascents on the ramp. Whereas in the gorilla
EMG activity decreased in the triceps brachii
muscle (Tuttle and Basmajian, 1974a),it usually remained the same or increased as the
chimpanzee ascended the ramp. This may be
related to the set elbow flexure in the chimpanzee and its absence in the gorilla. We
have not tested enough subjects to establish
whether osseoligamentously the elbow of Pan
gorilla is better adapted than that of Pan
troglodytes for the stresses of knuckle-walking.
The two African apes differed in the combinations of brachial flexors that acted to
move the forearm and hand forward at the
outset of the swing phase in knuckle-walking
cycles. In both species, the brachialis muscle
was the prime elbow flexor a t the outset of
swing phase. However, the brachioradialis
muscle acted with it more regularly in the
chimpanzee. Further, the biceps brachii muscle was quite often active a t the initiation of
swing phase in the chimpanzee, while it was
usually silent in the gorilla (Tuttle and Basmajian, 1974a). Many of the bouts of knuckle-walking were induced by tossing candies
about the room. The gluttonous chimpanzee
moved quickly in pursuit of these treats,
which may explain why more of his arm
muscles were recruited to inaugurate swings
of the distal forelimb.
EMG OF BRACHIAL MM. IN CHIMPANZEE AND ORANGUTAN
During regular quadrupedal progression,
the activity patterns of the branchial muscles of Pongo pygmaeus are not dramatically
distinct from those of the knuckle-walkers
(Pan gorilla and Pan troglodytes). Indeed we
could detect no regular EMG differences in
the brachial musculature between ordinary
fist-walking and modified palmigrade walking by Pongo or between fist-walking and
knuckle-walking by Pan trodoglodytes. Thus,
it appears that the range of hand postures
that are exhibited by the pongid apes do not
require different actions by the brachial muscles during quadrupedal walking.
At the outset of the forelimb swing phase
of quadrupedal walking the brachialis muscle is the prime elbow flexor in Pongo, as it
is in Pan. However, whereas the orangutan’s
brachioradialis muscle usually remains silent, like that of the gorilla, her biceps brachii muscle is typically active, like that of
the chimpanzee, a t the beginning of swing
phase.
The only difference that we discerned between the brachial EMG activity patterns of
the orangutan and the knuckle-walkers during quadrupedal and tripedal stances is that
the Asian ape more often exhibited moderate
potentials than the African apes did.
McHenry (1976) suggested that in African
Pongidae the steep, flattened, lateral wall in
the olecranon fossa is related to “locking”
the elbow in a hyperextended position during
knuckling behavior. However, Feldesman
(1982)has denied that any morphometric feature has been isolated on anthropoid distal
humeri that would distinguish knucklewalking African apes from orangutans. Our
EMG results are insuflFicient to resolve this
discrepency, though a t face value they do not
support the existence of osseoligamentous
features in the humeroulnar joint that would
demarcate the knuckle-walkers from orangutans.
The overall patterns of activity in the brachial muscles that we sampled are similar in
the gorilla and chimpanzee during bimanual
and unimanual hoisting and quiescent pendant suspension from the trapeze. During
hoists, their extensors are silent while their
flexors are highly active. In suspension, the
orangutan differed from the African apes
mainly in the lower level of EMGs in its
biceps brachii muscle as she hoisted herself
bimanually on the trapeze. Because of her
longer reach, the elbows did not approach
right angles during bimanual hoisting. However, during unimanual hoists to reach a pan
83
of food above the trapeze, the orangutan’s
elbow approached orthogonal flexure and her
biceps brachii muscle was prominently active.
We suspect that during at least some pongid hoisting actions, the brachioradialis muscle rivals the brachialis muscle as the prime
flexor of the elbow joint. This is best illustrated by the chimpanzee, in which the brachioradialis muscle acted a t high levels for
longer periods during unimanual and bimanual hoists on the food sill and trapeze than
did the brachialis and biceps brachii muscles.
We conclude that evolutionary transformations from brachial and elbow complexes
similar to those of Pongo to ones like Pan or
vice versa probably would be simpler to
achieve, vis-a-vis myological features, than
alterations of the shoulder complex (Tuttle
and Basmajian, 1978, p. 68).
ACKNOWLEDGMENTS
This study was supported mainly by NSF
grants GS-3209 and SOC75-02478 and by a
Public Health Service research career development award (1-K04-GM16347-01)from the
National Institutes of Health. Supplementary support was provided by NIH grant RR00165 to the Yerkes Regional Primate Research Center and the Marian and Adolph
Lichtstern Fund of the University of Chicago. We are especially thankful for the assistance of J. Malone, E. Regenos, J. Perry,
Dr. G.H. Bourne, R. Pollard, S. Lee, R.
Mathis, J. Roberts, Dr. M. Keeling, Dr. M.
Vitti, J. Hudson, K. Barnes, and L. Doan.
LITERATURE CITED
Feldesman, MC (1982) Morphometric analysis of the distal humerus of some Cenozoic catarrhines: The late
divergence hypothesis revisited. Am. J. Phys. Anthropol. 59:73-95.
MacConaill, MA, and Basmajian, JV (1969) Muscles and
Movements. Baltimore: Williams & Wilkins Co.
McHenry, HM (1976) Multivariate analysis of early hominid humeri. In E Giles and JS Friedlaender (eds): The
Measures of Man. Cambridge: Peabody Museum Press.
pp. 338-371.
Tuttle, RH, and Basmajian, JV (1974a) Electromyography of brachial muscles i n Pan gorilla and hominoid
evolution. Am. J. Phys. Anthropol. 41:71-90.
Tuttle, RH, and Basmajian, JV (1974b) Electromyography of forearm musculature in gorilla and problems
related to knuckle-walking. In: FA Jenkins, Jr. (ed):
Primate Locomotion. New York: Academic Press, pp.
293-347.
Tuttle, RH, and Basmajian, JV (1978)Electromyography
of pongid shoulder muscles, 111. Quadrupedal positonal
behavior. Am. J. Phys. Anthropol. 49:57-70.
Tuttle, RH, Basmajian, JV and Ishida, H (1979) Activities of oonpid thigh muscles during bipedal behavior.
Am. J. Phis. Anthopol. 50:123-136.
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