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. 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