AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 87:215-226 (1992) Electromyography of Pronators and Supinators in Great Apes RUSSELL H. TUTTLE, JOHN R. HOLLOWED, AND JOHN V. BASMAJIAN Department of Anthropology, University of Chicago, Chicago, Illinois 60637 (R.H.T., J.R.H.); Rehabilitation Centre, Chedoke Hospitals, McMaster University School of Medicine. Hamilton, Ontario. Canada L8N 3L6 (J.V.B.) Biceps brachii, Chimpanzee, Gorilla, Orangutan, KEY WORDS Positional Behavior, Manipulation ABSTRACT We obtained electromyographic recordings from the supinator, biceps brachii, pronator quadratus, and pronator teres muscles of a chimpanzee and a gorilla and from the supinator, pronator quadratus, and biceps brachii muscles of an orangutan as they stood and walked quadrupedally on horizontal and inclined surfaces, engaged in suspensory behavior, reached overhead, and manipulated a variety of foods and artifacts. In Pan troglodytes and Pangorilla, as in Homo sapiens, the supinator muscle is the prime supinator, with the biceps brachii muscle serving to augment speed or force of supination. Primacy of the pronator quadratus muscle over the pronator teres muscle during pronation is less clear in the African apes than in humans. Possibly, pongid radial curvature or forelimb elongation or both factors are related to the somewhat different patterns of activity that we observed in the pronator muscles of Pan versus those reported for Homo sapiens. In Pongo pygmaeus, as in P. troglodytes and P. gorilla, the pronator quadratus muscle acts as a pronator and the supinator muscle acts to supinate the hand a t the radioulnar joints. The biceps brachii muscle is active at low levels as the orangutan supinates its hand with the elbow flexed. Over the past half century, electromyography has revitalized, indeed revolutionized, human and other primate functional morphology (Basmajian and De Luca, 1985; Tuttle et al., 1979; Jouffroy, 1989).For instance, among the revised functional inferences in human anatomy, premised on EMG studies, is that the pronator quadratus, not the pronator teres, muscle is the prime pronator of our hands. Because the manual rotators-pronator quadratus, pronator teres, supinator, and biceps brachii muscles-are grossly similar to human ones in their bony attachments (Sonntag, 1924; Howell and Straus, 1961; Gregory, 1950; Tuttle, personal observations) and because the radioulnar joints of extant great apes and humans are more freely movable than those of most other mammals (Knussman, 1967; Lewis, 1989; OConnor, 1975; O’Connor and Rarey, 1979), we might expect them t o act similarly in @ 1992 WILEY-LISS,INC. comparable behavioral contexts. However, the different relative masses of the manual rotator muscles between apes and humans (Tuttle, 1969,197071972a,b),combined with the different proportional lengths and curvature of the upper limb bones (Schultz, 1956; Erikson, 1963; Hollowed, 1986; Swartz, 1990) and shapes of attachment points for the rotator muscles (Senut, 1989; Aiello and Dean, 19901, signal that they may not be closely comparable (Hollowed, 1986). Accordingly, we recorded, via electromyography (EMG),the supinator and pronator muscles of a common chimpanzee (Pan troglodytes), a western gorilla (Pan gorilla), and a Sumatran orangutan (Pongo pygmaeus) in order to determine which muscles were principally and otherwise active during manipulation, quadrupedal positional behavior, and suspensory activities. Received October 16,1989; accepted September 3,1991 216 R.H. TUTTLE ET AL. METHOD Eleven EMG experiments were conducted during a 16.5-month span on the right forelimb of a female western gorilla, which was between 4.5 and 6 years old and weighed between 34 and 42 kg. Eight EMG recording sessions were conducted during a 15.5month span on the right forelimb of a captive-born male chimpanzee, which was between 4 and 5.5 years old and weighed between 17 and 29 kg. Five EMG experiments were performed over a 14-month span on the right forelimb of a captive-born female orangutan, which was between 6 and 7.5 years old and weighed 24 and 28 kg. Indwelling fine-wire bipolar electrodes were used according to procedures developed by Tuttle and Basmajian (1974a). A subject was injected in the hip or left arm muscles with 13.5-15.0 mgkg body weight of Ketalar (ketamine hydrochloride)and 0.4 mg of atropine. Karma fine-wire bipolar indwelling electrodes were implanted in five muscles of the right forelimb with 26-gauge hypodermic needles. The electrodes were placed centrally in the bellies of the muscles. Needles for the pronator quadratus and supinator muscles were inserted until their tips contacted bone. Before withdrawing them, the radioulnar joint was rotated to verify electrode placement in these two muscles and in the pronator teres muscle, which, like the biceps brachii muscle, is readily located due to its size and superficial position in the limb. Rotations of the radioulnar joint and flexion-extension of the elbow joint served also to draw the electrodes more deeply into the muscles. Five channels of electromyogram, one channel of reference pulse (Tuttle and Basmajian, 1978a: p.731, and one-channel of narration were calibrated and recorded on an Ampex SP 300 8-channel tape recorder. Concurrently, 4 or 5 EMG channels were displayed on a Model 564B Tectronix oscilloscope. A Panasonic split-screen television unit was used to videotape the subject, the EMG display on the oscilloscope, and the narration. Behavior and EMG displays are precisely synchronous on the video tapes (Tuttle and Basmajian, 1978a,b). After the recording sessions, pen writer recordings of EMG activity were made from the FM tapes. Narration, time, and other referents were written along the bottoms of the records. After identifying the most prominent activities of each muscle on the record of an experiment, the relative magnitudes of all other EMG action potentials were graded marked, moderate, slight, negligible, and nil (Basmajian and De Luca, 1985). Behaviors that were accompanied by artifact-free EMG potentials on one or more channels were described on the basis of repeated examinations of the video tapes at normal and slow speeds with a Panasonic Tape-a-Vision model NV-3020. We recorded while the subjects were recovering from anesthesia and thereafter. Unless stated otherwise, the observations narrated below are from subjects that were well coordinated and showing no effects of anesthesia. The subjects could move freely in the 3.6 m by 3.9 m testing area and could climb on a trapeze and reach for foods suspended on strings from the ceiling, attached high on the walls, and held overhead by an investigator (Tuttle et al., 1983, Fig. 1;1979; Tuttle and Basmajian, 1978a,b). The subjects were in no sense “fettered,” as has been charged by Larson and Stern (1986). The ribbon cable, which conveys the EMG potentials to recording equipment, is highly flexible; and, it is quite easy for the investigator (R.H.T.) to keep the subjects and himself from getting tangled in it. Indeed, we used it in Japan with rapidly running, climbing, and brachiating gibbons without impairing their locomotion, except when the investigators chose to tug on it in order to encourage them to climb down from high supports near the ceiling. In the chimpanzee, the four forearm rotators were recorded simultaneously during one experiment; the supinator, biceps brachii, and pronator teres muscles were recorded simultaneously during a second experiment; and the supinator and pronator quadratus muscles were recorded together during a third experiment. In the gorilla, the four rotators were recorded together during one session and all except the pronator quadratus muscle were recorded during a second session. Only the pronator quadratus and biceps brachii muscles were simultaneously recorded in the orangutan. OBSERVATIONS Supinator The supinator muscle was recorded in four experiments for the chimpanzee, two for the gorilla, and one for the orangutan. In all subjects it was primarily responsible for supinating the hand at the radioulnarjoints. In EMG OF PONGID PRONATORS AND SUPINATORS 217 , Pt , 0 1 2 3 4 sec. Fig. 1. The chimpanzee picks up an M&M candy from the floor: initially he pronates the hand (sec. 0-2); then (sec. 2-41, he supinates the hand and flexes the elbow, carrying the sweet to mouth. (Symbols: pq, pronator quadratus muscle; su, supinator muscle; pt, pronator teres muscle; bb, long head of the biceps brachii muscle) the chimpanzee, the supinator muscle was active at variable levels in all suspensory behavior. It showed phasic EMG activity in most locomotor behavior in both African pongid subjects. In the chimpanzee, supination and flexion of the elbow during feeding bouts usually was accompanied by negligible EMG activity in the supinator muscle, but slight, moder- ate, and marked EMGs were recorded occasionally (Fig. 1). The level of activity depended on the degree to which the hand was supinated. Often there was negligible activity as the hand initially left the mouth. Supination while extending the elbow elicited slight and moderate EMGs. Activity was negligible when markedly supine hand positions were maintained, and it remained neg- R.H. TUTTLE ET AL. 218 bl I 1 1 . su bs I see. Fig. 2. The chimpanzee leaps from the floor to grab the trapeze (sec. 0) and slowly rotates clockwise unimanually beneath the bar (sec. 1-3). Then (sec. 3-81, he flexes his right elbow acutely and forages overhead with his left hand. His right hand is prone on the bar; but, there is intermittent subtle pronation and supination as he gropes overhead. Finally (sec. %9),he drops to the floor. (Symbols: bl, long head of the biceps brachii muscle; su, supinator muscle; bs, short head of the biceps brachii muscle; pt, pronator teres muscle) ligible as pronation was subsequently initiated. In all other instances of pronation, the supinator muscle was silent. The supinator muscle was active at moderate to marked levels as the chimpanzee held a large plastic ball against his chest with the hand in semiprone or slightly supine positions. Although initially moderate, EMG activity decreased to negligible as the subject reached up t o the handler with his hand semiprone. Activity dropped to nil as he held the handler around the shoulders and was carried. Holding the investigator around the neck with his hand semiprone elicited slight EMG activity in the supinator muscle. As the chimpanzee reached overhead for the trapeze bar or food sill, supinator activity varied from nil to marked. Standing and holding the trapeze bar yielded nil activity. As the subject hoisted himself unimanually or bimanually on the trapeze and sill, with the hand in diverse positions, slight-to-moderate and marked EMGs were recorded (Fig. 2). Unimanual and bimanual pendant suspension usually produced negligible and slight EMGs, but moderate levels were occasionally recorded also. During clockwise rotation about the right hand while hanging from the trapeze, EMG activity was marked or moderate. The supinator muscle was silent or active at low levels as the chimpanzee rotated counterclockwise. While the subject was still recovering from EMG OF PONGID PRONATORS AND SUPINATORS the effects of anesthesia, the supinator muscle was active at moderate and marked levels during knuckle-walking. Then EMGs decreased to negligible and slight as he became fully alert. The supinator muscle was commonly active throughout the swing and early stance phases of knuckle-walking. Usually the EMGs were phasic during the swing phase and decreased from mid-swing. The supinator muscle was occasionally active just before the hand was released from contact with the substrate. EMG activity was rarest during mid-stance, and if it occurred then, it was at low levels. During knuckle-walking, the chimpanzee positioned his hands more variably than the gorilla did. But the levels and pattern of EMG activity did not change as the chimpanzee knuckle-walked slowly and rapidly on the floor and as he knuckle-walked up the ramp. Knuckle-walking down the ramp was accompanied by negligible and slight activity in the supinator muscle. Further, EMG occurred immediately before release of the hand more commonly when he knucklewalked down the ramp than when he knuckle-walked up the ramp or on the floor. When the quadrupedal subject slid his mid phalanges along the floor, continuous, negligible-toslight EMGs were produced by the supinator muscle. The supinator muscle exhibited marked activity as the chimpanzee swung his hand up to a platform and climbed onto it. As he stepped headfirst off the platform or ramp onto his extended, knuckled forelimb, the supinator muscle was active at levels ranging from negligible to marked. It usually fell silent when his knuckles contacted the floor. Once, however, EMG activity increased as he markedly supinated his hand upon contact with the floor. Marked EMGs were recorded from the right supinator muscle as the subject exited the platform onto his left mid phalanges while his right palm remained on the platform. The activity ceased as his right hand left the platform. As the seated chimpanzee rose to quadrupedal postures, the supinator muscle was active at negligible-to-slight levels. When he rose from prone positions to quadrupedal stances, the supinator muscle exhibited marked EMGs. As the chimpanzee stood quadrupedally on fisted hands, the supinator muscle produced negligible-to-slight potentials. Knuckle-walking stances were accompanied by 219 negligible EMGs, except once when his forelimb was retracted. On that occasion, EMG activity was slight in the supinator muscle. As he sat resting his knuckles or fist on the floor there was nil, negligible, or occasionally slight EMG activity. EMGs were negligible as he sat with the dorsum of his hand resting on the floor. Generally, EMG levels decreased as a resting posture was maintained. But once the supinator muscle exhibited continuous slight activity as the seated subject leaned forward while supporting himself on the knuckled right hand. The gorilla’s supinator muscle exhibited variable levels of EMG activity as she supinated her hand during manipulation. Supination with concurrent elbow flexion or extension and supination with the elbow sustained in a flexed or extended position produced EMGs ranging from negligible to marked. The supinator muscle was inactive during pronation. Early in one session, EMGs ranging between slight and marked occurred in the supinator muscle as the subject stood bipedally and held the trapeze bar unimanually and bimanually. But, during similar behavior later in the session, EMG activity dropped to nil. Pulling on the trapeze bar elicited slight EMG activity. When the bipedal subject held the bar unimanually and alternately supinated and pronated her hand, EMGs, ranging from slight to marked, occurred during supination and nil EMG accompanied pronation. Clockwise rotation about the radioulnar joint as the subject stood bipedally and held the sill was accompanied by marked activity; and, counterclockwise rotation elicited nil activity in the supinator muscle. On one occasion, the supinator muscle was negligibly active as the subject stood bipedally and leaned against the facing wall with her palmed hand as a brace. Activity remained negligible as she laterally flexed her trunk to the left and rotated counterclockwise about her hand. Activity was marked as she kept her palm against the wall and returned to a more erect posture. The gorilla’s supinator muscle exhibited EMGs ranging from nil to slight during bimanual suspension on the trapeze. And activity was marked during clockwise rotation about the right hand while unimanually and bimanually suspended on the trapeze; it was nil during counterclockwise rotation. As the 220 R.H. TUTTLE ET AL. iments for the chimpanzee, two for the gorilla, and three for the orangutan. Each head was recorded separately once in the chimpanzee and in the orangutan. Only the activity of the biceps brachii muscle during rotatory actions is reported here. Further discussions on the electromyography of the biceps brachii muscle in pongid apes are in papers by Tuttle and Basmajian (1974) and Tuttle et al. (1983). The biceps brachii muscle was relatively inactive during bouts of supination, especially in the gorilla. Prominent activity was occasionally recorded in the chimpanzee. Both heads produced similar activity in the gorilla and orangutan. In the chimpanzee, activity of the long head of the biceps brachii muscle was often less than that of its short head. The chimpanzee’s biceps brachii muscle was active at negligible and slight levels as the subject supinated his hand while flexing his elbow (Fig. 1). Negligible to moderate activity was recorded during supination with the elbow flexed. Hoisting on the trapeze, while rotating clockwise about the right hand, was accompanied severally by slight, moderate and marked EMGs (Fig. 2). Clockwiserotation about the hand when suspended from the trapeze was accompanied sometimes by moderate and marked activity; counterclockwise rotation elicited negligible and slight activity in the biceps brachii muscle. The chimpanzee’selbow was usually flexed slightly during rotation beneath the trapeze. His biceps brachii muscle was commonly active during the initial swing phase of knuckle-walking. The gorilla’s biceps brachii muscle exhibited nil activity during supination with the elbow flexed. Supination, concurrent with elbow flexion, often elicited negligible and slight EMGs; and, it was occasionally accompanied by nil EMG. The biceps brachii muscle was rarely active during the early swing phase of knuckle-walking. But during knuckle-walking, the gorilla exhibited less radioulnar rotation than the chimpanzee did and her elbow was less flexed than his was in all phases of the knuckle-walking stride. The orangutan’s biceps brachii muscle was consistently active at slight levels as she supinated and flexed her elbow; the levels are similar t o those recorded during flexion without concurrent supination. The biceps Biceps brachii brachii muscle was active a t slight levels The two heads of the biceps brachii muscle during supination of the flexed elbow. It were recorded simultaneously in four exper- evinced negligible and slight EMGs during subject raised her feet to the bar during bimanual suspension, slight activity occurred. Rotation counterclockwise while hoisting elicited nil activity; bimanual hoisting without concurrent rotation produced slight activity. The supinator muscle was active at negligible and slight levels during the late swing and early stance phases of knuckle-walking on the floor and up the ramp and when the subject slid her knuckles or palm (with interphalangeal joints flexed) across the floor. It was occasionally active throughout the swing phase. When she raised her forelimb from the floor to the platform, activity varied from slight to marked. Knuckle-walking down the ramp was accompanied by EMGs ranging from nil to moderate, with a more variable pattern of muscle action than occurred while walking up the ramp or across the floor. EMGs ranging from negligible to moderate were seen as the subject stepped headfirst off the platform onto her knuckles. Nil and negligible activity was recorded in the gorilla’s supinator muscle during quadrupedal stances on the knuckles or modified palm and during tripedal knuckled stances on level surfaces or when facing uphill on the ramp. Tripedal knuckled stances while facing downhill elicited negligible and slight EMGs. Nil activity occurred in the supinator muscle as the subject sat with her fist resting on the floor. When the seated subject rose to quadrupedal stances, her supinator muscle exhibited negligible activity. On one occasion the groggy subject attempted to rise to a quadrupedal posture by repeatedly pushing against the floor with concurrent pronation of the hand. EMG activity ranged between negligible and slight during the pushing and pronation and between slight and marked during the recovery stroke with supination before the next push. In the orangutan, the supinator muscle was silent or negligibly active during quadrupedal stances and locomotion. Commonly, she slid her fist or modified palm instead of employig a true swing phase. When she did swing the fist free of the floor, movement occurred chiefly at the shoulder joint, with distal forelimbjoints held more or less stiffly in the positions of the preceding stance phase. EMG OF PONGID PRONATORS AND SUPINATORS 22 1 While standing quadrupedally on the knuckles, EMG activity was usually nil, but some negligible activity was recorded in the pronator quadratus muscle. Negligible activity ocPronator quadratus curred as the subject stood tripedally on his The pronator quadratus muscle was re- knuckles or quadrupedally on his fists. Sitcorded in two experiments for the chimpan- ting with the knuckles resting on the floor zee, one for the gorilla, and two for the elicited nil activity. In the gorilla, pronation of the forearm as orangutan. It was active at variable levels in pronation of the free hand and during sus- it rested on the floor was accompanied by pensory behaviors. It was also active during marked activity in the pronator quadratus at least some of the stance phase of quadru- muscle. Activity decreased as the pronated position was maintained; it ceased within 30 pedal locomotion in all subjects. As the chimpanzee pronated and extended seconds. Activity was moderate as the gorilla his forearm while feeding off the floor or while taking food from the investigator’s reached overhead for the trapeze with her hand, EMGs ranging from negligible to mod- hand prone. It dropped t o slight levels as she erate occurred in the pronator quadratus grabbed the trapeze and held it bimanually muscle (Fig. 1). Activity was nil during pro- with her hands prone and subsequently as nation from a markedly supinated to a semi- she raised her feet to the bar. During slow knuckle-walking, the gorilla’s prone position. Negligible and slight activity occurred as he sat holding his foot with the pronator quadratus muscle was usually active at negligible and slight levels, with occahand prone. EMG activity was initially negligible, but sional bursts of marked activity, during mid increased to marked as the chimpanzee and late stance and early swing phases of the reached up to the handler with the hand stride. The pronator quadratus muscle was semiprone. Activity dropped to nil as he held active more often at the onset of swing phase the handler with the hand semiprone. in the gorilla than in the chimpanzee. When Reaching overhead for the trapeze and grip- the subject stepped onto the stage, high acping the bar with the hand prone elicited tivity was occasionally recorded just before negligible, slight, and occasionally moderate knuckle contact. Sliding the knuckles off the activity. It dropped to nil as the bipedal ramp onto the floor elicited nil activity. Negsubject held the trapeze with the hand ligible and slight activity occurred as the gorilla walked quadrupedally while sliding prone. No data were recorded for the pronator her fist; but activity was nil as she slid her quadratus muscle during suspensory behav- hand in a modified palmigrade posture. ior by the chimpanzee because he did not Climbing onto the stage with the hand in a hang from the trapeze while we were moni- modified palmigrade position was accompanied by slight and moderate EMG activity. toring it. Activity was nil in the pronator quadratus Nil, and less often, negligible, levels of EMG were recorded in the pronator quad- muscle during knuckled quadrupedal stances, ratus muscle during the load-bearing phase but it varied from nil to slight when her hand of knuckle-walking as the chimpanzee was in a modified palmigrade posture. walked at various speeds on a level surface, Standing tripedally on the right knuckles walked up or down the ramp, or slid his while facing down the ramp elicited nil activknuckles along the floor during quadrupedal ity. Early in the experiment, as the groggy progression. Occasional slight activity oc- subject tried to rise from sitting to a curred early in one session as the subject quardupedal stance, EMG activity varied knuckle-walked. Highly variable levels of from negligible to marked; the activity was activity, ranging from nil to marked, were highest as she pushed on the floor and recorded as he stepped off the stage onto his slightly pronated her forearm several times right knuckles. While stepping off the stage while attempting t o rise. The EMG levels onto his left knuckles, activity in the right dropped as she relaxed. In the orangutan, pronating the free hand pronator quadratus muscle was negligible; it dropped to nil as the right hand neared the was accompanied by EMGs ranging from negligible to marked in the pronator quadrafloor. Occasionally nil, but usually negligible, tus muscle. Activity was usually greater activity occurred as the chimpanzee rose when the subject pronated her hand from a from a sitting to a quadrupedal posture. supine to a prone position than when her the early swing phase of fist-walking and crutch-walking. 222 R.H. TUTTLE ET AL hand was only slightly rotated. But occasionally full pronation elicited only negligible activity. Pronation was accompanied by similar activity levels whether the elbow was flexed or extended. Pronating the fist as it rested on the floor elicited negligible to slight activity when the forearm was also on the floor and marked activity when the forearm was off the floor. While the orangutan picked food off the floor, EMG activity was negligible in the pronator quadratus muscle as the hand approached the floor, marked as the food was grasped, and nil and the forearm was flexed and supinated. While lying supine with the right forelimb abducted, the pronator quadratus muscle was active in maintaining hand position; EMG activity was nil when the hand was supine, negligible when it was semiprone, and slight when it was prone. Negligible and slight levels occurred as she pushed a heavy box across the floor with the right palm while walking tripedally and while sitting and pushing the box with both palms. Once, the pronator quadratus muscle evinced a short burst of negligible activity as she supinated her free hand. Negligible and slight EMGs occurred as she supinated her hand and hit a large rubber ball with its dorsum. EMGs ranging from negligible to moderate occurred as the orangutan reached overhead with her hand semiprone. There was slight activity as she pulled weakly on the trapeze bar with her hand semiprone. When she reached into a food tray above the trapeze, activity varied from negligible to marked; but the degree of pronation could not be seen on the video tape. While the subject stood bipedally and rotated the bar of the trapeze overhead counterclockwise, activity was slight as her hand rotated from a supine to a seimiprone position. Then EMGs increased from moderate to marked as her hand continued from a semiprone to a prone position. Quiescent pendant suspension unimanually, bimanually, and with one or both feet on the bar usually elicited nil activity in the pronator quadratus muscle, but rare negligible activity also was recorded. Activity was slight during suspension with the hyperprone right hand and both feet on the bar. Counterclockwise rotation about the right hand during unimanual suspension produced slight activity. Activity was nil during clockwise rotation. The pronator quadratus muscle was silent as the subject swung uni- manually and bimanually on the trapeze. Bimanual hoisting elicited nil, negligible, or occasionally slight levels of activity. Negligible and slight activity was recorded as she rotated counterclockwise about the right hand while hoisting herself bimanually on the trapeze. The pronator quadratus muscle exhibited less consistently patterned activity during quadrupedal progression in the orangutan than in the African apes. EMG activity was usually negligible, but occasionally it reached moderate levels while she fist-walked. Activity was usually constant throughout the stride, but often it dropped during the swing phase. During slow fist-walking, activity occurred during the early and mid-stance phases, a s it does during chimpanzee knuckle-walking. Negligible and slight EMGs were elicited a s the orangutan shuffled her fists across the floor. When stepping off the stage or ramp onto the right fist, activity varied from negligible to marked. Slight and moderate activity was recorded during the load-bearing phase of quadrupedal walking with the hands in a modified palmigrade posture. On one occasion, the pronator quadratus muscle was active during the late swing phase. The orangutan frequently shuffled or slid her palms along the floor. This elicited negligible and slight EMGs throughout the stride. While shuffling down the ramp, EMG activity was continuous at negligible levels. Crutch-walking, with fisted hands, produced slight activity during the load-bearing phase of the stride. Modified palmigrade crutch-walking yielded negligible activity throughout the stride. On one occasion, the orangutan lowered herself from the trapeze onto her right knuckles with the elbow extended; negligible EMG was evinced by the pronator quadratus muscle. As the orangutan rose from sitting to quadrupedal stances, negligible EMG was elicited in the pronator quadratus muscle. Nil and negligible activity was exhibited during quiescent quadrupedal stances on fisted hands; nil activity occurred during modified palmigrade quadrupedal stances. Standing tripedally on the fist yielded nil activity, but modified palmigrade tripedal stances elicited nil to slight levels of activity. Negligible levels were usually recorded while the subject sat and rested the prone fist or palm on the floor, though the EMGs occasionally dropped to nil or rose to slight. EMG OF PONGID PRONATORS AND SUPINATORS Pronator teres The pronator teres muscle was recorded in two experiments each for the chimpanzee and the gorilla. In the chimpanzee it was active during the same behaviors that the pronator teres muscle was. In the gorilla, the pronator teres muscle acted less frequently than the pronator quadratus muscle. It appeared to augment the pronator quadratus muscle during some behaviors. In the pronator teres muscle of the chimpanzee, activity increased from negligible to marked levels as he reached up to the handler with his hand semiprone. Then activity decreased to moderate levels as he held the handler with the semiprone hand. There was no activity as he held a large ball against his chest with the hand semiprone or slightly supine. Reaching overhead for the trapeze with the hand prone elicited negligible and slight activity in the chimpanzee’s pronator teres muscle. Hoisting unimanually on the trapeze, with the hand prone, and bimanual hoisting, with a semiprone or prone hand, yielded EMGs ranging from slight to marked (Fig. 2). But unimanual and bimanual hoisting on the sill, with the hand prone, was accompanied by negligible activity. Quiescent unimanual pendant suspension on the trapeze with the hand prone also elicited negligible activity. Hanging bimanually recruited negligible activity when the hand was semiprone and negligible and slight activity when the hand was prone. Activity was initially negligible when the chimpanzee unimanually rotated clockwise beneath the trapeze from an initially prone manual position (Fig. 2). But occasionallythere were bursts of moderate and marked activity at the end of the rotation when the pronator teres muscle apparently acted as a brake. Early in sessions, the pronator teres muscle was active at levels ranging from slight to marked as the chimpanzee knuckle-walked. Later, activity decreased to negligible and slight levels. The pronator teres muscle was always active in early and midstance. Activity occasionally began just before knuckle contact and continued until release. Activity was uniform as the subject walked up and down the ramp. Levels of activity remained negligible and slight during fast knucklewalking on a level surface, but during the late swing phase it was more common than during slower knuckle-walking. Sliding the knuckles along the floor while walking for- 223 wards or backwards elicited negligible to slight activity throughout the stride. Stepping off the ramp or stage onto the right knuckles was accompanied by EMGs ranging from negligible to moderate, which commenced as the elbow was extended and decreased as a quiescent stance was assumed. Standing quadrupedally on his knuckles yielded negligible activity in the chimpanzees’spronator teres muscle; and fisted quadrupedal stances elicited nil activity. Negligible activity occurred as the sitting subject rested his knuckled semiprone hand on the floor; the EMG occasionally rose to slight when his hand rested in a prone position. Marked activity was recorded in the gorilla’s pronator teres muscle as she pronated her hand while leaning on her forearm with the elbow flexed; activity dropped to nil approximately 45 seconds after she pronated her hand. Pronation with the elbow extended was accompanied by negligible and slight activity. Slight EMG levels occurred during concurrent pronation and elbow flexion. Negligible activity was recorded once during concurrent supination and elbow flexion. Activity was nil during all other instances of supination with concurrent elbow flexion and extension and when the elbow was maintained in a variety of positions. One instance of extending the elbow with the hand prone produced negligible activity in the gorilla’s pronator teres muscle and a shorter burst of negligible activity was recorded in it during elbow extension with the hand semiprone. Holding the trapeze while bipedal elicited negligible and slight activity. Alternately supinating and pronating the hand in order to rattle the trapeze produced nil activity during supination and EMGs from slight to marked during pronation. As the gorilla rotated counterclockwise about her right hand while hanging bimanually, and when she hoisted herself bimanually, marked activity occurred in the pronator teres muscle. Knuckle-walking elicited slight activity in the gorilla’s pronator teres muscle early in recording sessions; but negligible activity predominated as she recovered from anesthesia. Activity was highest during the stance phase and occasionally early swing phase. Whereas supination was the norm at hand release in the more variable chimpanzee knuckle-walking pattern, the knucklewalking gorilla habitually pronated her hand at release. Walking quadrupedally while sliding the knuckles or fists on the 224 R.H. TUTTLE ET AL. forearm. During supination in the chimpanzee, the biceps brachii muscle was only recruited when the supinator muscle was prominently active. The biceps brachii muscle was active at lower levels than the supinator muscle, except when he revolved while hanging from the trapeze; during clockwise rotation about the right hand, moderate and marked EMG potentials were recorded in both muscles. But the biceps brachii muscle was probably acting to maintain the slight elbow flexion that was common during his suspensory behavior on the trapeze. This inference is supported by the fact that during the counterclockwise rotation, which immediately followed the clockwise rotation, the supinator muscle was silent, while the biceps brachii muscle remained active at negligible and slight levels. At knuckle release during quadrupedal locomotion,the biceps brachii and supinator muscles often fired synchronously. In the gorilla, only the supinator muscle was recruited to supinate the forearm at the flexed elbow. Usually the biceps brachii muscle was active at negligible and slight levels during supination with concurrent elbow flexion; but occasionally it was inactive. The biceps brachii muscle was sometimes silent during instances of supinator activity at knuckle release, possibly because of the quite extended position of the gorilla elbow during knuckle-walking. The pronator and supinator muscles acted alternately in the African apes as the subjects pronated and supinated their forearms during manipulation (Fig. 1) and suspensory behavior (Fig. 2). Synchronous antagonistic activity during rotation was rarely seen between the pronator teres and supinator muscles in the chimpanzee (Fig. 2). This never occurred between the pronator quadratus and supinator muscles. In the chimpanzee, the pronator teres muscle was active as a brake at the end of Simultaneity and discreteness several bouts of supination on the trapeze. In The pronators were usually simultane- one instance, his pronator quadratus muscle ously active in the chimpanzee and gorilla. did not act initially during pronation of the In both African apes, pronator quadratus forearm from a markedly supinated position; activity was relatively higher than that of the recoil of the supinator muscle may have the pronator teres muscle during pronation been sufficient t o initiate pronation. His prowith the elbow extended (Fig. 1).The prona- nator teres and supinator muscles were oftor quadratus muscle appeared to be the ten simultaneously active during hoisting primary pronator in the gorilla; the relation- behaviors. As he held the investigator ship between the two muscles is unclear for around the neck with his hand semiprone, the chimpanzee. the supinator muscle evinced slight EMGs, The biceps brachii muscle was secondary the pronator teres muscle produced moderto the supinator muscle as a rotator of the ate EMGs, and the pronator quadratus mus- floor elicited marked activity in the gorilla’s pronator teres muscle early, and slight activity later, in test sessions. Knuckle-walking up and down the ramp recruited negligible EMG activity in the pronator teres muscle. Activity during the stride showed less periodicity when she walked down the ramp than when she walked up the ramp or on a level surface. Stepping off the stage was accompanied by negligible and slight activity which often decreased when her knuckles contacted the floor. When the gorilla walked with her hands in a modified palmigrade posture, EMGs rangingfrom negligibleto marked occurred in the pronator teres muscle during stance phase. Activity was marked as she climbed onto the stage with her hand in a modified palmigrade posture. Nil and negligible EMGs were recorded as the subject stood quadrupedally on her knuckles or fists; but activity rose to negligible and slight levels when she used a modified palmigrade stance. Tripedal stances on the knuckles elicited nil and negligible activity in the pronator teres muscle Activity was negligible as the subject rose from a sitting to a quadrupedal posture with her hand supinated slightly. On one occasion the groggy subject rose from a sitting position to a quadrupedal stance by pushing and concurrently pronating the hand several times. Marked activity occurred as she pushed and pronated; activity was nil as she relaxed and supinated between pushes. The pronator teres muscle was silent as the gorilla stood bipedally facing a wall and rested her right palm against it. Activity was marked as she then leaned to her left and rotated counterclockwise about the right hand. Activity dropped to nil as she ceased to flex her body laterally; it remained nil as she stood erect and rotated clockwise about the right hand. EMG OF PONGID PRONATORS AND SUPlNATORS cle was silent. When he held a large ball against his chest with the hand semiprone or slightly supinated, moderate and marked EMG activity occurred in the supinator muscle and the pronator teres muscle was silent. The pronators usually alternated with the supinator muscle during quadrupedal progression by the African apes, though synchronous activity was often seen in these muscles as the subjects shuffled or slid their knuckles across the floor. Insufficient data were recorded on the activity of the pronators as the biceps brachii muscle acted during elbow flexion without concurrent supination. Elbow flexion with maintenance of a prone hand position was infrequent in all subjects. In the orangutan, the pronator quadratus muscle was not recruited to maintain a prone hand position during hoisting. No pattern could be discerned in the chimpanzee or the gorilla during hoisting behavior (Fig. 2). DISCUSSION Comparison with humans The electromyographyof the forearm rotators in humans has been summarized by Basmajian and De Luca (1985)and MacConnail1 and Basmajian (1969).In humans, as in African apes, the biceps brachii muscle is subordinate to the supinator muscle in various supinatory actions. The pronator quadratus muscle is the primary pronator in humans; but its predominance over the pronator teres muscle is less than that of the supinator muscle over the biceps brachii muscle. This relationship may hold also in the African apes. But the smaller difference in the activity levels of the pronator muscles and difficulties in obtaining frequent and consistent movements in the apes leaves this unclear. Antagonistic behavior, which would be expected at the beginning of rotation and in a braking capacity at the end of rotation (MacConnaill and Basmajian, 1969),was not seen in humans. Per contra, antagonistic behavior was occasionally evinced between the supinator and pronator teres muscles of the chimpanzee. Future studies Having documented basic action patterns of the pronator and supinator muscles in a limited arena of experimental challenges, we are left with questions about their mechanical roles in relation to the shapes and relative lengths of long bones in the forelimbs of 225 natural apes, modern humans, and prehistoric Hominidae. Inferences from Swartz’s (1990) thoroughgoing study on forelimb mechanics in suspensory anthropoid primates are suggestive here. Swartz (1990) found that although differences in muscle mass development as determined by dry weight (Tuttle, 1969,1972a,b), were correlated with differences in radial mediolateral curvature between highly suspensory anthropoid primates (gibbons and spider monkeys) and more quadrupedal monkeys, this correlation could not explain either the allometry of curvature or intertaxonal differences in curvature among them. In order to pinpoint the relationships of specific radioulnar rotator muscles to radial curvature (and the effects of forelimb elongation), we need concurrent strain gauge measurements on the radius and EMG recordings of the supinator and pronator muscles as subjects engage in a full spectrum of stationary and vigorous quadrupedal, suspensory, and manipulatory behaviors. The preliminary information that we have detailed herein should facilitate the construction of productive experimental protocols. We also recommend that the hypothesis of Swartz (1990) that the supinator muscle is prominently active during hylobatid brachiation be tested in vivo. These studies should allow us t o interpret radial curvature in Neandertals (Trinkaus, 1983) and other fossil hominoids with greater precision and bounded imagination. ACKNOWLEDGMENTS This investigation was supported mainly by NSF grants GS-3209, SOC75-02478, and BNS 8540290 and by a Public Health Service research career development award (l-KO4GM16347-01)from the National Institutes of Health. It was supported also in part by NIH grant RR-00165 from the Division of Research Resources to the Yerkes Regional Primate Research Center, which is fully accredited by the American Association of Laboratory Animal Care. We are especially grateful for the assistance of J. Malone, E. Regenos, J. Perry, Dr. G.H. Bourne, Dr. F.A. King, R. Pollard, S. Lee, R. Mathis, J. Roberts, Dr. M. Keeling, Dr. M. Vitti, J. Hudson, K. Barnes, and L. Doan. 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