Electromyography of knuckle-walking Results of four experiments on the forearm of Pan gorilla.код для вставкиСкачать
Electromyography of Knuckle-walking: Results of Four Experiments o n the Forearm of Pan gorilla RUSSELL TUTTLE, JOHN V. BASMAJIAN, ELEANOR REGENOS GLENN S H I N E Department of Anthropology, 1126 East 5 9 t h Street, University af Chicago, Chicago, Illinois 60637; Department of An a to m y , Emory University, Atlanta, Georgia 30322, (2nd Emory Regional Rehabilitation Research a n d Training Center, Emory Uniuersity, Atlanta, Georgia 30306 AND K E Y WORDS Electromyography Forearm muscles . Joints. . Knuckle-walking . Gorilla ABSTRACT Preliminary results of electromyographic (EMG) studies on the forearm of a gorilla provisionally support the hypothesis that special closepacked positioning mechanisms may characterize the wrist and metacarpophalangeal joints 11-V in extant knuckle-walkers (chimpanzees and gorillas). We recommend that once the bony features related to these close-packed positions are clearly identified, they may be employed strategically to discern evidence of a knuckle-walking heritage in the hands of extant hominoids, including man, and to trace the history of knuckle-walking in available fossils. This report contains results of the first successful employment of indwelling fine-wire electrode techniques to elucidate problems on the functional and evolutionary biology of great apes. Knuckle-walking is the characteristic hand posture employed by gorillas and chimpanzees during terrestrial locomotion and quiescent stance (Tuttle, '67, '69a,b, c,d, '70; Tuttle and Basmajian, in press). Some anthropologists have suggested that the late Tertiary antecedents of man were knuckle-walkers (Washburn, '67, '68a,b; Sarich, '71). In order to elucidate this fascinating riddle of hominoid evolutionary biology, we must achieve a thorough understanding of the biomechanics of knuckle-walking and define the morphological features that characterize the hands of knucklewalking apes (Tuttle and Basmajian, in press). Only then may we hope to discern vestiges of a knuckle-walking heritage should they exist in hominid hands. Electromyographic (EMG) studies of man have revealed that special relationships of ligaments and the articular surfaces of bones often produce close-packed positions i n joints. In close-packed position, the apposed articular surfaces in a joint are fully congruent and are held together tightly by the chief ligaments of the joint in such a way that they cannot AM. J. PHYS. ANTHROP.,37: 2 5 5 2 6 6 . be separated by traction across the joint (MacConaill and Basmajian, '69: 25-26; Basmajian, '72: 293). Either alone or synergetically with modest muscular activity, close-packed positioning mechanisms may suffice to maintain the integrity of joints during quiescent postures and during certain motile activities for which considerably greater activity of muscles might be incorrectly anticipated prior to EMG studies (Basmajian and Bazant, '59; Basmajian, '65, '67, '72; MacConaill and Basmajian, '69). Thus, elec tromyography is a valuable technique for indicating the extent to which close-packed positioning mechanisms versus (or synergetic with) contraction of muscles are vital to the integrity of human joints. If close-packed positioning mechanisms are indicated by EMG studies, then correlative osteological studies may reveal which bony features are especially implicated with them. The next task for the evolutionary anthropologist is to trace such bony features backwards in time to the extent that available fossils permit this exercise. On the basis of traditional comparative 255 256 TUTTLE, BASMAJIAN, REGENOS AND SHINE morphological studies and observations of passive joint movements in great and lesser apes, Tuttle ('67, '69a,b,c,d, '70; Tuttle and Basmajian, in press) inferred that close-packed positioning mechanisms might characterize the wrists and metacarpophalangeal joints 11-V in the knuckle-walking African apes, especially in contradistinction to their counterparts in the more exclusively arboreal, arm-swinging Asian apes (orangutans, siamangs, and gibbons). In order to test and to refine this hypothesis, we have launched a series of electromyographic studies on the forelimbs of great apes. We report results of our first four successful experiments on selected muscles in the right forearm of one fouryear old female western lowland gorilla (Pan gorilla gorilla). We also proffer several provisional inferential conclusions on the biomechanics of knuckle-w alking . This study constitutes the first successful employment of indwelling fine-wire bipolar electrodes in functional and evolutionary studies of the Pongidae. Such research was urged upon evolutionary anthropologists more than twenty years ago (Washburn, '5 1). METHODS The subject is injected in the hip muscles with 10 mglkg body weight of Ketelar [dl-2-(0-chlorophenyl)-2-(methylamino) cyclohexanone hydrochloride] so that she will remain passive while EMG apparatus is applied to her forearm. During each of the four experiments, two electrodes are employed to record activities of different muscles. The Karma fine-wire bipolar indwelling electrodes (Basmajian and Stecko, '62) are implanted with small gauge hypodermic needles. Before withdrawing the needles, we attempt to confirm placement of the electrodes by manipulating joints that the muscles cross. The needles are removed and the free (proximal) ends of the electrodes are connected by special encased metal springs (fig. 1) to differential preamplifiers. The preamplifiers are taped to the forearm at points distal to the electrodes. The entire system is grounded by a metal plate that is taped and cemented onto the shaved surface of the forearm. Lead wires from the preamplifiers are arranged along the dorso-lateral aspect of the arm and shoulder so as to permit free movement across joints. The forearm is wrapped loosely with a n elastic surgical bandage. The subject is attired in a onearmed denim jacket. The lead wires and one component of a connector are pulled through a n opening directly into a pocket on the back of the jacket (fig. 2 ) . The subject is transported to a specially designed trailer that contains a n exercise area (3.6 m by 3.9 m) and a recording booth. In the trailer, the two components of the connector are joined. A highly flexible ribbon cable extends from the connector through a slot near the ceiling and terminates at equipment in the recording booth. Two channels of electromyogram, one channel of reference pulse, and one channel of narration are calibrated and monitored on a Model 564B Tectronix oscilloscope. These are recorded permanently on a Hewlett-Packard four channel tape recorder. Concurrently, the narration of the investigator and behavior of the subject are recorded on a model VR 7000 Ampex Video Recorder. All systems run continuously from the time the subject is connected to the cable until she is fully recovered from anesthesia and numerous acceptable electromyograms are obtained or until both EMG channels on the oscillogram indicate that the electrodes are no longer operant. Subsequent to each experiment, visicorder records are produced from the FM tape. These are analyzed for details about EMG potentials. The entire visicorder record of each experiment is examined in order to locate the most marked activities of each muscle. We select EMG bursts that are relatively free from artifacts and which are characterized by high frequency and amplitude of potentials. These serve as referents for estimates on the magnitude of other potentials produced by the same muscle during that experiment only. All potentials that are relatively free of artifact are designated very marked, marked, moderate, slight, or negligible (Basmajian, '67: 46-47). Sound channels on the FM tape and ELECTROMYOGRAPHY OF KNUCKLE-WALKING 257 Fig. 1 Attachment of proximal end of electrode to spring connector of a differential preamplifier. 1 . Spring is ejected from protective encasement. 2. Electrode is placed between coils of the spring prior to its return into the encasement. (Symbols: p, second preamplifier; g, ground plate) 258 TUTTLE, BASMAJIAN, REGENOS AND SHINE Fig. 2 Subject standing tripedally while feeding with left hand. Note that right manual digit V i s not touching the floor. Also note ribbon cable exiting from the pocket on the right side of the back of the jacket. The photograph was taken late in the experiment when the subject showed no effects of anesthesia (photo by F. Keirnan, Yerkes Regional Primate Research Center). Video tape are synchronized so that particular behavioral episodes may be related to specific EMG potentials on the oscilloscope and visicorder records. The methods employed in our experiments may produce EMG records that are as good as the best electromyograms from normal human subjects. But the frequency with which we achieve such results is less than that in some human electromyographic laboratories since we cannot adjust many components of our equipment on the alert subject. We have been unable to quantify our results precisely and otherwise render them statistically since our subject does not follow a protocol of relatively stereotyped movements as human EMG subjects do. We believe that summary descriptions of major spontaneous behavioral events constitute the most informative rendering of our preliminary data. As our approach is refined and more experiments are conducted on larger samples of subjects, we may employ mathematical and computer renderings of pongid EMG data. RESULTS We have recorded EMG potentials from 259 ELECTROMYOGRAPHY OF KNUCKLE-WALKING the following five muscles: flexor carpi radialis, flexor carpi ulnaris, extensor carpi ulnaris, flexor digitorum superficialis, and flexor digitorum profundus. The muscles studied in each experiment and the quality and duration of results are listed in table 1. The subject is very groggy during the initial 15 minutes of each experiment, but she recovers rapidly thereafter. Approximately 45 minutes after transport to the trailer, the subject generally shows no effects of the anesthetic. She readily picks up small objects from the floor and her locomotion is indistinguishable from that which she executes when not anesthetized. She begins to knuckle-walk during the first 10 to 20 minutes after arrival in the trailer. The subject's coordination of locomotion is impaired markedly by anesthesia during the initial interval of each experiment. During this period, she employs not only knuckle-walking postures but also modified palmigrade, fist-walking, and other postures that are atypical of alert gorillas. During early and intermediate recovery intervals, the subject typically knucklewalks with her forelimbs outstretched (protracted and abducted at the shoulders) and with her elbows stiffly extended so that her hands are at sharp angles to the floor. Often there is notable wobbling, especially in side-to-side directions, at her wrists during early knuckle-walking. As the subject recovers more fully, she employs her forelimbs as well aligned columns beneath her shoulders. Her elbows now flex during many swing phases of the forelimb. She maintains her hands in positions more or less perpendicular to the floor during support intervals of the stance phase of knuckle-walking progression. Her wrists do not flex markedly during swing phase. Elbow flexion apparently provides adequate elevation of her hands so that they clear the floor during swing phases of progression at slow and moderate paces. The subject characteristically employs manual digits 11-IV during the support interval of the stance phase. Manual digits I and V generally do not touch the floor during knuckle-walking. During many support intervals of knuckle-walking progression and stance her wrists are adducted (cf. ulnar deviation) so that the load is deflected prominently onto digits I1 and 111. Initially the subject moves with very short erratic steps, often sliding her feet and sometimes also her knuckle-walking hands along the floor. As the effects of anesthesia wane, she takes longer strides, increases the tempo of her movements, and sits and reclines less frequently between bouts of locomotion. Towards the end of a n experiment, she may run, walk bipedally, and display stomp with her feet and hands in manners indistinguishable from behaviors reported for free-ranging gorillas (Schaller, '63). I n experiment 1, the flexor carpi ulnaris and flexor carpi radialis muscles generally acted concurrently during the stance phase of knuckle-walking though their activities did not always commence and terminate at exactly the same time. In different locomotive cycles, one or the TABLE 1 M u s c l e s a n d qualit3 a n d dtiration of records in e a c h E M G e x p e r i m e n t Experiment Muscle Quality of EMG Duration min 4 Flexor carpi ulnaris Flexor carpi radialis Flexor carpi ulnaris Extensor carpi ulnaris Flexor digitorum superficialis 111 Flexor digitorum profundus I V (?) Flexor digitorum superficialis I V Flexor digitorum profundus I11 or IV Excellent Excellent Excellent Excellent 75 75 75 75 Excellent 55 Good 55 Good 60 Poor 60 260 TUTTLE, BASMAJIAN, REGENOS AND SHINE other muscle exhibited the highest level of activity or they both showed the same relative activity. Moderate and marked potentials were prominent in the flexor carpi radialis muscle during the first 40 minutes of knuckle-walking and in the flexor carpi ulnaris muscle during the first 25 minutes of knuckle-walking. In both muscles, slight potentials increased in incidence after the first ten minutes of knucklewalking. Negligible potentials were not exhibited by them during the stance phase of progression. However, during relatively long intervals of quiescent stance, including some in which the left hand was no longer in contact with the floor, the right flexor carpi ulnaris exhibited negligible muscle potentials. In experiment 2, the flexor carpi ulnaris muscle again exhibited a greater prominence of moderate muscle potentials during the first 2 5 minutes of knucklewalking than during subsequent times. But unlike experiment 1 , it did not evidence any marked potentials during knuckle-walking, Further, the number of episodes of quiescent knuckle-walking stance in which the flexor carpi ulnaris exhibited negligible potentials was greater in experiment 2 than in experiment 1. In experiment 2, negligible potentials occurred in the flexor carpi ulnaris during several cycles of slow, erratic knucklewalking. During the swing phase of knucklewalking, the extensor carpi ulnaris muscle sometimes produced slight or moderate potentials. But it often exhibited negligible potentials in swing phase. By contrast, it showed moderate and marked activities during certain manipulatory activities such a s raising food to mouth with the wrist adducted and extended slightly. The duration of activity in the extensor carpi ulnaris was notably less than that of the flexor carpi ulnaris during knucklewalking of slow and moderate tempos. The extensor carpi ulnaris usually exhibited no activity during relatively quiescent resting and feeding stances. In experiment 3 , the flexor digitorum superficialis muscle evidenced slight, moderate and marked potentials during the stance phase of knuckle-walking pro- gression. Moderate potentials were predominant throughout the experiment. Marked potentials occurred during the initial 25 minutes of knuckle-walking. Marked potentials were exhibited not only during the stance phase of locomotion but also during quiescent stances while the subject was still groggy. Thereafter, feeding and resting stances were accompanied by moderate or slight potentials. By contrast with the flexor digitorum superficialis, the flexor digitorum profundus exhibited only slight and negligible potentials during knuckle-walking stance and progression, except one bout in which two successive steps were accompanied by moderate activities. The only other moderate or greater potentials in the flexor digitorum profundus muscle were produced during manipulatory behaviors, for example when the subject tightly grasped cloth. I n experiment 4, the flexor digitorum superficialis chiefly exhibited negligible potentials during knuckle-walking progression and stance; but slight, moderate and marked potentials also occurred during stance phases in some bouts of brisk locomotion. On several occasions, prehensile actions of the fingers were accompanied by larger potentials in the flexor digitorum superficialis than during immediately antecedent bouts of knucklewalking. During one sequence in which moderate and marked activities were prevalent, there is evidence for notable propellant flexion of metacarpophalangeal joints 11-IV just prior to release into swing phase. In experiment 4, the flexor digitorum profundus muscle exhibited negligible or nil potentials during stance phases of knuckle-walking progression and during quiescent stances. In summary, higher EMG potentials generally are produced in the flexor carpi ulnaris, flexor carpi radialis, and flexor digitorum superficialis muscles early in our experiments when the subject is groggy and executes a sprawling pattern of knuckle-walking than subsequently when she employs well coordinated knucklewalking in a n alert state. The extensor carpi ulnaris and flexor digitorum profundus muscles appear not to be promi- ELECTROMYOGRAPHY OF KNUCKLE-WALKING nantly active during most quiescent knuckle-walking stances and during progression at slow and moderate paces. DISCUSSION Gorillas and chimpanzees possess limited capacities for dorsiflexion (extension) and adduction (ulnar deviation) of the wrist by comparison with orangutans and hylobatid apes (Tuttle, ’67, ’69a,b,c,d, ’70). The African apes have capacities for hyperextension of metacarpophalangeal joints 11-V that are remarkably greater than those of the Asian apes (Tuttle, ’69a e t seq.). Students employing classic descriptive and comparative morphological approaches to hominoid hands inferred severally that different morphological complexes might be involved fundamentally in the biomechanics of knuckle-walking. Virchow (’29: 494) reported the palmaris longus, flexor carpi radialis, and flexor carpi ulnaris muscles to be important structures limiting dorsiflexion in the wrist of Pan troglodytes but he noted that other restraints must also be considered. Schreiber (‘36) believed that Virchow overemphasized the role of the proper flexor muscles as factors limiting dorsiflexion in the chimpanzee wrist. He proposed instead that the shapes and articular surfaces of bones and the structure of the palmar ligaments must be important factors, if not the principal ones, in limiting dorsiflexion in wrists of Pan troglodytes. Straus (’40) assumed a position intermediate between those of Virchow and Schreiber though he clearly favored greater emphasis on intrinsic features of the wrist instead of the flexor muscles as factors limiting marked dorsiflexion in the chimpanzee (Tuttle, ’67: 192). These authors conducted their studies on single or few embalmed, unembalmed, and alert subjects of various ages. On the basis of a much larger sample of embalmed, unembalmed, and anesthetized African apes, Tuttle (’67 et seq.), in agreement with Schrieber, concluded that although synergetic action of flexor and extensor muscles across the wrist joint may provide some support during knucklewalking, the interaction of bones and 261 ligaments within the wrist are probably the primary factors which prevent it from dorsiflexing during load-bearing phases of knuckle-w alking . Schreiber (’36) and Tuttle (’67 e t seq.) considered that the palmar carpal ligaments, which limit dorsiflexion on the lateral side of the wrist, also limit adduction in Pan troglodytes. Marked adduction would appear to be likely in load-bearing wrists of African apes because of the notable proximal positioning (termed “retreat” and “withdrawal” by Lewis, ’65) of the distal end of the ulna relative to the distal end of the radius and proximal row of carpal bones. Several different factors are posited to explain the capacity for African apes to walk with the backs of middle phalanges as the sole manual contact with the substrate. Wilder (1861) and Straus (’40) emphasized permanent shortness of long digital flexor muscles, especially the flexor digitorum profundus muscle, as the principal feature related to typical knucklewalking hand postures. Straus (‘40: 205) believed that shortened digital flexor muscles were primarily related to “brachiation,” enabling “the great apes to hang by their hands for a considerable length of time, without expenditure of great muscular energy, merely through slight dorsiffexion of the wrist.” In two unembalmed subadult chimpanzee specimens, Straus (’40) found that sectioning the tendons of the flexor digitorum superficialis muscle lessened the digital flexion that normally accompanies dorsiflexion of the wrist. When he severed tendons of the flexor digitorum profundus muscle, the digits did not flex at all when the wrists were dorsiflexed. Straus also studied the progressive development of restricted digital flexion during the first eight weeks of a chimpanzee’s life. He concluded that shortening of the digital flexor muscles developed very early and progressed rapidly postnatally. Tuttle and Basmajian (in press: fig. 3) suggest that the long digital flexor muscles may be “tendonized’ more extensively in the African apes than in orangutans. This would explain the greater degree of permanent shortness of the digital flexor muscles in chimpanzees and gorillas. Fig. 3 Fresh preparations of the right forearm and hand of an adult male Pan troglodytes (a) and a juvenile P o n g o p y g m a e u s (b). The flexor carpi radialis, flexor carpi ulnaris, and pronator teres muscles have been removed to expose the flexor digitorum superficialis muscle (s) and parts of the flexor digitorum profundus muscle (p) that are not covered by it. Note the remarkable “tendonization” of the flexor digitorum superficialis and indicia1 component of the flexor digitorum profundus muscle in P u n . Contrast the condition in P u n with that in P o n g o . This “tendonization” may be responsible for the permanent shortness of the long digital flexor muscles in the knuckle-walkers. ELECTROMYOGRAPHY OF KNUCKLE-WALKING Napier ('59) noted the presence of special articular shelves on the posterior aspects of metacarpal bones 11-V in chimpanzees and gorillas. He speculated reasonably that they are related to hyperextension of the proximal phalanges during knuckle-walking. Tuttle ('67) suggested that the pronounced bony ridges at the bases of the articular shelves might act synergetically with the shortened long digital flexor muscles to prevent hyperextension to a degree that the metacarpophalangeal joints might be stressed traumatically when the knuckle-walking hand is load bearing. Tuttle ('67 et seq.) discussed the possibility that the well developed lumbrical and interosseous muscles also might be important factors in effecting knucklewalking postures. But he noted that electromyographic studies would have to be employed to test this hypothesis. Subsequently, Tuttle ('69a: 345; et seq.) specifically mentioned that full hyperextension of the proximal phalanges constituted special close-packed positions of metacarpophalangeal joints 11-V in the knuckle-walking African apes. However, Tuttle ('69a: 346) still maintained that some activity in the flexor digitorum superficialis and flexor digitorum profundus muscles probably is required even during quiescent knuckle-walking stances. Tuttle ('67, e t seq.) proposed that shortness of the long digital flexor muscles is a primary adaptation of knuckle-walkers since among apes they alone possess remarkable development of this feature. Results of our initial EMG studies on the proper flexor muscles of the wrist and the long flexor muscles of the digits will be considered now in reassessment of hypothetical biomechanical premises of knuckle-walking that were based on passive joint movements and comparative morphological studies. This employment of EMG results may lead to novel insights, though these must be further refined by EMG experiments on more subjects, improved EMG techniques, and further detailed studies on bone-ligament-muscle relationships in hominoid hands (Tuttle and Basmajian, in press). EMG results on the flexor carpi radialis and flexor carpi ulnaris muscles are consistent with the hypothesis that close- 263 packed positioning mechanisms may be operant to safeguard the wrists of knucklewalkers against dorsiflexion during quiescent stance and progression at slow and moderate tempos on level substrates. This is particularly likely on the medial side of the wrist joint; during post-recovery in our subject, the flexor carpi ulnaris evidences very low potentials during quiescent quadrupedal and even tripedal stances. Activity of the flexor carpi radialis muscle during post-recovery knuckle-walking by our subject is probably related more to limitation of adduction than of dorsiflexion in the load-bearing wrist. Previous authors may have overemphasized exclusiveness of the lateral ligaments in preventing extreme adduction of the wrist during knuckle-walking. The flexor carpi radialis and the lateral ligaments probably act synergetically to subserve this function. The fasciculus of the flexor digitorum profundus muscle to digit IV (and perhaps also that to digit 111) is remarkably inactive during knuckle-walking in our subject. This could indicate either that digit IV is relatively unimportant during her knuckle-walking or that activity of the flexor digitorum profundus muscle is not requisite to the maintenance of many knuckle-walking postures in our subject and perhaps also in sther knuckle-walkers. Several observations favor the latter explanation. Although digits 11 and 111 appear to be the principal load-bearing fingers during many bouts of knuckle-walking progression, digit IV often is in contact with the substrate. Further, during other bouts of progression and stance, the subject's weight may shift prominently onto digit IV without an increase in the activity of the flexor digitorum profundus muscle. During some stances in particular, the tips of the subject's fingers are not tightly flexed, indicating that the flexor digitorum profundus is not prominently active then. The flexor digitorum superficialis muscle may be more fundamentally involved in support of the hyperextended metacarpophalangeal joints than the flexor digitorum profundus muscle is, though the inconsistency of results between experiment 3 and experiment 4 make this suggestion tentative. Since the flexor digitorum superficialis muscle attaches directly 264 TUTTLE, BASMAJIAN, REGENOS AND S H I N E onto the middle phalanges, it might be expected to be principally involved in support of hyperextended metacarpophalangeal joints when intrinsic structures do not suffice. The fasciculus of the flexor digitorum superficialis to digit I11 produced potentials that are distinct from those produced by the flexor digitorum profundus muscle in experiments 3 and 4. The behavior of fasciculus IV of “the flexor digitorum superficialis muscle” in experiment 4 is remarkably like that of the flexor digitorum profundus muscle. Thus, we suspect that the implantation needle may have penetrated into the flexor digitorum profundus muscle in experiment 4. Alternatively, the lesser potentials exhibited by the “flexor digitorum superficialis muscle” to digit IV in experiment 4 might indicate that this fasciculus is less involved in knuckle-walking than its counterpart to digit 111. Placement of electrodes in particular fasciculi of the long digital flexor muscles is considerably more difficult than implantation in more superficial and discrete muscles like the proper flexor muscles of the wrist. This is especially true of the flexor digitorum superficialis muscle. We are quite certain of implants into the flexor digitorum profundus muscle since the needle may be inserted until it contacts bone. Although initial identification of a fasciculus may be established readily, there is a chance that the electrode will be pulled into a neighboring fasciculus after the subject becomes active. Thus, we proffer the above revisions of the biomechanical premises of knucklewalking with a note cautioning that further revisions may be anticipated on the basis of future experiments. CONCLUSIONS Preliminary EMG studies on the flexor muscles in the forearm of a gorilla suggest that future comparative morphological studies on the wrists of African apes may reveal special bony features related to certain close-packed positions imperative to knuckle-walking. These features may then be employed to discern evidence of knuckle-walking heritage in the wrists of other extant hominoids and to trace the history of knuckle-walking in available fossils. The fact that Lewis (‘69, ’72a,b) has been unable to discover particular bony features in the wrists of knucklewalkers (that would distinguish them from other hominoids) should not dissuade other workers from searching for them (Tuttle and Basmajian, in press). The fact that the flexor digitorum profundus muscle, which constitutes approximately 44% of total forearm musculature in the gorilla, is relatively inactive during many knuckle-walking behaviors indicates that special close-packed positioning mechanisms may be operant in the metacarpophalangeal joints of digits 11-V. But these mechanisms probably are not exclusive of muscle activity since the flexor digitorum superficialis and perhaps also the lumbrical and interosseous muscles may participate severally in knucklewalking episodes. The relative inactivity of the extensor carpi ulnaris muscle during knuckle-walking is probably related to the fact that the same basic posture of the wrist is maintained in the swing and stance phases of most slow and moderately paced progressions. During swing phase, when activity of the wrist extensors might be anticipated, elbow flexion elevates the hand clear of the floor and shoulder movements are probably chiefly responsible for its placement anteriorly. ACKNOWLEDGMENTS This investigation was supported mainly by NSF grant GS-3209 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 RR-00165 to the Yerkes Regional Primate Research Center. We are especially indebted to Mr. Robert Pollard for his assistance with the gorilla. We also thank J. Perry, J. Malone, and G. Super at the Rehabilitation Research and Training Center of Emory University; Dr. G. H. Bourne (Director), Gen. G. Duncan (Assistant Director), Dr. D. Rumbaugh (former Associate Director), Dr. M. Keeling, J. Roberts, F. Keirnan, and E. van Ormer of the Yerkes Regional Primate Research Center; and S. Toibin ELECTROMYOGRAPHY OF KNUCKLE-WALKING and K. Barnes at the University of Chicago for their assistance and cooperation. LITERATURE CITED Basmajian, J. V. 1965 Man’s posture. Arch. Phys. Med. and Rehab., 46: 2&36. 1967 Muscles Alive: Their Functions Revealed by Electromyography. Second ed. Williams and Wilkins Co., Baltimore. 1972 The Functional and Evolutionary Biology of Primates. R. Tuttle, ed. Aldine-Atherton, Inc., Chicago. Chap. 13,pp. 29%304. Basmajian, J. V., and F. J. Bazant 1959 Factors preventing downward dislocation of the adducted shoulder joint: an electromyographic and morphological study. J. Bone and Joint Surg., 41-A: 1182-1186. Basmajian, J. V., and G . Stecko 1962 A new bipolar indwelling electrode for electromyography. J. Appl. Physiol., 1 7 : 849. Lewis, 0. J. 1965 Evolutionary change in the primate wrist and inferior radio-ulnar joints. Anat. Rec., 151: 275-286. 1969 The hominoid wrist joint. Am. J. Phys. Anthrop., 30: 251-268. 1972a The Functional and Evolutionary Biology of Primates. R. Tuttle, ed. AldineAtherton, Inc., Chicago. Chap. 9, 207-222. 1972b Osteological features characterizing the wrist of monkeys and apes, with a reconsideration of this region in Dryopithecus (Proconsul) nfricanlts. Am. J. Phys. Anthrop., 36: 45-58. MacConaill, M. A , , and J. V. Basmajian 1969 Muscles and Movements: a Basis for Human Kinesiology. Williams and Wilkins, Co., Baltimore. Napier, J. R. 1959 Fossil metacarpals from Swartkrans. Fossil Mammals of Africa. No. 17, Brit. Mus. (Nat. Hist.), London. Sarich, V. 1971 Background for Man. Readings in Physical Anthropology. P. Dolhinow and V. M. Sarich, eds. Little, Brown and Co., Boston. Chap. 5, pp. 60-81. Schaller, G. B. 1963 The Mountain Gorilla: Ecology and Behavior. University of Chicago Press, Chicago. 265 Schreiber, H. 1936 Die Extrembewegungen der Schimpansenhand. Morph. Jb., 77: 2%60. Straus, W. L., Jr. 1940 The posture of the great ape hand in locomotion, and its phylogenetic implications. Am. J. Phys. Anthrop., 27: 199204. Tuttle, R. H. 1965 The Anatomy of the Chimpanzee Hand, with Comments on Hominoid Evolution. Ph.D. Dissertation, University of Chicago. University Microfilms, Ann Arbor. 1967 Knuckle-walking and the evolution of hominoid hands. Am. J. Phys. Anthrop., 26: 171-206. 1969a Quantitative and functional studies on the hands of the Anthropoidea. I. The Hominoidea. J. Morph., 128: 309-364. 1969b Knuckle-walking and the problem of human origins. Science, 166: 953-961. 1969c Terrestrial trends in the hands of the Anthropoidea: a preliminary report. Proc. Second Int. Congr. Primat., Atlanta, Ga. Karger, BasellNew York. Vol. 2. 192-200. 1969d The way apes walk. Science J., SA(5): 6 6 7 2 . 1970 The Chimpanzee: Physiology, Behavior, Serology, and Diseases of Chimpanzees. G. H. Bourne, ed. S. Karger, Basel/New York. Vol. 2, 167-253. Tuttle, R. H. and J. V. Basmajian 1972 Electromyography of forearm musculature i n gorilla and problems related to knuckle-walking. Advances in Primatology. Vol. 3. Appleton-CenturyCrofts, Inc., New York. In press. Virchow, H. 1929 Das 0 s centrali carpi des Menschen. Morph. Jb., 63: 48&530. Washburn, S. L. 1951 The analysis of primate evolution with particular reference to the origin of man. Cold Spring Harbor Symp. Quant. Biol., 15: 67-78. 1967 Behavior and the origin of man. Proc. Roy. Anthrop. SOC.,pp. 21-27. 1968a The study of human evolution. Condon Lectures, Oregon State System of Higher Education, Eugene. 1968b Changing Perspectives on Man. B. Rothblatt, ed. University of Chicago Press, Chicago. Chap. 8, pp. 193-206. Wilder, B. G. 1861 Contributions to the comparative myology of the chimpanzee. Boston J. Nat. Hist., 6: 352-384.