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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 77:519-528 (1988) Gluteus Maximus Muscle Function and the Origin of Hominid Bipedal ity MARY W. MARZKE, JULIE M. LONGHILL, AND STANLEY A. RASMUSSEN Department of Anthropology, Arizona State University, Tempe, Arizona 85287 (M. WM.); Columbia, Ohio (J.M. L.); Department of Health, Physical Education and Recreation, Northern Arizona University, FlugstaK Arizona 86011 (S.A.R.) KEY WORDS: locomotion Electromyography, Hominid evolution, Bipedal ABSTRACT Bipedality not only frees the hands for tool use but also en- hances tool use by allowing use of the trunk for leverage in applying force and thus imparting greater final velocity to tools. Since the weight and acceleration of the trunk and forelimbs on the hindlimbs must be counteracted by muscles such as m. gluteus maximus that control pelvic and trunk movements, it is suggested that the large size of the cranial portion of the human gluteus maximus muscle and its unique attachment to the dorsal ilium (which is apparent in the Makapan australopithecine ilium) may have contributed to the effectiveness with which trunk movement was exploited in early hominid foraging activities. To test this hypothesis, the cranial portions of both right and left muscles were investigated in six human subjects with electromyogra- phy during throwing, clubbing, digging, and lifting. The muscles were found to be significantly recruited when the trunk is used in throwing and clubbing, initiating rotation of the pelvis and braking it as trunk rotation ceases and the forelimb accelerates. They stabilize the pelvis during digging and exhibit marked and prolonged activity when the trunk is maintained in partial flexion during lifting of heavy objects. The gluteus maximus muscle of humans is distinctive in its size and attachments. The specializations are in its cranial portion, which is much thicker than in other pri- mates and has a firm attachment to the dor- sal ilium, between the posterior gluteal line and the sacrum. Some attachment fibers run to the overlying fascia, which continues to the fascia latae (Stern, 1972). To understand why humans differ from other primates in the relative size and attachments of the glu- teus maximus muscle, one must determine when it is recruited in movements of the trunk and hindlimbs. Electromyographic studies during voluntary exercises have shown that the cranial portion of the muscle functions in extension, lateral rotation, and abduction of the femur on the trunk and in controlling flexion and rotation of the trunk on the femur (Karlsson and Jonnson, 1965). It is recruited only minimally during slow walking, but contracts during jogging and running, probably functioning as an abduc- tor in maintaining lateral stability of the hip joint (Stern et al., 1980). It apparently has a similar function during fast walking and stair-climbing as each limb is loaded and en- ters the single support phase (Lyons et al., 1983). All these investigators, and also Fur- lani et al. (19741, find that the cranial portion of the muscle is distinguished from the cau- dal portion in its function of abduction of the thigh. The experiments reported here investigate the role of the cranial portion of m. gluteus maximus in controlling the trunk on fixed hindlimbs during activities by the forelimbs that exploit the leverage of the trunk. These activities include throwing, clubbing, dig- ging, lifting, and gathering of objects below waist level, all of which should have contrib- uted to the effectiveness of foraging by early Received February 17, 1988; accepted June 22, 1988. @) 1988 ALAN R. LISS, INC

Gluteus maximus muscle function and the origin of hominid bipedality

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 77:519-528 (1988)

Gluteus Maximus Muscle Function and the Origin of Hominid Bipedal ity

MARY W. MARZKE, JULIE M. LONGHILL, AND STANLEY A. RASMUSSEN Department of Anthropology, Arizona State University, Tempe, Arizona 85287 (M. WM.); Columbia, Ohio (J.M. L.); Department of Health, Physical Education and Recreation, Northern Arizona University, FlugstaK Arizona 86011 (S.A.R.)

KEY WORDS: locomotion

Electromyography, Hominid evolution, Bipedal

ABSTRACT Bipedality not only frees the hands for tool use but also en- hances tool use by allowing use of the trunk for leverage in applying force and thus imparting greater final velocity to tools. Since the weight and acceleration of the trunk and forelimbs on the hindlimbs must be counteracted by muscles such as m. gluteus maximus that control pelvic and trunk movements, it is suggested that the large size of the cranial portion of the human gluteus maximus muscle and its unique attachment to the dorsal ilium (which is apparent in the Makapan australopithecine ilium) may have contributed to the effectiveness with which trunk movement was exploited in early hominid foraging activities. To test this hypothesis, the cranial portions of both right and left muscles were investigated in six human subjects with electromyogra- phy during throwing, clubbing, digging, and lifting. The muscles were found to be significantly recruited when the trunk is used in throwing and clubbing, initiating rotation of the pelvis and braking it as trunk rotation ceases and the forelimb accelerates. They stabilize the pelvis during digging and exhibit marked and prolonged activity when the trunk is maintained in partial flexion during lifting of heavy objects.

The gluteus maximus muscle of humans is distinctive in its size and attachments. The specializations are in its cranial portion, which is much thicker than in other pri- mates and has a firm attachment to the dor- sal ilium, between the posterior gluteal line and the sacrum. Some attachment fibers run to the overlying fascia, which continues to the fascia latae (Stern, 1972). To understand why humans differ from other primates in the relative size and attachments of the glu- teus maximus muscle, one must determine when it is recruited in movements of the trunk and hindlimbs. Electromyographic studies during voluntary exercises have shown that the cranial portion of the muscle functions in extension, lateral rotation, and abduction of the femur on the trunk and in controlling flexion and rotation of the trunk on the femur (Karlsson and Jonnson, 1965). It is recruited only minimally during slow walking, but contracts during jogging and

running, probably functioning as an abduc- tor in maintaining lateral stability of the hip joint (Stern et al., 1980). It apparently has a similar function during fast walking and stair-climbing as each limb is loaded and en- ters the single support phase (Lyons et al., 1983). All these investigators, and also Fur- lani et al. (19741, find that the cranial portion of the muscle is distinguished from the cau- dal portion in its function of abduction of the thigh.

The experiments reported here investigate the role of the cranial portion of m. gluteus maximus in controlling the trunk on fixed hindlimbs during activities by the forelimbs that exploit the leverage of the trunk. These activities include throwing, clubbing, dig- ging, lifting, and gathering of objects below waist level, all of which should have contrib- uted to the effectiveness of foraging by early

Received February 17, 1988; accepted June 22, 1988.

@) 1988 ALAN R. LISS, INC

520 M.W. MARZKE ET AL

bipedal hominids. To date, the muscle has not been monitored during these behaviors except by Toyoshima and Hoshikawa (19741, who recorded strong action potentials for the muscle during normal throwing (in which the ball is thrown overhand while the contra- lateral foot steps forward and the trunk ro- tates). Unfortunately, the portion of the muscle monitored was not noted in their report.

MATERIALS AND METHODS Electrode placement

Six subjects (four men and two women) be- tween the ages of 22 and 50 years volun- teered for the study. No gait abnormalities or physical handicaps were found for these subjects.

Paired fine-wire silver enamel (100 pm) electrodes were implanted into the cranial portion of the right and left gluteus maximus muscles using a 25-gauge sterile needle. Each electrode was connected to a preamplifier at- tached to long cables inserted into a four- channeled Gilson Strip pen recorder, which transformed the myoelectrical signal onto paper. The amplification of the recorder was standardized for each subject at 20 or 50 mvl cm. A 16-mm camera was used to film the subjects in color at 32 frames per second. Synchronization of the EMG s i p a l with movements recorded on film was accom- plished by a reed switch mechanism (Chan et al., 1977). Several steps were taken to min- imize movement artifact and to ensure cor- rect placement of the electrodes. Styrofoam cups were placed over the implantation areas to prevent displacement of the electrodes by overlying clothing. Subjects walk.ed for 15 min after electrodes had been inserted, prior to recording, to allow the electrodes to settle within the muscle tissue. The position of the cranial portion of the gluteus maximus mus- cle was initially identified through palpation and by locating the dimple overlying the pos- terior superior spine. A muscle stimulator was used to generate a contraction from the upper portion of the muscle, after which the location was marked for electrode placement. Back-stimulation was performed after the electrodes had been inserted to verify that the muscle had been penetrated. For five sub- jects, the electrode insertion point was marked by a vial of antifreeze, and magnetic resonance images were taken in the trans- verse plane, showing the location of the an- tifreeze relative to the muscle. Slight vari-

ation was found between subjects in location of electrode insertion, but all images showed the marker in the region of the cranial por- tion of the gluteus maximus muscle, near its origin from the ilium.

Experimental procedure The muscles were monitored first during

walking as the subject took several steps across the laboratory floor. For throwing, a small ball of yarn (1.5 in. in diameter and weighing less than 1 gm) was aimed at a moving target. Subjects were instructed to throw vigorously to compensate for the light weight of the object being thrown. They all used an “overhand-with-step” normal throw, which produced the highest ball velocities of all throwing patterns analyzed by Toyoshima and Hoshikawa (1974). For clubbing, a base- ball bat was provided as substitute for a stick. In one-handed clubbing, the motions were similar to those of the overhand-with-step throw, in which weight is shifted from the right to the left leg as the subject clubs with the right hand. (The reverse weight distri- bution was assumed by the single left-handed subject.) For two-handed clubbing, two of the subjects kept their weight evenly on the two legs; the remaining four duplicated the one- handed clubbing posture. Digging was per- formed with a digging stick shaped according to a description of those commonly used by the !Kung San (Lee, 1982). Five subjects as- sumed a stance with feet parallel; the sixth placed one foot in front, contralateral to the hand used for digging. For the lifting activ- ity, the subject leaned down to secure the second author under the arms, raised her from the floor, and dragged her several feet, moving backward. The subject’s knees and hips were flexed and the burden was held as close to the body as possible, in an effort to limit stress on the back (Caillet, 1978; Ek- holm et al., 1982). Gathering involved flexion at the hips and knees to reach for small ob- jects scattered in patches on the floor. The degree of flexion at the joints and positioning of the limbs varied as the subjects moved between patches.

Each subject’s attire consisted of a T-shirt and loose-fitting shorts, which accommo- dated the electrodes and cables. A stiff flag was strapped to the waist on the back. Move- ment of the flag relative to a background grid permitted observation of trunk rotation in the films as the subjects performed the activities. Two trials of each activity were

GLUTEUS MAXIMUS MUSCLE AND BIPEDALITY 521

performed and recorded after the subjects had practiced each behavior several times and had become familiar with the equipment. Subjects began each activity from a relaxed position with hindlimbs together and re- turned to this position after completing the activity. Prior to a session, each subject per- formed isometric contractions of the gluteus maximus muscle and other exercises, such as lifting, to produce a large signal with which signals during trials could be compared in relative magnitude.

Data analysis Duration of the EMG signal was measured

from a point where the pen leaves the base- line to the point where the pen returns. Am- plitude of each EMG burst was determined by measuring the height of the average spike from the baseline within the burst. The av- erage spike was determined by visual inspec- tion, and its height was measured by the number of horizontal graph lines inter- cepted. Each burst was then compared to the observed height of the maximum burst pro- duced by the subject’s muscle. (In some cases, the largest signal was produced during the contractions elicited prior to the trials; in others, the maximum signal occurred during the trials.) The following scale of amplitudes was assigned (see Fig. 1): 0 = none (0-6% of the maximum burst), 1 = minimal (7-33%), 2 = moderate (34-67%), and 3 = marked ac- tivity (68-100%).

Each film was analyzed frame by frame with a stop-action projector, along with vi- sual analysis of the raw EMG data. Since complex activities like these are performed with slight differences among individuals in timing and vigor, the following procedure was followed to ensure that comparison of subjects in EMG activity was made for a similar sequence of movements. Figure drawings were made from the projected im- ages of one subject performing the full se- quence of movements for each activity. The figure drawings were attached to a viewing screen so that films of other subjects could be projected onto them, superimposing similar postures to allow direct comparison of EMG activity. A few estimates of segment acceler- ation and deceleration were made by compar- ing the number of frames required to record the distance covered by the segments during movement from one position to the next in the sequence.

I O NONE -

1 M I N I M A L -- 2 MODERATE

Fig. 1. Traces of EMG signals from the cranial portion of the gluteus maximus muscle illustrating variations in the relative magnitude of activity for one subject during clubbing with one hand. Scale (based on percent- age of maximal burst): 0 = no activity (0-6%), 1 = minimal activity (7-33%), 2 = moderate activity (34- 67%), 3 = marked activity (68-100%). Actual percent- ages for the signals illustrated here are 3%, 20%, 46%, and 100% respectively.

RESULTS Walking

Among the six subjects, only three pro- duced EMG signals during walking, and in them both duration and amplitude of EMG were minimal, occurring immediately before and during heel strike.

Throwing The sequence of movements of the body

segments of right-handed individuals during throwing and the accompanying activity of the gluteus maximus muscles are shown in Figure 2. (Movements and signals of the left- handed individual were the reverse of those shown in Fig. 2.) A striking feature of the sequence in subject 1 was the acceleration of the forelimb that accompanied decel- eration of trunk rotation as the arm reached the level of the ear prior to release of the ball (F-G). The right gluteus maximus muscle became active as the pelvis rotated counter- clockwise toward the left hindlimb prior to or at the moment when the left foot touched the ground. (This counterclockwise rotation of the pelvis on the thigh is the counterpart of lateral rotation of the thigh on the pelvis.) It remained active throughout rotation of the trunk until the right foot was lifted from the floor (G-H). The left muscle was recruited soon after the right, following left foot con- tact and initial counterclockwise rotation, just prior to acceleration of the throwing arm

522 M.W. MARZKE ET AL

J I H G F E D C

I I I I I I 1 I I I I I I I I !

I

I I I I I I I I I I I I

Fig. 2. Sequence of body movements with accompany- ing EMG activity in five right-handed subjects during throwing. Graph reads from right to left. ]Duration of EMG bursts from onset to offset is represented by the length of the rectangular bars. Amplitude is depicted with shading: white, minimal; gray (stippled), moderate;

when the arm began its upward lateral movement. Moderate or marked activity was maintained in four individuals during the follow-through after release of the ball, prob- ably to control the anterior tilt of the pelvis and then to extend it, since pelvic rotation had ceased. EMG signals decreased. to a min- imal level or even disappeared as the trunk was raised from 70" to 30" of flexion. In the most skilled thrower (11, whose arm exhib- ited the greatest acceleration, the left muscle ceased its contraction earlier than in the other subjects.

Onehanded clubbing During this behavior, all subjects main-

tained most of their body weight on the left hindlimb, eliciting strong EMG signals from the left gluteus maximus muscle and no ac- tivity from the right Pig. 3). The left muscle was recruited as the pelvis rotated counter- clockwise toward the left leg and the arm wielding the club approached the level of the ear or began to advance downward. Moder- ate to marked potentials continued through ground strike and activity ceased. abruptly

black, marked. The left leg is lifted to take a step (posi- tion B). As the left foot contacts the ground (C,D), the trunk begins to rotate in a counterclockwise direction, followed by the upward and lateral movement of the throwing arm (E,F). The ball is released between F and G.

after the trunk had returned to approxi- mately 30" of flexion.

Twehanded clubbing Four subjects used essentially the same

movements of the limbs for two-handed club- bing as they did in the one-handed clubbing session, producing a similar recruitment pat- tern of the left gluteus maximus but also some activity by the right muscle. The re- maining two subjects distributed their weight more evenly over both hindlimbs, eliciting a different pattern of activity from both right and left gluteus maximus muscles (Fig. 4). The magnitude of the signal was typically less than that produced during throwing and one-handed clubbing, showing minimal to moderate amplitudes. Initial recruitment of the muscles varied among subjects, but al- ways occurred as the trunk was flexing and the club was moving rapidly downward. EMG activity continued through ground strike, decreasing as the rising trunk moved through a range from approximately 60" to 10" of flexion.

GLUTEUS MAXIMUS MUSCLE AND BIPEDALITY 523

I 4 9 5 ~~~ .̂ ..,...... >..> ....... I i 5 q

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I I I

I I I I I I

I I I I

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Fig. 3. Sequence of body and tool movements with accompanying EMG activity in five right-handed sub- jects during one-handed clubbing (see legend for Fig. 1).

The trunk rotates counterclockwise and flexes to approx- imately 80" (B-D). The club strikes the ground at E. The right gluteus maximus muscle is silent in all subjects.

n G F E D C A

I I I I I I

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Fig. 4. Sequence of body and tool movements with accompanying EMG activity in two subjects during two- handed clubbing, with the legs approximately parallel

(see legend for Fig. 1). The trunk flexes until the club strikes the ground (C-E). Both left and right muscles are active.

Twehanded digging

One subject moved the limbs as he did in one-handed clubbing and elicited similar EMG activity. Another did not engage the

trunk enough to trigger gluteus maximus muscle contraction. The remaining four ex- hibited the same movements and associated EMG activity as were shown in the two- handed clubbing sequence, with the feet ap-

524 M.W. MARZKE ET AL.

H G F E I I I I 1 1 1 I I I I I 1 1

Fig. 5. Sequence of body and tool movements with accompanying EMG activity in five subjects during dig- ging with the legs approximately parallel (see legend for

Fig. 1). The trunk flexes until the stick strikes the dirt (D-F).

proximately parallel (Fig. 5). InitLal recruit- and maintaining some flexion at the hip and ment of both muscles typically occurred as knee during walking between patches. Since the trunk flexed between 30" and 90". Mod- they varied considerably in the timing of erate activity was maintained through movements, only one subject is represented ground strike and decreased to minimum as in Figure 7. EMG activity was similar for the trunk was lifted onto the hindlimbs. similar movements. In all subjects, minimal

to moderate signals were produced by the left Lifting and/or right muscle as the trunk reached

The gluteus maximus muscle was more ac- approximately 40" to 50" of flexion. The larg- tive (both in duration and in amplitude) in est potentials occurred when subjects moved all subjects during lifting than in any other between patches of objects with their trunks activity (Fig. 6). There was minimal to no flexed. Duration and magnitude of contrac- signal as they flexed the trunk approxi- tion varied as weight shifted from one foot to mately 90" to reach the load, but a dramatic the other, with cessation during all or most increase in signal amplitude occurred as they of the swing phase. One or both of the mus- lifted the load, with the majority of subjects cles contracted as the trunk was returned to showing marked potentials. The high mag- an erect position at the end of the activity. nitude was maintained throughout the activ- ity until the load was released. Contraction of the left and right muscles was not contin- The EMG data from all sessions demon- uous, but there was considerable overlap in strate that the cranial portion of the gluteus phases, with variability among individuals maximus muscle is only minimally recruited in the duration and amount of over- during walking but generates moderate to lap. large signals when the hindlimbs are fixed

and the trunk is used 1) for leverage to en- Gathering hance the force and thus impart greater final All subjects moved in a way illustrated in velocity to the hand-held tools in throwing,

Figure 7, bending the spine, flexing the hip clubbing, and digging; and 2) to control and joints, flexing the knees to reach the objects, move the trunk during bending and lifiing.

DISCUSSION

A

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GLUTEUS MAXIMUS MUSCLE c cxc vcvcfkdfd' d 525

I I I 1

Fig. 6. Sequence of body movements with accornpany- ing EMG activity in subject 1 during lifting. This se- quence reads from left to right (see legend for Fig. 1).

Fig. 7. Sequence of body movements with accompany- ing EMG activity in six subjects during gathering. Read from right to left (see legend for Fig. 1).

It is not surprising, therefore, that the mus- cle is large in humans, since in addition to moving the hindlimb it also functions regu- larly to move, and to resist movement of the trunk, forelimbs, and objects manipulated by the forelimbs. When the forelimbs are being moved vigorously and rapidly, the moments to be resisted by the muscle are particularly large. In explaining the abundance of human trunk musculature, in excess of that neces- sary to raise external loads, Farfan (1978) notes that accelerated motions of the spine that occur in activities such as striking a baseball may double the effective body weight, thus requiring the excess of muscu- lature to stabilize the trunk. Attachment of gluteal fibers to the ilium, further cranially from the hip joint than the skeletal attach- ments in nonhuman primates (Stern, 1972), provides mechanical advantage when the muscle is producing and controlling move- ment of the trunk on the hindlimbs. Drawing upon previous observations of recruitment

patterns of the gluteus maximus muscle dur- ing voluntary exercises, it is possible to ana- lyze the role played by the muscle in the complex activities monitored in the present experiment.

Throwing In the skilled thrower (11, contraction of the

left muscle began as the left foot contacted the floor. The forelimb segments then accel- erated, and soon afterward muscle contrac- tion and rotation ceased at approximately the same time. Toyoshima and Hoshikawa (1974) have shown that approximately 46.9% of the velocity of the overhand throw comes from the step and rotation of the pelvis and trunk. Deceleration of trunk rotation allows the throwing forelimb to accelerate, thus in- creasing the speed and distance of the thrown object (Atwater, 1970). Atwater (1970) found that skilled throwers tend to decelerate the trunk to zero velocity prior to ball release, whereas counterclockwise rotation tends to

526 M.W. MARZKE ET AL.

continue in average women throwers. It thus appears that the gluteus maximus muscle in subject 1 is functioning as a brake to stop trunk rotation and to thus allow the forelimb segments to accelerate. [Karlsson and Jonn- son (1965) demonstrated that the cranial por- tion of the gluteus maximus muscle con- tralateral to the direction of rotation is acti- vated when the pelvis and trunk rotate on fixed hindlimbs.] "he same pattern of fore- limb acceleration accompanied by a rela- tively short period of gluteus maximus muscle contraction was observed in a base- ball player during a pilot study for the proj- ect reported here. The remaining subjects (anthropologists with more experience in dig- ging than in throwing) did not exhibit the same amour?t of acceleration of the forelimb accompanied by brief contraction (of the glu- teus maximus muscle. The muscle continued to contract during the follow-through and partial return of the trunk to the vertical, probably controlling flexion of the trunk and then extending it. The tendency in all sub- jects toward the disappearance of muscle sig- nal in the final stages of extension is consistent with the normal sequence in which pelvic extension precedes spinal extension (Farfan, 1978).

In all subjects, the right gluteusi maximus muscle fires as the trunk begins to rotate counterclockwise on the right hindlimb, be- fore weight is transferred to the left foot. [Atwater (1970) also found that the pelvis begins rotation in this direction before the left foot makes contact during throwing.] Since the majority of the muscle lies behind the hip joint axis, it is in a position favorable for rotating its attachment area on the ilium toward its insertion on the femur. Logan and McKinney (1982) note that of all thLe hip mus- cles involved in lateral rotation of the thigh (of which counterclockwise rotation of the pelvis is the counterpart), m. gluteus maxi- mus has the greatest potential strength.

Subject 1 is distinctive in having a shorter period of contraction for the left muscle and a longer period for the right. Possibly follow- ing left hip rotation, continued flexion of the trunk between stages G and H occurred in the spine rather than at the hip. The right muscle may have been extending the thigh as the limb was held off the ground.

Onehanded clubbing Fig. 2). Since the hindlimb movements are similar

to those during throwing, the left gluteus

maximus muscle appears to act initially to brake rotation and flexion of the trunk as the clubbing limb accelerates. After ground strike, the muscle probably functions as an extensor as the weight of the trunk, forelimbs, and club is raised. The higher magnitude of muscle activity recorded during this activity compared with activity during throwing is probably related to the greater weight of the club.

Twehanded clubbing and digging Subjects who moved one leg forward as they

clubbed or dug with two hands exhibited muscle activity comparable to that in one- handed clubbing. In subjects maintaining more even distribution of weight on the two hindlimbs Fig. 4), both the right and left gluteus maximus muscles fired, apparently controlling rapid flexion of the trunk before ground strike, stabilizing the hip joints at ground strike, and then raising the trunk again on the hindlimbs. Activity of the mus- cle during trunk flexion and extension has been well documented in EMG analyses of controlled exercises (Farfan, 1978). The up- per portion of the muscle is in a more favor- able position to control anterior pelvic tilt than the lower, because its attachment to the dorsal ilium is more cranial to the hip joint and thus closer to the center of gravity, where it exploits a longer power arm. Karlsson and Jonnson (1965) found that the cranial portion slightly exceeded the caudal in activity dur- ing maintenance of trunk flexion at 15 O , 30°, and 90".

In activities involving the striking of ob- jects by hand-held extensions of the forelimb, the best performer accelerates the fastest or through the greatest distance, creating the greatest striking force (Cooper et al., 1982). It is possible, then, that the hip extensor musculature in subjects monitored here was functioning in part to brake the trunk, allow- ing forelimb acceleration prior to ground strike.

Lifting and gathering Many of the movements and signals gen-

erated by the muscles in the two activities are similar, but the spine bends less during lifting, with the result that more bending occurs at the hip joint. The larger signals from the gluteus maximus muscle during lifting no doubt reflect the large force neces- sary to overcome the large internal and ex- ternal loads. Muscle activity does not begin

GLUTEUS MAXIMUS MUSCLE AND BIPEDALITY 527

until the trunk has partially flexed, probably reflecting the tendency of the spine to flex before the pelvis during forward bending (Farfan, 1978). When the muscles contract, they are probably acting as extensors, con- trolling anterior pelvic tilt, and as abductors when weight shifts primarily to one hind- limb. Karlsson and Jonsson (1965) observed activity of the upper portion of the gluteus maximus muscle in subjects that maintained a crouched position with both hips and knees flexed at about 60" and go", respectively, and suggested that it was functioning as a hip extensor. In an analysis of m. gluteus maxi- mus fiber length, Stern and Susman (1983) found that when the thigh is flexed more than 60" the muscle assumes a position an- terior to the hip joint. In this position it would be expected to function as a medial rotator of the thigh or in lateral rotation of the trunk on the thigh. Both right and left muscles were active in our subject when the hip joints were at maximum flexion shown in Figure 6. EMG analysis of the muscle during exercises that test these flexed postures and move- ments separately are necessary to clarify its functions in the more complex activities.

During some stages of lifting, the muscle also may be rotating the thigh laterally, to a position known to enhance the extensor ac- tion of the muscle in weight lifters by in- creasing the distance between the muscle and the axis of the hip joint (Farfan, 1978).

Bending of the knees during these activi- ties requires contraction of the quadriceps muscle, which tightens the iliotibial band across the knee joint, providing a stable ori- gin of gluteal muscle fibers and thus enhanc- ing their function in drawing the pelvis toward the femur during lifting. The effec- tiveness of the gluteus maximus muscles in abduction of the trunk depends on the degree of flexion at the hip joint. In all subjects, hip flexion during lifting was maintained be- tween 45" and 90" while the load was moved, an angle at which all fibers of the muscle, and particularly those of the cranial portion, are in an optimal position to abduct the trunk on the thigh (Kapandji, 1983). The advantage of having a large cranial portion with an attachment to the ilium is that both the greater force and longer power arm facilitate prolonged control of the heavy trunk and its external loads.

CONCLUSION

It was found that the cranial portion of the human gluteus maximus muscle plays two

significant roles during activities involving use of the forelimb. First, it positions the trunk and maintains it at angles optimal for retrieving and carrying loads. Second, it con- tributes to controlled rotation of the trunk, initiating and then braking the trunk against acceleration when it is used for leverage in accelerating hand-held tools. The distinctive features of the cranial portion of the human gluteus maximus muscle (its large size and its attachments to the ilium and the fascia latae) seem to find an explanation in these functions requiring considerable torque to re- sist the forces and momentum generated by the weight and movements of the trunk, fore- limbs, and hand-held loads. The large size of the muscle contributes force, its attachment to the ilium near the center of gravity of the body provides leverage, and the attachment t o the iliotibial tract adds fibers that act as a fixed origin of the muscle when the trunk is being moved on flexed hips and knee joints.

Stern (1972) suggested that the distinctive features of the human gluteus maximus mus- cle may have developed in response to loco- motor behaviors involving marked shifts in the center of gravity, and the muscle's func- tion in stabilizing the hip during jogging and running was later demonstrated (Stern et al., 1980). The experiment reported here corrob- orates their evidence that the size and at- tachments of the muscle are compatible with requirements in humans for control of the trunk on the hindlimbs in activities involv- ing major shifts in the center of gravity. It also demonstrates that activities involving use of the forelimbs and bending of the trunk during bipedal posture and slow walking, as well as jogging and running, are enhanced by the uniquely human features of the mus- cle.

Two sources of evidence indicate that effec- tive use of the trunk for leverage in foraging activities, such as lifting and gathering ob- jects from the ground and low bushes, dig- ging, pounding, clubbing, and throwing, was within the potential of hominids early in the course of their evolution. The first source is the skeleton of AL 288 from Hadar, represen- tative of the species Australopithecus afaren- sis, in which the forelimbs are smaller relative to body size than they are in pongids (Jungers and Stern, 1983). These proportions indicate that the total weight that had to be balanced by trunk and pelvic musculature was proportionately less than in pongids. The second indication that the trunk could be used effectively for leverage is the presence

528 M.W. MAR

of an attachment area for M. gluteus maxi- mus on the australopithecine ilium from Makapan (Dart, 19491.1

The occurrence in Australopithecines of morphological features that enhance control and movement of the trunk on fixed hind- limbs is consistent with the hypothesis that foraging behaviors exploiting the leverage of the trunk, forelimbs, and hand-held tools may have been a factor in the origin olf hominid bipedality (Marzke, 1986).

It is likely that differences from modern humans in the vertebral column, rib cage, and pelvis of early hominids, Homo erectus, and Neandertals (Tague and Lovejoy, 1986; Schmid, 1983; Brown et al., 1985; Rak and Arensberg, 1987) had functional ramifica- tions for the intrinsic back muscles and ab- dominal muscles, which also may find an explanation to some extent in activities that involve rapid and forceful movements of the trunk and forelimbs. These differences are presently under investigation.

ACKNOWLEDGMENTS

We thank E. Rasmussen, M. Castro, S. Ka- plan, D. Pierotti, J. Marshall and L. Davis for their technical assistance, anonymous re- viewers for helpful comments, and our sub- jects from Arizona State University and Northern Arizona University for their partic- ipation in the study. Dr. J. Levy, Scottsdale Memorial Hospital, kindly arranged for the magnetic resonance imaging. Funding was provided by the Northern Arizona Univer- sity Organized Research Committee.

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