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Electromyography of knuckle-walking Results of four experiments on the forearm of Pan gorilla.

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Electromyography of Knuckle-walking: Results of Four
Experiments o n the Forearm of Pan gorilla
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
K E Y WORDS Electromyography
Forearm muscles . Joints.
Knuckle-walking . Gorilla
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
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
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).
The subject is injected in the hip muscles with 10 mglkg body weight of Ketelar
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
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
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
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)
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.
We have recorded EMG potentials from
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
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
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
Quality of EMG
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
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
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-
nantly active during most quiescent
knuckle-walking stances and during progression at slow and moderate paces.
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
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.
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-
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
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.
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.
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
and K. Barnes at the University of Chicago for their assistance and cooperation.
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