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Chimpanzee and human feet in bipedal walking.

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CHIMPANZEE AND HUMAN FEET IN BIPEDAL
WALKING
HERBERT ELFTMAN AND J O H N MANTER
Columbia University
S I X FIGURES
There is general agreement today among most competent
investigators that man numbers among his ancestors some
form, which, if living, would be classed with the anthropoid
apes. There is still divergence of opinion a s t o the closeness
of relationship of this ancestral form to the apes now living..
But the most important problem confronting students of
mail’s evoliition is no longer the adjudication of taxonomic
claims, but the elucidation of the changes, anatomical and
physiological, which have occurred in the transformation of
the anthropoid ape ancestor into man.
One of the parts of the human body which has undergone
radical changes in the evolution from ape to man is the foot.
Previous investigators who have studied the evoliition of the
foot were handicapped by the lack of a method for studying
accurately the changes which it undergoes while it is being
used by the animal. We have been able to investigate the
physiology of the foot by means of an apparatus recently described (Elftman, ’34). I n the present paper we shall compare the function of the chimpanzee foot in bipedal walking
with the human. I n a later paper these results will be correlated with the morphology of the foot and the general problem of the eyolutioii of the foot considered.
Of the living great apes, the Chimpanzee, gorilla and gibbon
will, on occasion, walk bipedally. The orang is so characteristically arboreal in its natural habitat that it can contribute
69
AMERICAN JOURNAL O F PHYSICAL A N T H R O P O U M Y . VOL. XY, N O . 1 A N D SUPPLEMENT
APRIkJUNE,
1935
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Fig. 1 Dintribution of pressure in the human foot during bipedal walking.
Fig. 2 Distribution of pressurr in the chimpanzee foot during bipedal walking. Thr six rrcorda f r o m racah stel) which are
st rod need hrro n.r e. Rrniirated 1)v eaunl tinic intervrtln and are takrn from I( inore ronlplcte cinrmatic ~ ~ I I I I .
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CHIMPANZEE AND H U M A N FEET
71
little to our present study. We were fortunate in being able
to make records in our laboratory of the distribution of pressure in the foot of a chimpanzee, Meshie, a t the time approximately 5 years old. Her participation in this research was
made possible by the kind cooperation of Mr. H. C. Raven,
to whom we are grestly indebted. Observations were made of
the chimpanzees, gorilla and gibbon at the New York Zoological Gardens, and of skeletons in the collections of the American Museum of Natural History and of Columbia University.
Prof. J. H. McGregor generously lent us plaster casts which he
had made of impressions made by the gorilla, John Daniel 11,
while walking over a soft surface, These casts have been of
great value in comparing the method of use of the gorilla
foot with that of the chimpanzee and of man.
Throughout the discussion which follows it must always
be borne rigorously in mind that there is considerable variation in the manner in which any individual uses his foot at
different times. There is also considerable diversity in the
structure of feet. Many normal human feet do not possess as
high an arch as the particular foot illustrated. The chimpanzee does not always hold its foot in exactly the manner
used when our records were taken. I n the particular record
reproduced the animal turned slightly toward one side at the
completion of the step. But the conclusions which we reach
have been based on a study of all of our records, not merely
those illustrated here. We believe that we have been successful in allowing for these various disturbing factors.
To facilitate a comparison of the action of the chimpanzee
foot with that of the human, figures 1and 2 contains, in parallel arrangement, records of the distribution of pressure in the
foot of these two forms at six equally spaced intervals in the
course of a n ordinary step. In these records the pressure is
registered as dark dots, the size of the dot varying with the
pressure. A description of the apparatus and an account of
the general results obtained with the human foot were published by Elftman ( ’34).
AMEBIOAN JOUBNAL OF PHYSICAL ANTSIROPOLOGY, VOL. IX, NO.
1 AND SUPI’LXNENT
72
HERBERT ELFTMAN AND J O H N MANTER
SEQUENCE OF EVENTS I N TKE STEP
I n the sequence of events taking place i n the foot during a11
ordinary step there are several noteworthy differences hetween the chimpanzee and the human. Figure 3 shows stages
comparable to those of figures 1 and 2 in lateral view. In
these illustrations it will be seen that in man the heel is definitely the first part of the foot to touch the ground, while in the
chimpanzee there is a tendency for the fore-part of the foot
to come into contact almost at the same time a s the heel. This
gives the characteristic shuffling appearance to the animal as
it walks. The lateral border of the chimpanzee foot gains contact with the ground first, and the foot then rotates about this
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Fig.3 Fulcra of the human and chimpanzee feet in the course of a step.
border until the medial border also touches. In the human,
contrary to the views of Keith ('23) and others, the medial
and lateral borders of the anterior part of the foot touch the
ground sychronously.
The heel of the chimpanzee remains in contact with the
ground longer, relatively, than does the heel of man. This is
correlated with a difference in the position of the center of
gravity and consequent differences in the use of the leg
muscles.
When the chimpanzee lifts its heel off the ground, the transverse tarsal joint becomes, for a moment, the fulcrum about
which the body moves. Upon further lifting, the fulcrum is
transferred to the toes. There is no ball of the foot in the
73
CHIMPANZEE A N D HUMAN FEET
chimpanzee, the metatarso-phalangeal joints being so arranged that the foot cannot be flexed as a whole in this region.
In the human, on the contrary, the transverse arch is flattened
anteriorly, and the metatarso-phalangeal joints so arranged
that the foot can be flexed. Instead of transferring the fulcrum immediately to the toes, it is consequently transferred
from the heel to the ball of the foot and then to the toes, as
the human foot is lifted from the ground.
CLiI
NAK
CAL.
Fig.4 Medial view of human and chimpanzee feet, showing the bones transmitting pressure to the ground before the heel is lifted. The dorsi-flexion of the
chimpanzee foot about the transverse tarsal joint is apparent. T.T., transverse
tarsal joint ; CAL., calcaneus ; NAV., navicular ; CU I, first cuneiform.
REGIONS TRANSMITTING PRESSURE TO THE GROUND
When we consider the distributing of pressure in the sole
of the chimpanzee foot, as in figures 1 and 2, we a r e immediately struck by the large area of contact in the heel region.
I n the normal human foot, the only bone which exerts pressure
in this region is the calcaneus. In the chimpanzee not only
74
HERBERT ELFTMAN AND JOHN MANTER
the calcaneus, but also the base of the fifth metatarsal on the
lateral side and the navicular and first cuneiform on the
medial side (fig. 4), exert pressure on the substratum. This
proves that the chimpanzee foot is not inverted in bipedal
walking and that there is not even an adumbration of a longitudinal arch. A transverse arch is, however, obviously
present, as shown by the lack of pressure under the second and
third cuneiforms.
I n the presence of a transverse arch in the tarsus, the chimpanzee resembles both the human and its pithecoid forebears.
Eut in the forward extension of this arch so a s to include the
metatarsal heads, the chimpanzee resembles the monkeys and
differs from man. I n man the transverse arch has become so
flattened in this region that it is no longer functional during
weight-bearing.
TRANSVERSE TARSAL JOINT
The extreme mobility of the transverse tarsal, or Chopart’s,
joint in the chimpanzee is demonstrated in figure 2. When the
heel is lifted from the ground, there is an increase of pressure
under the base of the fifth metatarsal, which remains in contact with the ground. I n this respect the chimpanzee resembles the monkeys and differs from the normal human situation, in which movement about the transverse tarsal joint is
extremely limited.
PATH OF THE RESULTANT
The relative distribution of pressure between the lateral
and the medial portions of the foot is a crucial matter, not
only for the functioning of the foot, but also for theories concerning its evolution. This matter can best be investigated
by determining the change in position of the resultant of foot
pressure in the course of a step. The resultant of the pressure
at any moment is a force equal to the total pressure exerted by
the foot a t the moment, and so located that it will have the
same effect as the pressures of the various parts of the foot
CHIMPANZEE AND HUMAN FEET
75
taken together. The relation between the path of the resultant and the ‘axis’ of the foot has been considered in a
recent paper (Elftman and Manter, ’34), in which the path
of the resultant for the human foot was first published.
In figure 5 are shown the paths of the resultant for the
human and chimpanzee steps recorded in figures 1 and 2. If
one considered only points one and six, one could easily conclude that the ‘axis’ of the human foot was a line through the
Fig.5 Path of the resultant of pressure in the human and chimpanzee feet
during one step, superimposed on ti dorsal view o f the bones of the feet in the
position they occupy before the heel is lifted.
heel, bisecting the space between the first and second toes,
essentially as figured by Morton (’24). It is obviously impossible, however, to neglect the intermediate points, since they
represent the position of the resultant during two-thirds of
the step.
As the resultant passes over the metatarsal region of the
foot, it follows, in the human, the medial border of the third
metatarsal. I t s path in this region of the chimpanzee foot
is roughly comparable to that in the human. I n the tarsal
76
HERBERT ELFTMAN A N D J O H N MANTER
region the chimpanzee differs markedly from the human in
that the path exhibits a medial convexity, as would be expected from the everted condition of the foot.
The sharp curvature of the path of the resultant in the
region of the metatarsal heads in the human is due to the relationship between the axis of contact of the ball of the foot
with the ground and the axis of the lower ankle joint. The
presence of a well-developed transverse arch in the anterior
p a r t of the foot and a difference in orientation of the lower
ankle joint in the chimpanzee enter into a n explanation of the
differences between the terminal portions of the paths of the
resultants in the two forms. The position of this portion of
the path when the chimpanzee holds its toes in positions different from those used during our recording is subject to
conjecture.
POSITIOS OF THE TOES
The foot prints of the chimpanzee present an unexpect.ed
similarity in contour to the human, in that the lateral toes do
not project f a r ahead of the hallux, as might be expected from
the morphology of the foot. This is due to the fact that the
four lateral toes a r e curled under and toward the hallus
(figs. 4 and 5 ) . On some occasions the chimpanzee walks with
the toes extended, but the curled position is at least as characteristic as any other. This position of the toes compensates
to a certain extent for the great length of the phalanges, which
makes walking with the toes extended more awkward.
The hallux is held in various degrees of adduction with respect to the other toes as the chimpanzee walks. The position
illustrated in figure 5 represents what our observations lead
us to believe is a n average condition. A t times the hallux is
directed obliquely away from the foot; frequently it is to be
seen a t the other extreme, snugly flexed underneath. Often
the hallux of one foot is more adducted than is the hallux of
the other. The greater the abduction of the hallux, the more
marked is the tendency f o r the concentration of pressure on
the lateral portion of the foot. The variability in position of
the hallux is made possible by the importance of the lateral
portion of the foot in the transmission of pressure.
CHIMPANZEE AND HUMAN FEET
77
EQUILIBRIUM I N STANDING
It is in the act of standing that the advantage of the rigidity
of the tarso-metatarsal portion of the human foot is most
easily apparent. It is possible f o r the human body to sway
backward and forward between the positions in which the
center of gravity is directly over the heel o r the ball of the
foot, without danger of toppling over. I n the chimpanzee
the latitude of movement is much less, due to the mobility of
the transverse tarsal joint.
F i g . 6 Equilibrium in standing.
A study of the problem of static equilibrium must also take
into account the possibility of rotation of the body about various joints. It is possible for the human to stand (fig. S), a s
Braune and Fischer (1889) so nicely showed, with all of the
major joints and the centers of gravity of the various parts
of the body situated in a vertical plane passing through the
ankle joint. This position would involve the minimal amount
of muscular contraction. I n actual practice the posture assumed by the human differs, but only slightly, from this position. When the chimpanzee stands (fig. S), its limbs are
78
HERBERT ELFTMAN AND JOHN MANTER
flexed a t hip and knee, necessitating continuous muscular contraction to prevent movement in those joints. To remove this
necessity the animal either rests on all fours o r squats. It is
not surprising that the art of prolonged standing represents a
definitely human accomplishment.
FOOT OF TEIE GORILLA
Although this paper is concerned with a comparison of the
chimpanzee foot with that of man, the foot of the gorilla is,
in its main features, so similar to that of the chimpanzee that
it may appropriately be mentioned. We have not, as yet,
made records of the pressure distribution in gorilla feet.
SUMMARY
We may summarize our comparison of the foot as used in
bipedal walking by the chimpanzee and man in the following
manner :
1. The chimpanzee places the lateral border of the foot on
ground almost immediately after the heel has touched and then
rotates the inner border downward until the foot comes into
complete contact. I n the human the heel precedes the rest of
the foot markedly, and when the fore-part of the foot comes
into contact both medial and lateral borders touch simultaneously.
2. The chimpanzee foot is everted when on the ground and
does not possess a longitudinal arch. I n consequence, the
navicular, first cuneiform and base of the fifth metatarsal
transmit pressure to the ground. I n the human foot a longitudinal arch is present, preventing the navicular and first
cuneiform from making contact, and frequently the arch is so
high that even the base of the fifth metatarsal does not come
into proximity with the ground.
3. The transverse tarsal joint is freely movable in the chimpanzee but not in the human foot with a longitudinal arch.
4. The traiisverse arch of the chimpanzee foot continues
through the region of the metatarsal heads, so that only the
first and fifth metatarsal heads transmit pressure to the
CHIMPANZEE AND HUMAN FEET
79
groiind. I n the human all of the metatarsal heads bear on the
ground directly, giving rise to the ‘ball of the foot,’ a useful
fulcrum in man but not in the chimpanzee.
5 . The position of the hallux in the chimpanzee varies from
extreme abduction to a position of adduction and flexion
underneath the other toes. With greater abduction, a larger
proportion of pressure is transmitted by the lateral border
of the foot and the precedence in time of contact of lateral
border over medial is increased. The human hallux is ah-ays
adducted.
CONCLUSIONS
From a detailed study of the actual method of use of the
chimpanzee foot in bipedal walking in comparison with the
human foot nnder the same circumstances, it is evident that
the chimpanzee foot is still primarily adapted to arboreal life
and has not undergone any noticeable evolutionary changes
toward the human type of terrestrial foot. The gorilla foot,,
likewise, although the shortening of the four lateral metatarsals a n d phalangeal series is an adaptation for terrestrial
progression, presents no evidence of the appearance of a
longitudinal arch o r an immobilization of the transverse tarsal
joint. That the immobilization of the transverse tarsal joint
in a plantar-flexed position is the crucial factor in the evolution of the human foot and that this allies the human foot
more closely to the ape foot as used in arboreal than in bipedal
progression we shall show in a subsequent paper.
LITERATURE CITED
RRAUKE,1%‘. A N D 0. FISCHER1889 Uher den Sclrwerpunkt des meiischlichen
Korpers. A4bh.K. Sachs. Gesell. Wiss., Math.-Phys. KI., XV, 559-670.
ELFTMAN,H. 1934 A cinematic study of the distribution of pressure in the
huinan foot. Anat. Rec., L I X , 481-491.
H . A N D J. T. MANTER 1934 The axis of the huntan foot. Science,
ELFTMAN,
L S X X , 484.
KEITH,A. 1923 Man’s posture. Brit. Med. J., 669-672.
hloRms, D. J. 1924 Evolution of the human foot. P t . 11. Am. J. P h p .
Anthrop., V I I , 1-52.
WEIDENREICH,
F. 1921 Der Mensrheiifusz. Z. Morph. 8; Aiitlirop., X X I I ,
51-28?,
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