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Dynamic Pressure Patterns in the Hands of Olive Baboons Papio anubis During Terrestrial LocomotionImplications for Cercopithecoid Primate Hand Morphology.

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THE ANATOMICAL RECORD 293:710–718 (2010)
Dynamic Pressure Patterns in the Hands
of Olive Baboons (Papio anubis) During
Terrestrial Locomotion: Implications for
Cercopithecoid Primate Hand
Morphology
1
BIREN A. PATEL1* AND ROSHNA E. WUNDERLICH2
Department of Anatomical Sciences, Stony Brook University, Stony Brook, New York
2
Department of Biology, James Madison University, Harrisonburg, Virginia
ABSTRACT
Habitually terrestrial monkeys adopt digitigrade hand postures at slow
speeds to increase effective forelimb length and reduce distal limb joint
moments. As these primates move faster, however, their hands transition to
a more palmigrade posture, which is likely associated with the inability of
wrist and hand joints to resist higher ground reaction forces (GRF) associated with faster speeds. Transitioning to a palmigrade posture may serve to
distribute GRFs over a larger surface area (i.e., increased palmar contact),
ultimately reducing stresses in fragile hand bones. To test this hypothesis,
dynamic palmar pressure data were collected on two adult baboons (Papio
anubis) walking, running, and galloping across a runway integrated with a
dynamic pressure mat (20 steps each; speed range: 0.46–4.0 m/s). Peak GRF,
contact area, peak pressure, and pressure-time integral were quantified in
two regions of the hand: fingers and palms (including metacarpal heads). At
slower speeds with lower GRFs, the baboons use digitigrade postures resulting in small palmar contact area (largely across the metacarpal heads). At
faster speeds with higher GRFs, they used less digitigrade hand postures
resulting in increased palmar contact area. Finger contact area did not
change across speeds. Despite higher GRFs at faster speeds, metacarpal
pressure was moderated across speeds due to increased palmar contact area
as animals transitioned from digitigrady to palmigrady. In contrast, the pressure in the fingers increased with faster speeds. Results indicate that the
transition from digitigrady to palmigrady distributes increased forces over a
larger palmar surface area. Such dynamic changes in palmar pressure likely
moderate strain in the gracile bones of the hand, a structure that is integral
not only for locomotion, but also feeding and social behaviors in primates.
C 2010 Wiley-Liss, Inc.
Anat Rec, 293:710–718, 2010. V
Key words: digitigrade; palmigrade; metacarpal;
terrestrial; pressure; force
Many terrestrial mammals are characterized by digitigrade distal limb postures. In digitigrade postures, the
proximal ends of the metapodials are raised off the
ground while the metapodial heads and the phalanges
remain in contact with the substrate. The majority of
mammals that adopt digitigrade distal limb postures do
so in both their fore- and hind limbs (Howell, 1944).
However, there are some exceptions to this generalization, such as bears that have plantigrade feet but adopt
C 2010 WILEY-LISS, INC.
V
phalanx;
Grant sponsor: National Science Foundation; Contract grant
numbers: BCS 0524988, BCS 0509190.
*Correspondence to: Biren A. Patel, Department of Anatomical Sciences, Health Sciences Center, Tower A, 8th Floor, Stony
Brook University, Stony Brook, NY 11794-8081.
E-mail: biren.patel@stonybrook.edu
Received 7 January 2010; Accepted 11 January 2010
DOI 10.1002/ar.21128
Published online in Wiley InterScience (www.interscience.wiley.
com).
BABOON HAND PRESSURE
digitigrade forepaws (Brown and Yalden, 1973). Similarly, some of the terrestrial Old World monkeys, including baboons (Papio), geladas (Theropithecus), mandrills
and drills (Mandrillus), patas monkeys (Erythrocebus),
and some species of macaques (Macaca) and mangabeys
(Cercocebus), digitigrade postures are only adopted in
the hands while the feet assume semidigitigrade postures with only the proximal heel elevated (Napier and
Napier, 1967; Tuttle, 1969; Brown and Yalden, 1973;
Meldrum, 1991; Whitehead, 1993; Hayama et al., 1994;
Schmitt and Larson, 1995; reviewed in Patel, 2008,
2009). For primates, this asymmetry in distal limb postures may be related to fundamental kinetic differences
between the fore- and hind limbs during quadrupedal
locomotion (Patel, 2010).
In general, quadrupedal primates support more
weight on their hind limbs and thus experience relatively lower peak ground reaction forces (GRFs) on their
forelimbs during locomotion (Demes et al., 1994; Schmitt
and Lemelin, 2002; Schmitt and Hanna, 2004; Franz
et al., 2005; Hanna et al., 2006). This fore- versus hind
limb difference is generally considered to be one component of forelimb compliance among primate quadrupeds
(Schmitt, 1999, but see Raichlen et al., 2009). The ‘‘compliance model’’ suggests that the relatively reduced loads on
the forelimb during locomotion allow primates to retain
relatively gracile forelimb bones (especially in the hand)
and the capacity for increased forelimb mobility during arboreal locomotion (Schmitt, 1999). When traveling on the
ground, primates generally experience higher forelimb
GRFs compared to when utilizing arboreal (and simulated
arboreal) supports (Schmitt, 1994, 1999; Schmitt and
Hanna, 2004; Franz et al., 2005; Young, 2009). Because
higher GRFs during ground locomotion could result in
higher musculoskeletal stresses for an animal, adopting a
digitigrade hand posture may help moderate these higher
forces and potential stresses (Patel, 2010; see below). In
fact, Schmitt (1994, 1995) reported that olive baboons
(Papio anubis) change between a palmigrade hand posture
on simulated branches to a digitigrade hand posture during terrestrial locomotion, and he related this change to
the higher GRFs experienced by the baboon during terrestrial locomotion compared to arboreal locomotion. Zeininger et al. (2007) evaluated the ontogeny of digitigrade
hand postures in yellow baboons (Papio cynocephalus) and
found that they become increasingly more digitigrade with
age. Because larger individuals will experience relatively
higher GRFs on terrestrial substrates, these results further imply that adopting a digitigrade hand posture may
help moderate higher GRFs.
Digitigrade distal limb postures are often suggested to
be biomechanical adaptations for cursorial (i.e., highspeed) locomotion on terrestrial substrates for a variety
of mammalian lineages (Howell, 1944; Gambaryan,
1974; Hildebrand and Goslow, 2001). Digitigrady likely
serves as a component of an overall more extended limb
posture adopted by terrestrial mammals which may help
to moderate the negative effects of higher GRFs acting
on the musculoskeletal system during fast locomotion
(Biewener, 1983, 1989; Polk, 2002). Because fast locomotion is associated with higher peak GRFs (Rubin and
Lanyon, 1982; Biewener, 1983, 1989; Demes et al., 1994;
Polk, 2001, 2002; Patel, 2010), which in turn induces
high levels of strain in long bones (Rubin and Lanyon,
1982; Biewener, 1983; Biewener and Taylor, 1986;
711
Demes et al., 2001), adopting a digitigrade posture
aligns the metapodials with the GRF vector and likely
subjects the metapodials to a higher degree of axial compression, rather than the bending forces that predominate in many mammalian long bones (e.g., Biewener,
1991). Additionally, digitigrade postures help moderate
wrist and ankle joint moments that antigravity muscles
(e.g., wrist and digital flexor muscles; plantar flexor
muscles) must resist by decreasing the moment arm
between the GRF vector and the center of the wrist and
ankle joints (e.g., Biewener, 1983, 1989; Polk, 2002;
Patel, 2010). This will ultimately reduce the mass-specific amount of muscle force needed to prevent these
joints from collapsing into dorsiflexion when subjected to
higher forces at faster speeds. Consequently, animals
that generate relatively higher GRFs at faster speeds
are expected to adopt more digitigrade postures.
This hypothesis has been experimentally evaluated in
three species of habitually terrestrial monkeys: Papio
anubis, Macaca mulatta, Erythrocebus patas (Patel,
2010). As predicted, these monkeys exhibit lower wrist
joint moments when they adopt digitigrade hand postures compared to when they adopt more palmigrade
hand postures. Therefore, these primates may need less
activity from wrist and digital flexor muscles when using
a digitigrade hand posture during locomotion. These
monkeys were also predicted to adopt more digitigrade
hand postures at faster speeds. However, contrary to
expectations, these monkeys become more palmigrade
when they experience higher forces at faster speeds
(Patel, 2010). Thus, these results suggest that primates
do not alter their hand postures to reduce rising joint
moments at faster speeds, and instead, they allow their
hands to be forced into palmigrady. The most likely explanation for this is that these primates are simply unable
to resist being forced into palmigrady as the weight of the
body passes over the supported forelimb. This, in turn, is
due to the highly mobile (i.e., compliant) wrist and hand
joints these primates have evolved to effectively navigate
arboreal substrates (e.g., Yalden, 1972). Because habitually terrestrial cercopithecine monkeys are digitigrade
when moving slowly and are more palmigrade when moving at fast speeds, it is clear that a digitigrade hand posture in primates is not a cursorial adaptation (Patel,
2008, 2009, 2010, in press; Patel and Polk, 2010).
One possible benefit of changing from a digitigrade to a
palmigrade-like hand posture at faster speeds may be to
distribute the higher GRFs across the entire palmar surface of the hand. Increasing contact area would lower the
stresses in individual hand bones and related soft tissues,
which would be especially important when forces are high
at faster speeds. Unlike most digitigrade mammals, primates have relatively large and thick thenar and hypothenar
pads covering their metacarpals as they extend proximally
from the base of their interdigital pads. These palmar pads
may offer a larger area to distribute GRFs when adopting
a palmigrade hand posture at fast speeds. This potential to
dynamically moderate stresses in the hand is especially
important for primates because they depend on their
hands for not only locomotion, but also for manipulative
behaviors (e.g., feeding, grooming).
In this study, we evaluate the relationship between
speed, GRF, contact area, and pressure in the hands of
two habitually digitigrade olive baboons (Papio anubis)
during terrestrial locomotion. Peak GRF, contact area,
712
PATEL AND WUNDERLICH
peak pressure, and pressure-time integral are assessed
in two regions of the hand: 1) the fingers and 2) the palm
including metacarpal heads. Three predictions are made.
First, as animals increase locomotor speed, their hands
will be subjected to higher peak GRFs. Second, as animals increase locomotor speed, their hands will change
from a digitigrade to a palmigrade hand posture, and contact area will increase in both the fingers and palm
region of the hand. Third, as animals increase locomotor
speed and experience higher GRFs, pressure in both the
fingers and palm will not change substantially, and by
extension, stresses and strains will be moderated.
MATERIALS AND METHODS
Olive baboons (Papio anubis Lessen 1827) were chosen
for this study because they are highly terrestrial (Jolly,
1967) and have been observed to use digitigrade hand
postures in both the wild and in captivity (e.g., Napier
and Napier, 1967; Schmitt, 1994; Patel, 2009, 2010). One
adult male (28 kg) and one adult female (23.1 kg) were
housed separately in large rooms where they were permitted to move freely on the ground and on ‘‘arboreal’’
supports between experiments. All experiments were conducted at the Stony Brook University Primate Locomotion Laboratory (Stony Brook, NY), and all protocols were
approved by the Institutional Animal Care and Use Committee of Stony Brook University (Stony Brook, NY).
Each animal was filmed using a video-based motion
analysis system (Peak Performance Technologies, Inc.,
Englewood, CO) as it moved over a plywood platform
(10.5 m long 0.7 m wide) within a tunnel enclosed by
clear Lexan (Fig. 1; Polk, 2001). One HSC -180NS video
camera fitted with a Cosmicar/Pentax TV Zoom [8–48
mm] lens, shuttered at 1/2000 sec to avoid motion blur,
and operating at 60 Hz, was positioned adjacent to the
tunnel in lateral view. Video signals were time stamped
using a GL-250 time-code generator (J.C. Labs, La
Honda, CA). The video served two functions. First, it
allowed for the proper identification of steps in which
the hand made full contact with the pressure mat (see
below). Second, the video was used to calculate the animal’s speed as it traversed the pressure mat. Speed was
calculated as the time interval required for a fixed anatomical marker (either the tip of the nose or the base of
the tail) to pass between two markers spaced 1 m apart
and located on either side of the pressure mat. Froude
numbers, v2/gh, where v is velocity, g is the gravitational
constant, and h is the cube-root of body mass (Hof, 1996;
Biewener, 2003), were calculated to standardize and
combine data from the two animals of different body
size. The cube-root of body mass was used rather than
hip height because the animals never stood still in front
of the camera.
Dynamic palmar pressure distribution was collected
using an EMED-SF platform (Novel, Inc., St. Paul, MN).
The platform consists of a matrix of 0.5 0.5 cm capacitive sensors that quantify load normal to their surface;
the platform does not quantify horizontal forces (e.g.,
fore-aft and mediolateral). A minimum load of 1 N/cm2
or 10 kPa was considered contact. Four variables were
measured in the fingers and palm (which included the
metacarpal heads): 1) peak GRF (%bw), 2) contact area
(cm2), 3) peak palmar pressure (N/cm2), and 4) pressuretime integral ([N*s/cm2]). Peak GRF and pressure are
Fig. 1. Schematic of the experimental tunnel with integrated
dynamic pressure mat and video capture system.
obtained from the sensor that reports the highest force
or pressure value across an entire step in each anatomical region (fingers and palm). Contact area for an anatomical region is not involved in the calculation of peak
pressure for that area because each sensor’s peak pressure is calculated, and the peak of these is used to determine the peak for an anatomical region. Pressure-time
integral is a quantity similar to impulse that accounts
for both pressure and time over which the pressure is
applied and thus provides a valuable means to compare
how much total pressure is supported by each anatomical region. Relative contact area for each region of the
hand was calculated by taking the square-root of contact
area and dividing this by hand length (cm) for each animal subject. Hand length was measured in each animal
as the distance from the proximal end of the palmar pad
(i.e., the wrist joint) to the distal end of the third digit
(female baboon: 14.0 cm; male baboon: 15.4 cm). Finally,
dynamic center of pressure (CoP) location was evaluated
when possible to observe the pattern of total hand pressure throughout the step.
Twenty steps for each subject (total N ¼ 40) at a range
of speeds (0.68–4.0 m/s) were analyzed. Steps from both
symmetrical and asymmetrical gaits were analyzed together and these included ‘‘kinematic’’ walks, runs,
ambles, and gallops (e.g., Hildebrand, 1985; Biknevicius
and Reilly, 2006; Schmitt et al., 2006). Combining data
from symmetrical and asymmetrical steps was justified
because Patel and Polk (2010) found no significant differences in hand postures when olive baboons use different gaits at the same speed (i.e., symmetricalasymmetrical transition speeds). These authors also
showed only little change in hand posture across galloping speeds. Because of the small number of gallops
(female baboon: two steps; male baboon: five steps; all
left hands), we did not differentiate between leading and
trailing forelimbs, despite the potential for differences in
peak vertical forces that can exist between these limb
pairs when mammals gallop (Demes et al., 1994; Walter
and Carrier, 2007). For similar reasons (i.e., small sample size), we did not distinguish different types of symmetrical gaits such as walks and ambles (e.g., Schmitt
et al., 2006).
BABOON HAND PRESSURE
713
Fig. 2. Representative pressure diagrams illustrating peak pressure
over one entire step. a) Slow female baboon (2.1 m/s) using a digitigrade hand posture. b) Fast male baboon (4.0 m/s) using a palmigrade-like hand posture. At slow speeds, peak pressures are located
under the metacarpal heads of the palm. At fast speeds, peak pressures are located under the metacarpal heads of the palm, but contact area also increases to include the thenar and hypothenar regions
of the palm. Across all speeds, peak pressure is greater in the palm
region compared to the finger region. Also across all speeds, there is
minimal contact between the ground and the area underlying the proximal interphalangeal joint (i.e., white space between the finger and
palm regions). Center of pressure (CoP) migrates from the fingertips at
touchdown to the metacarpal heads at lift-off across all speeds.
Two sets of analyses were performed. First, we qualitatively and quantitatively described (with descriptive
statistics) the pressure distribution patterns in the fingers and palm during a typical step and across steps of
different speeds. Second, we quantitatively examined
(using Pearson product-moment correlation and ordinary
least squares regression statistics) the bivariate relationships between Froude number, peak GRF, relative contact area, peak pressure, and pressure-time integral. All
statistics were calculated using SPSS 16.0 for Mac OS X
(SPSS Inc., Chicago, IL).
down), the metacarpal heads of Digits II–V make contact
with the ground, typically within 1–2 frames. As a result,
CoP migrates proximally from the distal phalanges
towards the metacarpal heads. CoP remains under the
metacarpal heads until the hand lifts off the ground.
Summary statistics of Froude number, peak GRF, relative contact area, peak pressure, and pressure-time integral are presented in Table 1. All correlation coefficients
of the bivariate relationships are summarized in Table 2.
Across all speeds, peak GRF and peak pressure is larger
in the palm region of the hand compared to the finger
region of the hand (Table 1). Thus, a larger proportion of
weight is supported by the palm than by the fingers
with any given step. Within the palm region of the
hand, both GRF and peak pressure are greatest in the
metacarpal heads across all speeds, and these peak pressures are consistently located in Digits III and IV,
although the metacarpal head of Digit II can also experience high pressures (Fig. 2).
Froude number is significantly positively correlated
with peak GRF in both the fingers (r ¼ 0.624, P < 0.001)
and the palms (r ¼ 0.757, P < 0.001) making these
results consistent with those reported for other studies
on cercopithecine primates (Demes et al., 1994; Polk,
2002; Patel, 2010). Therefore, at higher Froude numbers,
these primates have higher forelimb forces than at low
Froude numbers (Fig. 3).
RESULTS
Irrespective of speed, both baboons make initial contact
with the ground using their distal phalanges. Thus, the
center of pressure (CoP) is consistently located on or near
the finger-tips at the beginning of forelimb support (Fig.
2). From the pressure distribution patterns, it is also
apparent that the other phalanges (middle and proximal)
only occasionally make full contact with the ground (Fig.
2). This is not surprising because baboons (and cercopithecine monkeys in general) often flex their proximal interphalangeal joints and extend their metacarpophalangeal
joint during locomotion, effectively shortening the functional length of the fingers (Nieschalk and Demes, 1993;
Richmond, 1998). After this initial contact (i.e., touch-
714
PATEL AND WUNDERLICH
TABLE 1. Summary statisticsa
Mean
Median
Minimum
Maximum
Froude #
Peak
force:
fingers
Peak
force:
palm
Contact
area:
fingers
Contact
area:
palm
Peak
pressure:
fingers
Peak
pressure:
palm
Pressure-time
integral: fingers
Pressure-time
integral: palm
0.106
0.044
0.016
0.538
15.65
14.89
8.88
24.63
66.51
61.29
43.98
109.16
0.246
0.247
0.180
0.290
0.283
0.262
0.240
0.420
171.75
155.00
85.00
400.00
366.38
350.00
245.00
645.00
3.71
3.43
1.26
6.45
11.55
11.22
2.63
24.35
a
Force in %bw; pressure in N/cm2; pressure-time integral in N*s/cm2; Froude # and contact area are dimensionless.
TABLE 2. Pearson correlation coefficientsa
Froude #
Peak force: fingers
Peak force: palm
Peak
force:
fingers
Peak
force:
palm
Contact
area:
fingers
Contact
area:
palm
Peak
pressure:
fingers
0.624a
0.757a
0.238
0.066
0.810a
0.583a
0.719a
0.800a
Peak
pressure:
palm
0.090
Pressure-time
integral: fingersb
Pressure-time
integral: palmb
0.326a
0.619a
0.052
Values significant at the 0.05 level (2-tailed) are in bold. Results of Kolmogorov-Smirnov tests (P > 0.05) for each variable
and each animal subject justified the use of parametric correlation.
Correlation based on log (base 10) transformed data.
a
b
Fig. 3. Scatter plot of Froude number against peak GRF (%bw).
Gray triangles: palm region. Black circles: finger region. Solid lines
represent least-squared regression lines.
Fig. 4. Scatter plot of Froude number against relative contact area.
Gray triangles: palm region. Black circles: finger region. Solid lines
represent least-squared regression lines.
Froude number is significantly positively correlated
with contact area for the palm region of the hand (r ¼
0.810, P < 0.001; Fig. 4). As a result, peak GRF in the
palm is also significantly positively correlated with palmar contact area (r ¼ 0.580, P < 0.001; Fig. 5a). At
slower speeds, with lower peak forces, the baboons use
stereotypical digitigrade hand postures and only the
metacarpal heads make contact with the pressure mat
(Fig. 2a). As speed and peak forces increase, the hand
becomes less digitigrade (i.e., more palmigrade) and a
greater portion of the palm (proximal to the metacarpal
heads) makes contact with the pressure mat (Fig. 2b).
More specifically, the thenar and hypothenar regions
make contact with the ground. Even with this larger
contact area, however, CoP does not appear to extend
proximally beyond the area of the metacarpal heads
(Fig. 2b).
In contrast, contact area of the fingers is not significantly correlated with Froude number (r ¼ 0.238, P ¼
0.139; Fig. 4) despite the hand transitioning between a
digitigrade and palmigrade posture. This finding, along
with the resulting pressure maps (Fig. 2b), suggests that
the digits are not becoming significantly more extended
at faster speeds and that the skin underlying the middle
and proximal phalanges are only making minimal contact with the ground. Thus, even at faster speeds, the
digits retain their short functional lengths (see above).
Peak GRF in the fingers is not significantly correlated
with finger contact area (r ¼ 0.066, P ¼ 0.686; Fig. 5b).
Froude number is not correlated with peak pressure
in the palm region of the hand (r ¼ 0.090, P ¼ 0.581;
Fig. 6). This is not surprising since both peak force and
palmar contact area increase at faster speeds (see
above). Also, peak vertical force is not correlated with
BABOON HAND PRESSURE
Fig. 5. Scatter plots of peak GRF (%bw) against relative contact
area in a) palm region and b) finger region. Solid lines represent leastsquared regression lines.
Fig. 6. Scatter plot of Froude number against peak pressure
(N/cm2). Gray triangles: palm region. Black circles: finger region. Solid
lines represent least-squared regression lines.
peak pressure in the palm (r ¼ 0.052, P ¼ 0.748; Fig.
7a). Therefore, pressures in the palm region of the hand
are moderated across a large range of speeds and forces.
715
Fig. 7. Scatter plots of peak GRF (%bw) against peak pressure
(N/cm2) in a) palm region, and b) finger region. Solid lines represent
least-squared regression lines.
In contrast, Froude number is significantly correlated
with peak pressure in the fingers (r ¼ 0.583, P < 0.001;
Fig. 6), as is peak force (r ¼ 0.719, P < 0.001; Fig. 7b).
Accordingly, pressure is not moderated in the fingers as
they are in the palms as these animals transition
between digitigrade and palmigrade hand postures at
faster speeds.
Further evidence of differences between the finger and
palm regions in peak pressure is provided by the pressure-time integral results; pressure-time integrals are
analogous to impulse (force-time integral) and represent
the total pressure throughout a step in each anatomical
region. At all but the fastest observed speeds, pressuretime integrals are typically larger in the palms compared to the fingers, indicating that the palms experience higher total pressure throughout the step (Table 1,
Fig. 8a). As speed increases, peak pressure in the palm
is moderated, but peak pressure in the fingers increases
and approaches the levels seen in the palm region of the
hand (Fig. 8a). For both the palm and finger regions of
the hand, the relationship between Froude number and
pressure-time integral is negative (Table 2), but the
slope of the ordinary least-squares regression line for
the palm is significantly greater than that for the fingers
(P < 0.05; Fig. 8b).
716
PATEL AND WUNDERLICH
Fig. 8. Pressure/time graphs. a) Relationship between time (ms)
and peak pressure (N/cm2) in the finger and palm regions for a representative slow female baboon step (1.712 m/s) and fast male baboon
step (3.75 m/s). b) Scatter plot of log Froude number against log pressure-time integral (N*s/cm2). Gray triangles: palm region. Black circles:
finger region. Solid lines represent least-squared regression lines. The
slope for the palm region is significantly more negative than the slope
for the finger region (P < 0.05).
DISCUSSION
This study provides a quantitative analysis of pressure
distribution in two different functional regions of the
hands of a nonhominoid primate. Previous studies have
examined manual digital pressures in primates (Richmond, 1998; Wunderlich and Jungers, 2009), but these
studies did not specifically evaluate speed effects on
pressure patterns. Previous kinematic studies of terrestrial locomotion have shown that baboons (and other
habitually terrestrial cercopithecoid primates including
patas monkeys and rhesus macaques) can transit
between a digitigrade hand posture typical of most terrestrial mammals to a more palmigrade hand posture
across speeds (Patel, 2008, 2009, in press; Patel and
Polk, 2010). In these studies, the angle between the
metacarpals and the ground significantly decreased as
these animals walked, ran, or galloped with faster
speeds and experience higher GRFs. The larger palmar
contact area at faster speeds and with higher peak
forces reported in the present study further supports the
conclusion that digitigrady in the primate forelimb is
not a cursorial adaptation.
Pressure is equal to force per unit area. Thus, moderation of pressure can be achieved if both force and area
increase with faster speeds. This is the pattern that is
observed in the palm region of the hand. As baboons
move at faster speeds on the ground, their forelimbs experience higher GRFs, and palmar contact area
increases. Peak pressure in the palm region of the hand
does not change significantly as these animals transit to
a palmigrade hand posture from a digitigrade hand posture. Although in vivo bone strain in the hand was not
measured in this study, the attenuation of peak pressure
in the palm region of the hand likely attenuates strains
in hand bones, specifically in the metacarpals.
The possibility of moderating strains in the metacarpals across different hand postures has implications for
metacarpal morphology in cercopithecoid primates.
Hands are in direct contact with the substrate and therefore their morphology is likely to reflect different loading
regimes associated with different locomotor and postural
behaviors (e.g., Etter, 1973; Lemelin, 1999; Wunderlich,
1999; Jungers et al., 2005; Patel et al., 2009). Patel
(2008), unpublished observations, found that the metacarpals (II, III, and IV) of habitually digitigrade cercopithecoid monkeys (e.g., Papio, Mandrillus, Theropithecus,
Erythrocebus) could only weakly be distinguished from
those that only adopt palmigrade postures (i.e., habitually arboreal cercopithecine and colobine monkeys).
Although digitigrade taxa were found to have relatively
shorter metacarpals, they did not have relatively more
robust metacarpal heads or midshaft cross-sectional
areas. Furthermore, multivariate discriminant analyses
resulted in several digitigrade taxa being classified as
palmigrade and some palmigrade taxa being classified as
digitigrade, suggesting some degree of morphological similarity in the palm bones of these primates (Patel, 2008,
in press, unpublished observations). This similarity
between hand posture groups could be attributed to the
fact that even habitually digitigrade cercopithecoid monkeys adopt palmigrade hand postures in different situations such as terrestrial running (Patel, 2008, 2009, in
press; Patel and Polk, 2010). Specifically, when forces are
higher during running, the potentially higher pressures
that could result in increased strain on the metacarpals
are moderated by adopting a palmigrade hand posture
with greater contact area in this region of the hand (Figs.
5a, 7a, 8a). Although obtaining strain data from these
hand bones is unlikely due to the invasive nature of such
experiments (e.g., Richmond, 1998; Demes et al., 2001),
additional palmar pressure data from more baboons and
other digitigrade taxa are necessary to see if these patterns are consistent.
Terrestrial cercopithecoids have a significantly smaller
phalangeal index (middle phalanx III length þ proximal
phalanx III length/metacarpal III length) compared to
their arboreal and semiterrestrial counterparts (Fig. 9;
Midlo, 1934; Etter, 1973; Patel, in press), a pattern that
is seen in other primates and nonprimate mammals
(e.g., Lemelin, 1999; Kirk et al., 2008). Thus, terrestrial
digitigrade primates have relatively short fingers. Relative elongation of phalanges serves as an arboreal adaptation because longer fingers facilitate the ability to
grasp branches during arboreal locomotion (Napier,
1993; Lemelin, 1999). Because peak GRFs are higher
717
BABOON HAND PRESSURE
Fig. 9. Box-and-whiskers plot of phalangeal index [(middle phalanx
III length þ proximal phalanx III length)/metacarpal III length] for arboreal (N ¼ 100), semiterrestrial (N ¼ 45), and terrestrial (N ¼ 39) cercopithecoid individuals based on unpublished data (B. Patel). Horizontal
lines within each box illustrate the median of the distribution. Boxes
envelop the interquartile range (50% of values) of the sample distribution, and whiskers encompass the range excluding outliers. Filled
circles beyond whiskers indicate outliers. Terrestrial cercopithecoid
primates (all habitually digitigrade) have the shortest fingers relative to
palm length. Arboreal taxa include: Allenopithecus nigroviridis, Cercopithecus ascanius, C. campbelli, C. cephus, C. diana, C. mitis, C.
mona, C. neglectus, C. nictitans, Colobus guereza, C. polykomos,
Lophocebus albigena, M. assamensis, M. fascicularis, M. tonkeana,
Miopithecus talapoin, Nasalis larvatus, Piliocolobus badius, Presbytis
comata, P. frontata, P. melalophus, P. rubicunda, Pygathrix nemaeus,
Rhinopithecus roxellana, T. cristata, T. obscura, T. phayrei. Semi-terrestrial taxa include: Cercocebus agilis, C. torquatus, Chlorocebus
aethiops, M. fuscata, M. nemestrina, M. nigra, M. sylvanus, Semnopithecus entellus. Terrestrial taxa include: Erythrocebus patas, Mandrillus
leucophaeus, M. sphinx, P. anubis, P. cynocephalus, P. hamadryas, P.
ursinus, Theropithecus gelada. Additional details of the data source
can be found in Patel (2008) and Patel et al., (2009).
during terrestrial compared to arboreal locomotion
(Schmitt, 1994, 1999; Schmitt and Hanna, 2004; Franz
et al., 2005; Young, 2009), shorter phalanges of terrestrial digitigrade monkeys may help to lower bending
moments by effectively shortening the load arm on
which the higher GRF vector acts (Nieschalk and
Demes, 1993). This is also important since peak pressures are not moderated in the fingers at faster speeds
and with hand postural changes as they are in the palm
(Figs. 6, 8a). Although these patterns are robust,
dynamic pressure data during terrestrial locomotion
from primates with long fingers are necessary to further
evaluate this issue. Our preliminary (unpublished) pressure data from two arboreal New World monkeys with
relatively long fingers (Cebus and Ateles) do suggest that
the finger region of the hand is not loaded as highly in
these taxa as they are in the digitigrade baboon.
It is necessary to recognize that knowing the location
of CoP is necessary for accurately measuring joint
moments (e.g., Fowler et al., 1993; Carrier et al., 1998;
Witte et al., 2002). For any given hand posture, a CoP
located further away from a joint center will effectively
result in large joint moments (due to longer GRF
moment arms). Because contact area increased at faster
speeds, resulting in a more palmigrade hand posture, it
would be expected that CoP would also migrate proxi-
mally towards the carpus and will help lower wrist joint
moments (Patel, 2010). Effectively, this proximal migration will shorten the GRF moment arm and will help
moderate wrist joint moments when the hand is subjected to higher forces at faster speeds. Although the
present study did not focus on hand CoP, our preliminary observations presented above do not suggest a
proximal migration of CoP. While CoP does move from
the fingertips of the hand at touchdown to a region
underlying the metacarpal heads for all speeds, there is
little proximal movement past the metacarpal heads
even with increased palmar contact area (Fig. 2b). The
lack of CoP movement despite increased palmar contact
area may be a result of simultaneous increase in finger
pressure. This supports the idea that transitioning from
a digitigrade hand posture to a palmigrade hand posture
may occur in these habitually terrestrial monkeys at the
expense of rising wrist joint moments and increased
wrist and digital flexor muscle activity (Patel, 2010). It
is important to note, however, that these CoP patterns
may not be representative of all primate hands during
terrestrial locomotion. Additional dynamic pressure
data, including CoP patterns, from other habitually digitigrade primates, as well as strictly palmigrade primates
are necessary to evaluate this issue further.
CONCLUSION
Primates are well known to exhibit a number of morphological and behavioral features associated with arboreal lifestyles. Among these are their gracile limbs with
long digits, high levels of forelimb protraction, compliant
gaits, and higher hind limb than forelimb peak GRFs.
However, catarrhine primates specialized for life on the
ground are faced with higher peak GRFs acting on the
forelimb, a substrate that is not compliant, and the need
to move fast for long distances and/or to avoid predators.
With faster locomotor speeds, peak vertical GRFs increase,
and skeletal and soft tissue integrity can be compromised.
To reduce peak loads on the hand, a structure that is integral for not only locomotion but also feeding and social
behaviors, baboons distribute these higher forces over a
larger palmar surface area, thereby attenuating palmar
pressure. Such dynamic changes in palmar pressure likely
moderate strain on their gracile hand bones. As far as we
know, this is a unique adaptation to speed-related
increases in GRFs and musculoskeletal load.
ACKNOWLEDGMENTS
The authors thank Jason Organ and Qian Wang for
inviting us to contribute to this volume. The baboons
used in this study were kindly loaned to us by Daniel
Schmitt and Robert Davis (Duke University). Kristin
Fuehrer assisted with the experiments and training of
the animals.
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