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First published online August 22, 2008
Journal of Experimental Biology 211, 2735-2751 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018820
The movements of limb segments and joints during locomotion in African and Asian elephants
1 Structure and Motion Laboratory, Department of Veterinary Basic Sciences, The
Royal Veterinary College, University of London, Hatfield, Hertfordshire AL9
7TA, UK
2 Institut fuer Spezielle Zoologie und Evolutionsbiologie, mit Phyletischem
Museum, Jena 07743, Germany
* Author for correspondence (e-mail: jrhutch{at}rvc.ac.uk)
Accepted 15 May 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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Key words: elephant, proboscidea, joint, locomotion, biomechanics, speed, gait, kinematics
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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Ulysses in Act II, Scene iii
(Shakespeare, 1609
)
Elephants run straight-legged, thigh lined up with shank and upper arm with lower arm, so their legs look rather like mobile Doric columns.
p. 214 in (Bakker, 1986)
As the above quotes exemplify, humans have long recognized the distinctive
columnar (straight-legged) limb posture of elephants. This recognition has
generated classical misconceptions, such as elephants having no joints, no
knees or four knees [e.g. pp. 101-109 in
(Tennent, 1999)]. Ridiculous
as those fallacies may seem to contemporary scientists, elephant posture and
gait remain misunderstood, partly because of their strange anatomy and partly
because of little rigorous measurement of elephant locomotion. Hence,
potentially misleading oversimplifications persist, even in recent
comparative/functional studies (e.g.
Bakker, 1986;
Paul, 1998;
Paul and Christiansen, 2000),
and have become integrated into textbook and popular media accounts. Elephants
are unusual among terrestrial animals not only in their enormous size [up to
7000 kg in adult African elephants
(Christiansen, 2004;
Wood, 1972)] and
apomorphically long limbs (Alexander et
al., 1979a) but also in their limited speed range [
7 m
s–1 maximum (Hutchinson
et al., 2003)], continuous changes of kinematic patterns with
increasing speed (Hutchinson et al.,
2006) and smooth, relatively low-speed transition to bouncing or
`running' hindlimb mechanics (Ren and
Hutchinson, 2008).
Earlier studies (Gambaryan,
1974; Hildebrand and Hurley,
1985; Marey and Pagès,
1887) provided basic descriptions of elephant segment and joint
motions that are widely cited and useful; however, these were largely
qualitative, were based on small sample sizes, and utilized ambiguous or
technically limited methodology [e.g. 24 Hz video in
(Hildebrand and Hurley, 1985);
unknown elephant speeds in most studies]. Our initial studies
(Hutchinson et al., 2003)
showed that elephants shift from vaulting to bouncing hip motion, from slow to
fast speeds; our later studies (Hutchinson
et al., 2006) observed that hindlimb flexion seemed to increase
concurrently, suggesting a shift from vaulting to bouncing limb mechanics
(Ren and Hutchinson, 2008) as
previously predicted. This limb flexion has not yet been quantitatively
measured or related directly to speed changes but does indicate marked
differences in joint motion at least between the forelimbs and hindlimbs. The
differences between fore- and hind-foot posture and dynamics in elephants also
relate to altered loading and scaling of the bones and tendons
(Miller et al., 2008).
Our previous analysis of stride parameters
(Hutchinson et al., 2006)
demonstrated that elephants change speed by increasing stride frequency, more
than stride length, up to a dimensionless speed
[û=v/(hg), where
û is dimensionless speed, v is velocity, h is
hip height, and g is acceleration due to gravity] of 1.0.
Beyond this speed, stride frequency approaches its maximum and stride length
contributes relatively more to speed increase. Correspondingly, stance time
decreases most steeply with speed (continuing beyond
û1.5), with swing time reaching its plateau at
û1.0; concurrently, duty factor decreases sharply and then
levels off. From the perspective of motion within the limbs, it is thus
expected that, as elephants increase speed, they mainly increase joint/segment
angular velocities (required for larger stride frequencies) until running
quickly (û>1.0) when they rely more on greater joint
rotations (larger ranges of motion, perhaps including greater limb flexion) to
generate the longer strides observed.
In the present study, we analyze the coordination patterns of the limb
segments and joint movements during steady-state locomotion in Asian
(Elephas maximus Linnaeus 1758) and African (Loxodonta
africana Blumenbach 1797) elephants. As noted above, elephant limbs are
the archetype for columnar, graviportal animals [large, relatively slow, with
long proximal and short distal limb segments
(Gregory, 1912)] and hence are
of comparative interest, particularly for deciphering the relationships
between body size and locomotor form and function. Therefore, our major aim is
to determine how elephant limb motions compare with those of other mammalian
species – are elephants always relatively restricted in their
joint/segment ranges of motion, having less mobile joints than other animals?
Are their limb motions fundamentally distinct from all other animals',
especially cursorially specialized forms, as they often have been
characterised (e.g. Bakker,
1986; Gregory,
1912; Paul, 1998;
Paul and Christiansen, 2000)?
We sought to carefully examine how useful and accurate the term `columnar' is
when applied to elephants.
Thus, we pose two fundamental questions here, related to the aims above.
Firstly, what are the limb segment and joint angular and angular velocity
changes across a normal walking stride in elephants, and how do motions differ
within and between limbs? Particularly, just how columnar are elephants (i.e.
what are their segmental and joint angles during locomotion)? Secondly, how do
elephant limb motions change with speed, from slow walking to fast running;
i.e. does their `columnarity' change with speed (are their legs always
legs for necessity, not for flexure)? At all speeds, are all joints
similarly columnar and of low flexibility/mobility or is there
intra-/inter-limb diversity in joint flexibility
(Gambaryan, 1974;
Hildebrand, 1984;
Hildebrand and Hurley, 1985)?
We test the null hypothesis that joints all use a similar fraction of their
maximal range of motion (i.e. that is allowed by joint surfaces and ligaments)
by comparing maximal in vivo (fast locomotion) vs in vitro
(cadaver manipulation) joint range of motion and discuss alternative
hypotheses.
We used motion analysis with 15 elephants covering a sevenfold body mass
range to quantitatively answer these questions. Accurate baseline joint and
segment kinematic data are vital for further biomechanical analyses, e.g.
`internal work' [movements of segments relative to the body's center of mass
(Hildebrand and Hurley,
1985)], joint powers (e.g.
Dutto et al., 2006), or
inverse dynamics analysis of muscle/bone stresses (e.g.
Alexander et al., 1979b;
Biewener, 1989;
Biewener, 1990) of locomotion.
We expected that there would be no major size/species differences in elephant
kinematics (as in Hutchinson et al.,
2006; Ren and Hutchinson,
2008) but here search for any previously overlooked differences
within limbs, as earlier studies have focused on whole-body or whole-limb
kinematics. Another subsidiary goal of our analysis was to quantify how
`normal' elephant limbs move in order to establish a comparative dataset for
identifying foot, joint or other limb pathologies, which are a major concern
for elephant keepers (Csuti et al.,
2001; Egger et al.,
2008).
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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5 years old; 521 and 688 kg body mass), two
sub-adults (1740 and 2072 kg) and four adults (
20 years old;
3149–3684 kg body mass). For African elephants, we used one juvenile
(930 kg), three sub-adults (2550–3230 kg) and three adults
(3100–3512 kg). Ages were known to trainers ±1 year (or better
for juveniles), and body masses were obtained using truck scales (±5
kg) or a custom-made force platform apparatus (for Thailand elephants;
constructed by Arsalis Inc., Louvain-la-Neuve, Belgium; 7000 kg max. load, 16
1 mx1 m plates). However, for the three adult African elephants mass was
unknown so it was estimated from published mass–shoulder height
equations (Laws et al., 1975;
Christiansen, 2004). Hip and
shoulder heights were measured from the ground to the approximate hip joint
center or to the top of the scapula (Fig.
1) respectively, with flexible measuring tape (±1 cm) when
the animal was standing still. All studies were approved by The Royal
Veterinary College's animal welfare and ethics board.
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Trials
Elephants were led by their handlers, using positive reinforcement such as
food rewards and vocal commands, with the goal of maintaining a steady speed
across a straight distance of 15 m. They also had at least 5 m before and
after this distance to accelerate and decelerate to a steady speed, which,
from previous experience, we knew to be sufficient
(Hutchinson et al., 2006).
Trainers randomly varied the speed from slow walking to fast running across
trials and allowed ample rest and food between trials to prevent fatigue.
Experiments were cancelled if animals showed musculoskeletal pathology,
fatigue or any other artifacts that would cause discomfort or adversely affect
our measurements. Handlers sought to build speed up to the near-maximal speed
that the animal could achieve, which as usual among captive elephants was
below the top speeds observed in sleeker, more active elephants [e.g. in
Thailand 6.8 m s–1
(Hutchinson et al., 2003;
Hutchinson et al., 2006)].
Data collection was conducted outdoors (during cloudy/twilight periods to
reduce sunlight interference with our infrared motion capture) except at the
Colchester Zoo site, when data were collected inside an elephant barn. All
animals were moving over very firm (concrete, asphalt, packed dirt or force
platform) and level substrates.
Marker placement and motion capture
Infrared-reflective tape (Scotchlite 8850; 3M, Manchester, UK) covering
styrofoam hemispheres (7 cm diameter for all elephants, except the juveniles,
for which 3.5 cm diameters were used) were attached to the skin with
double-sided carpet tape, over palpable landmarks.
Fig. 1 explains these landmarks
and positions. The ears (particularly in African elephants) hid the shoulder
marker in some trials so data on the upper arm segment and elbow joint angle
are scarcer. Due to time constraints (sunlight, animal and trainer
availability, and manageability) the number of markers we could use was
limited. This was exacerbated by the tendency of elephants to intentionally or
accidentally dislodge or destroy markers. We considered using multiple-marker
clusters to rigorously quantify 3D limb motions (e.g. Cappozo et al., 2005;
Rubenson et al., 2007) but
this was judged to be impossible under the constrained conditions.
A synchronized six-camera Qualisys (Gothenburg, Sweden) MCU 500 system (240
Hz) was used to record elephant limb marker motions, digitally triggered at
the start of each trial. The system was calibrated before and after trials to
a 3-D measurement accuracy of 1 mm. Total capture volume varied with
ambient light and other conditions but was generally 12 mx3
mx4 m. Animal forward velocity for each stride was measured by
calculating the averages of the hip and shoulder marker velocities. We defined
steady-state trials as those in which the absolute difference between the
forward velocities at two consecutive heel strikes was less than 20% of the
average forward velocity. Trials with greater or smaller values of
acceleration/deceleration were discarded. Froude numbers
[Fr=v2/(hg)] and
dimensionless speed (û=Fr0.5) were
calculated to normalize speeds (e.g.
Alexander and Jayes, 1983) for
comparison between elephants of different sizes and with previous stride
parameter data (Hutchinson et al.,
2006).
To estimate maximal joint ranges of motion, we conducted in vitro
studies with fresh cadaveric limb material (joint capsules and ligaments
intact, skin and muscles removed) of one juvenile Asian elephant (830 kg body
mass; 3 years old at death) that had no significant musculoskeletal
pathologies. We used the same motion capture system described above but with
the cameras in a ring around the specimen, and 2.5 cm diameter markers
attached on bony landmarks and anatomical reference points (total of at least
five markers per segment) to calculate the sagittal plane joint motions
(detailed below) of the hip, shoulder, knee and elbow joints. We captured a
static pose emulating standing posture (for calibration of distances between
markers), then manually flexed and extended each joint, one at a time, through
its maximal range of motion for five trials per joint. Measurements on an
additional adult elephant cadaver were performed but we were unable to safely
apply sufficiently large loads to cover a plausibly large range of motion. For
comparisons with other species, we conducted the same in vitro
measurements with the forelimbs and hindlimbs of one Dutch Warmblood horse
(adult, previously healthy, 500 kg body mass) using the same markers as Back
et al. (Back et al., 1995a;
Back et al., 1995b) and
collated literature data for cats, dogs and humans (see Discussion).
Angle and angular velocity calculations
All motion analysis 3-D coordinate data were first filtered using a
low-pass, zero-lag Butterworth digital filter [fourth order, cut-off frequency
8 Hz (Winter et al., 1974)].
We calculated the sagittal plane (approximated as the plane parallel to the
mean direction of motion in each trial) motions of the upper arm, forearm,
forefoot (manus), thigh, shank and hindfoot (pes) segments and the elbow,
wrist, knee and ankle joints. Segmental angles were `external' angles measured
with respect to the vertical. Joint angles were the `internal' angles between
two articulated segments. As there were no repeatable landmarks on the scapula
or pelvis that we trusted to have minimal skin motion, we did not measure
shoulder or hip joint angles, although our upper arm and thigh segmental
angles are roughly comparable to these. More detailed analyses of 3-D
kinematics and skin motion for these two joints and the scapula segment [a
critical component of mammalian limb motion
(Fischer and Blickhan, 2006;
Fischer et al., 2002)] are
still needed. Qualitative estimates of segment abduction and adduction can be
made using our data (as the motion capture is inherently 3-D; see Discussion
for qualitative descriptions of these motions) but here we focus primarily on
sagittal plane motions as these are clearly the dominant component of elephant
limb motion. Angular velocities of joints and segments were calculated using a
first-order finite differentiation method
(Pezzack et al., 1977).
We divided trials into their component strides by identifying the stance
and swing phases for each limb. Limb touch-down was defined as when the
vertical position (extracted from our motion capture data) of the fore- or
hindfoot carpal or tarsal marker reached its minimum. Limb lift-off was
defined as when the fore or hind limb middle toe marker
(Fig. 1) reached its lowest
position (Hutchinson et al.,
2006) (Fig. 2). The
resulting duty factors matched those at comparable speeds from video data
(Hutchinson et al., 2006),
validating this approach and avoiding a reliance on lower-resolution video
footfall identification.
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As suggested by our marker placement repeatability assessment (see Results), we identified (post hoc) moderate offsets in some angle measures vs stride time (i.e. % gait cycle) that were probably caused by inaccurate marker placements, although most of the angle measures were repeatable. In these cases, to minimize the bias these offsets would bring into our statistical analyses, the whole angular displacement curves for a stride were shifted so that the mid-stance angles matched the mean mid-stance angle for all elephants. As the angular velocity is insensitive to this marker placement offset, this error does not affect those measurements. Similar relative offset errors are presumably present in most other studies of animal limb motion but are seldom discussed or investigated.
Our in vitro cadaveric studies calculated sagittal plane joint motions more rigorously as they used multiple markers attached directly to the skeleton. To quantify the total ROM of each joint we simply used the 3-D coordinates of each joint marker to quantify the sagittal plane joint angles in maximal flexion and extension.
Marker placement repeatability assessment
One might expect that the positioning of skin-mounted motion capture
markers would be hard to reproduce on multiple animals or experiments with the
same animal. Therefore, we conducted a basic initial analysis of how
consistently we were positioning the skin markers. The same investigator
(J.R.H.) placed all 10 markers on two African elephant subjects (B and C in
Table 1). The elephants did
four trials of normal walking at the same speed, then the markers were removed
and replaced with a new set. This was repeated 10 times for subject B and five
times for subject C. We calculated the mid-stance segment (for upper arm and
thigh) or joint angles as above and analyzed the results statistically as
below.
Soft tissue artifacts (caused by the skin/muscles moving with respect to
the underlying bones) are also certainly a problem for studies of elephant
joint motion, perhaps more so than in smaller animals. Absolute error in
estimating skeletal joint centers using skin markers is large, and the mobile,
thick skin of elephants may also cause large relative errors. Invasive bone
pin measurements (Reinschmidt et al.,
1997; van Weeren et al.,
1988; van Weeren et al.,
1990) are impossible, and elephants are too large for
cineradiographic imaging studies of joint motion (e.g.
Cappozzo et al., 1996;
Filipe et al., 2006;
Gatesy, 1999). However,
studies of human and horse skin marker accuracy show that errors in
calculating flexion and extension angles are relatively minor, especially for
distal joints (Back et al.,
1995a; Back et al.,
1995b; Leardini et al.,
2005; Reinschmidt et al.,
1997; van Weeren et al.,
1988; van Weeren et al.,
1990). Therefore, we assume that our flexion/extension
measurements are reasonably accurate, pending more exhaustive in
vitro and in vivo analyses.
Statistical analysis
All statistical analyses were conducted using SPSS 15.0 software (SPSS,
Inc., Chicago, IL, USA). The effects of locomotor speed, species and body mass
on angles and angular velocities were analyzed using analysis of variance
(ANOVA) with repeated measurements via a linear mixed model approach
by taking into account of the intra- and inter-subject variability. The
different speed ranges, elephant species and body mass ranges were the fixed
effects, and elephants were random effects. Differences between each pair were
tested using Fisher's least significant difference (LSD) multiple comparison
based on the least-squared means. This approach was chosen to maximize the
amount of usable data, as opposed to regression techniques.
Our marker replacement repeatability test involved an independent simple t-test for each segment or joint at mid-stance (between elephant differences), a one-way ANOVA to test for the effect of trial number, and a one-sample Kolmogorov–Smirnov test for normal distribution of segment or joint angles between marker sets. The effect of speed was removed via a Pearson's two-tailed t-test for correlation between speed and mid-stance angle. A linear curve fit equation was then used to remove speed effects if a speed–angle correlation was present. For normally distributed angles we used a two-way ANOVA to test the influence of marker set on angle (non-normally distributed angles were omitted). Statistical significance was considered as P<0.05.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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As the two subjects showed statistically significant differences (P<0.05), they were treated separately. No effect of trial number was found (P>0.05). The mid-stance angle data were all normally distributed except for the thigh and wrist from subject B (which were not subsequently analyzed). Speed only had a significant effect on subject B's knee joint angle and subject C's thigh segment angle (P<0.01), as the speed range was narrow. Except for these latter two angles, which showed a significant difference between marker sets (P<0.05), and the two excluded for non-normally distributed data, the remaining six angles showed a repeatable pattern at mid-stance (P>0.05).
Post hoc examination of the significantly different or
non-normally distributed angles (and outliers) showed that, in most cases,
markers had been replaced in positions that were slightly offset from the
normal position, offsetting the entire segment/joint angle vs stride
time curves upward or downward 5–10 deg. (cf. Figs
2,
3,
4); if manually shifted back to
lie over the mean values (see Materials and methods), these differences
disappeared. These results indicated that the placement of the skin markers
was repeatable when conducted by the same experienced investigator and also
that we needed to correct for offset angle vs stride time curves.
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0.10), elephant limb
segment and joint motions throughout the stride were generally quite smooth
(Fig 2 and
Fig 3). At mid-stance, the
forearm, forefoot and thigh segments were all relatively vertical (within 4 11
deg.), whereas the upper arm, shank and hindfoot segments were less vertically
inclined (–19, –23 and 38 deg. to vertical, respectively).
Correspondingly, the elbow, wrist and knee joints were fairly extended at
mid-stance (157 deg., 186 deg. and 152 deg.) but the ankle joint was more
flexed (117 deg.). Touch-down and lift-off angles varied by <20 deg. from
these values.
During the stance phase of the forelimb (Table 2; Fig. 2), the upper arm was retracted throughout stance, along with the forearm (which sometimes showed increasing segment angles very late in stance) and forefoot. The elbow and wrist joints both extended slightly, remaining almost static (but with hints of a shift from early flexion to extension in some strides), and then flexed late in stance (Fig. 3). Similarly, decreasing segmental angles in stance [shifting to negative (i.e. behind vertical) angular values at lift-off] prevailed for the hindlimb. The thigh was protracted slightly before lift-off, the shank showed a smooth rotation toward negative values throughout stance, whereas the hindfoot angle decreased steeply in late stance. The knee joint flexed throughout stance (most steeply in late stance but with a flexion–extension–flexion sequence in some strides) and the ankle joint flexed past mid-stance, with some extension very early in stance in some strides, then extended in the last third of stance (Fig. 3).
In earliest swing, segment angles continued to decrease (except for the thigh), then slightly later reached their minimum angles (Fig. 2), although more proximal segment angles tended to decrease little, or no further, from their lift-off values (0–3 deg.; Table 2). Segmental maximal swing angles occurred just before touch-down in the forelimb (angles decreasing gradually before touch-down) but closer to mid-swing in the hindlimb. Therefore, the segmental angles began decreasing from late swing through touch-down, rather than initiating this decrease in early stance. However, the presence of this late swing `retraction' was more variable for the upper arm and shank, especially at slower speeds. Elbow and wrist joint flexion during swing (Fig. 3; Table 2) exceeded the values for the knee and ankle (mean flexion of 20 deg. and 24 deg. from lift-off to minimal swing vs 12deg. and 4deg., respectively). The elbow and wrist joints flexed and then extended steeply, beginning to shift back toward flexion in late swing. The knee joint flexed in early swing, then extended for most of swing with a brief flexion before touch-down. The ankle joint was unusual in that it remained almost static (near touch-down angle, with more gradual flexion in some strides) for the last half of swing after an early swing extension–flexion shift (Fig. 3).
The greatest ROM in the forelimb (Table
2) was for the forefoot segment and wrist joint (90 deg. and 66
deg.), with smaller values for more proximal segments and joints
(36–52deg.). This is expected as the rotations of proximal segments
contribute to the rotation of distal segments. In the hindlimb, the pattern
was similar; the hindfoot segment was the most mobile (76 deg.), followed by
the shank (55 deg.) and thigh (29 deg.), and the knee joint was slightly more
mobile (42 deg. ROM) than the ankle (30 deg.). Overall, the upper arm segment
was less vertical and had 50% larger ROM than the thigh segment, during
stance phase and across a whole stride; some of this presumably relates to
scapular motion.
Effects of speed on limb motion
As the elephants increased their speed across an almost eightfold speed
range, from the slowest speeds we recorded (0.62 m s–1;
Fr=0.026) to the fastest (4.92 m s–1;
Fr=1.66), their segment and joint motions mostly changed
continuously. Relatively small increases of joint and segment rotation
(Fig. 4) and large changes of
angular velocity (Fig. 5;
discussed further below) relate to increased stride length and especially
frequency [i.e. decreases of stance and swing time
(Hutchinson et al., 2006)].
Movies 1 and 2 in supplementary material show representative motion capture
data in real time for added comparison. Many segments and joints did not show
significant changes of their angles with speed even from slow walking to
faster running. Shifts between joint flexion and extension during stance
became more obvious in many trials at higher speeds (cf. Figs
3 and
4; elbow, knee and ankle joint
angles) but otherwise the sequence and timing of segment and joint motion did
not markedly change.
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5 deg. changes) existed for other cases, not outlined here (cf. Figs
6,
7,
8,
9).
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Forelimb segmental angles changed most markedly with speed (Figs
4 and
6; supplementary material Table
S1) for the upper arm segment at mid-stance (–15 deg.), the forearm
segment at minimal (–7 deg.) and maximal (+8 deg.) swing, and the
forefoot segment angle at lift-off (+10–15 deg.), minimal (–13
deg.) and maximal (+21 deg.) swing. Joint angles reflected these changes
(Fig. 6; supplementary material
Table S2). At speeds past the presumed gait transition point
[(Ren and Hutchinson, 2008)
û>0.50; speed range E–G in
Fig. 6], notice that the trend
for mid-stance angles in particular became markedly steeper, indicating
increased limb flexion, especially for the upper arm and elbow.
In the hindlimb (Figs 4 and
7; supplementary material Table
S1), the thigh segment angle showed only moderate changes (7–8 deg.),
the shank angle decreased at mid-stance (–13 deg.) and minimal swing
(–5deg.), and the hindfoot angle only changed in swing phase: –16
deg. for minimal swing and +14 deg. for maximal swing. Overall, the knee joint
exhibited marginally larger increases toward flexion, and greater ROM, than
the elbow, whereas the wrist shifted to become strikingly even more flexed
during swing than the ankle joint, consistently with more than twice the ROM.
By contrast, during stance, the ankle angle (changing from 116 deg. to 108
deg.) at mid-stance became even more flexed than the wrist angle
(186–189 deg.). Again, at speeds past the presumed gait transition
point (û>0.50; speed range E–G in
Fig. 7), we observed steeper
trends for thigh segment, knee joint and ankle joint mid-stance angle changes
with speed; the hindlimb was becoming appreciably more flexed.
Joint and proximal segment angular velocities changed sharply with
increasing speed (Figs5,
8,
9; supplementary material
Tables S3 and S4). Some noisy fluctuations are present but the data do show
the shifts of segment or joint angular velocities from positive to negative
values, especially during stance phase (e.g. elbow, knee and ankle joints).
The largest relative increases (as a multiple of values at
û<0.25 vs û>1.0) of angular velocity were
during stance, often around 7–9x (but 203x for shank
touch-down), compared with 3x increases for swing-phase angular
velocity and RAV. These changes concur with rapid decreases of stance (and
swing) time in faster-moving elephants
(Hutchinson et al., 2006).
Thigh segment lift-off angular velocity showed the only shift of sign, from a
mean value of 16 deg s–1 at the slowest speed to –17
deg s–1 at the fastest speed
(Fig. 9).
Unsurprisingly, the largest absolute increases of angular velocity were for
distal segments (supplementary material Table S3), especially RAV. Maximal
swing velocity increases (positive values) were comparable for upper arm and
thigh and for forearm and shank, but the increase in the forefoot was >50%
larger than the increase in the hindfoot. There was a similar trend for
velocity decrease – the largest decreases were for distal segments, but
with comparable values among serially homologous segments (<–100
proximal, <–200 middle, >–300 deg. s–1
distal). We measured very similar trends for the joints
(Figs8 and
9; supplementary material Table
S4). Few, if any, limb segment/joint angular velocity values seemed to plateau
at faster speeds (Figs 8 and
9; supplementary material
Tables S3 and S4). As stride frequency and swing time reach their maxima and
minima at Fr>1.0 (Hutchinson
et al., 2006), this is unsurprising; elephants at the top
locomotor speeds measured in the present study
(Fig. 5; speed range G in Figs
8 and
9) should have reached close to
their peak angular velocities. At the fastest speeds, wrist joint angular
velocity ranged from –772 to +773 deg. s–1 whereas the
ankle ranged from –180 to +390 deg. s–1; the elbow and
knee velocity ranges were more similar at –258 and +319 deg.
s–1 and –223 and +328 deg. s–1,
respectively. Hence, overall, the maximal wrist RAV remained at least twice
the RAV values of the other joints.
Maximal vs utilized ROM in elephant joints
Maximal ROM values were 127, 115, 122 and 67 deg. for the elbow, wrist,
knee and ankle for the elephant (identical qualitative patterns were also
observed in the adult elephant). We estimate that, during high-speed
locomotion, the more proximal joints used 31–40% of their available ROM
whereas the more distal joints used 55–77%
(Table 3).
Body mass: relationship with limb motion
Elephants did not significantly change their limb segment or joint angles
across the sevenfold size range we observed (P>0.05). There were
slight statistical differences among sizes (supplementary material Table S5),
particularly <1000kg vs >3000kg, for the angles of the forefoot
segment at maximal swing, thigh segment at touch-down, shank segment at
mid-stance and lift-off and knee joint at minimal swing. Yet only the
forefoot, thigh, shank and knee joint's minimal swing angles had consistent
trends (slightly more extended joints in larger elephants) across the whole
size range, and even these differences were slight. Total ROM also did not
change significantly with size for any segments or joints (supplementary
material Table S5).
Segment and joint angular velocities decreased markedly with increasing
elephant size (Table 4), as
expected from measured size-related differences in stride frequencies
(Hutchinson et al., 2006).
These reductions (most evident between <1000 kg and >3000 kg animals)
occurred at different points in the stride for different segments and joints.
Values for 1000–3000kg and >3000kg animals differed only
marginally.
For the forelimb, we measured angular velocity decreases with size (toward zero values; supplementary material Table S5) for the upper arm at mid-stance and its RAV, the forearm at mid-stance, minimal and maximal swing and RAV; and the forefoot at maximal swing. No forelimb joints showed size-related decreases. For the hindlimb, angular velocity decreases were more widespread, occurring for the femur at mid-stance, maximal swing and RAV; the shank at all events except touchdown, and the hindfoot at mid-stance. In contrast to the forelimb, the knee joint showed large decreases for lift-off, minimal and maximal swing and RAV, although the ankle joint merely reduced its mid-swing angular velocity. Notably, touch-down angular velocity never showed a statistical size-related difference or clear trend for any segment or joint.
African and Asian elephants: limb motion comparison
As with general footfall patterns, we found some slight statistical
differences between limb segment and joint angles and angular velocities
during normal walking in African and Asian elephants (supplementary material
Tables S6 and S7). We detected a few statistically significant differences for
the angular velocities of two segments and one joint: the forearm segment at
minimal swing, forefoot segment at minimal swing and RAV, and knee joint at
minimal swing (supplementary material Table S7). The mean differences (African
minus Asian angular velocity values) were –25 deg. s–1
(25% of African value), 99 deg. s–1 (26%), 116 deg. s
–1 (15%) and –39 deg. s –1 (31%),
respectively.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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Limb motion: how columnar are elephants?
Considering other studies of footfall patterns
(Hutchinson et al., 2006) and
evidence for bouncing gaits at higher speeds
(Hutchinson et al., 2003;
Ren and Hutchinson, 2008), our
results are changing how elephants are viewed: no longer simply the
straight-limbed, inflexible juggernauts of classical literature. The present
study shows that elephant limbs are more than just columnar legs for
necessity, not for flexure. Flexion increases gradually with speed
(Fig. 10), so the posture of
an elephant at top speed is somewhat different from a slow-walking elephant
– there is no single columnar posture adopted at all speeds. A fully
columnar limb would incur large, potentially damaging, transient impact forces
and jarring, expensive center of mass motions due to its infinite stiffness
(Fischer and Blickhan,
2006).
|
;
Hildebrand, 1984;
Hildebrand and Hurley, 1985);
for example, in swing phase the wrist and knee joints move through moderate
ROM arcs (89deg. and 49deg., respectively, vs <40 deg. for other
joints/segments).
Our study refutes the notion that elephants have `inflexible' or `rigid'
ankles (Gambaryan, 1974;
Hildebrand, 1984;
Hildebrand and Hurley, 1985;
Paul, 1998). The ankle ROM
(mean 37deg. vs 89deg.) and peak angular velocities (mean
–180/+390degs–1 vs
–772/+773degs–1 in ankle vs wrist) are less
than half those of the wrist [not less than one-fifth as in Hildebrand
(Hildebrand, 1984)] even at
Fr>1 but the movement is appreciable and not purely passive (but
see Gambaryan, 1974). For
example, 27deg. of ankle dorsiflexion occurs during swing, which is
likely to be actively controlled in order to achieve ground clearance. As the
ankle dorsiflexes and rebounds 15–20deg. during stance in running, and
extensive elastic tissues cross the ankle joint
(Gambaryan, 1974), there is
the potential for elastic energy storage – it is probably incorrect to
characterize elephants as having ankles that are not spring-like
(Paul, 1998). Although the
wrist uses a large ROM during swing phase, its stance phase motion is actually
less spring-like than that of the ankle. Unlike plantigrade primates
(Pike and Alexander, 2002) or
digitigrade felids (Day and Jayne,
2007), elephants do not hold their ankles (or knees in the
additional case of felids) relatively static throughout the stride; the ankle
is a dynamic structure. However, our in vitro study (see below) of
elephant joint ROM shows that elephant ankles have low mobility relative to
cats, dogs and horses (but not humans), which may simply correlate with their
increased plantigrady, a speculation that deserves testing with data from
other species.
A brief mention of the patterns of the segment/joint abduction and
adduction we observed during locomotion in elephants is warranted here as,
despite their size and graviportal, fairly upright limb structure, they
clearly do use appreciable non-sagittal limb motions (see also
Schwerda, 2003) that are
obfuscated by labels like `columnar'. Although our markers were positioned
lateral to joint centers and hence would generally lead to quantitative
overestimates of abduction that are unreliable, we can safely infer
qualitative patterns of motion orthogonal to flexion/extension motions that
can apply to all speeds, sizes and species observed. Elephants smoothly
adducted their upper arm and thigh from mid-swing through late-stance phase,
then abducted. The elbow tended to be fairly static in adduction during
stance, then abducted and adducted in swing, whereas the knee showed a similar
motion to the upper arm and thigh but with markedly large swing-phase
abduction [note that the knee joint's helical axis passively contributes to
this motion (Weissengruber et al.,
2006)]. The wrist remained quite static (more so than the elbow)
in slight abduction during stance (much like its stasis in extension) and then
quickly abducted, then adducted during swing. The ankle adducted throughout
stance then abducted in late swing after being static in early swing. Overall,
the magnitudes of swing phase abduction in the hindlimb tended to be markedly
larger than those of the forelimb. Like Schwerda
(Schwerda, 2003), we infer
that elephants achieve foot-ground clearance during swing largely via
flexion of the wrist (which is much greater than ankle flexion) and abduction
of the whole hindlimb (especially hip and knee). Thus, elephant limbs are
often not as parasagittal as might be assumed.
We have also found that the `columar, graviportal vs crouched,
cursorial' dichotomy also breaks down somewhat when other mammalian species,
large and small, cursorial and not, are compared with elephants (see further
below). Surprisingly, the columnar, graviportal limbs of elephants and the
flexed, cursorial limbs of horses are posed rather similarly at comparable
speeds, excepting differences in foot posture
(Dutto et al., 2006;
Marey and Pagès, 1887),
a similarity that many functional studies have overlooked (see below). We feel
that the differences between cursorial and graviportal limb structure and
function have often been overstated – both are common in larger land
animals and both tend to involve more straightened limbs as a size-related
consequence (Biewener, 1989),
along with other features (Coombs,
1978).
Joint ranges of motion: how `overdesigned' are elephant joints for locomotion?
Our results for elephant and joint ROM
(Table 3) reject the null
hypothesis that all joints use a similar fraction of their maximal ROM, as the
amount of maximal ROM actually used in running (Fr>1) varies from
31 to 77%. Alternatively, perhaps more distal joints (wrists and ankles) of
elephant limbs use a larger fraction of their maximal range of motion than
more proximal joints (elbows and knees). This could be because, as in many
animals, more distal joints of elephants use a wider absolute range of motion,
presumably related to their lower segmental inertia
(Gambaryan, 1974;
Hildebrand, 1984;
Hildebrand and Hurley, 1985;
Marey and Pagès, 1887).
This hypothesis is tentatively supported
(Table 3).
A second alternative hypothesis is that, for reasons of safety, perhaps
larger animals such as elephants stay far from their limits of joint motion
(i.e. show positive allometry of the maximal ROM vs utilized ROM
ratio). Few published data on maximal joint ROM in vivo and in
vitro exist for other mammalian species that would draw elephants into
such a broader context, but Table
3 shows comparative data for cats, dogs, humans, horses and
elephants (4–4000 kg body mass). These data do not fit the ideal
criteria of being drawn from the same individuals/breeds using identical
methods (except for the elephant and horse methodology) and therefore demand
careful interpretation. We infer that the proximal–distal decline of
joint ROM found for elephants is not ubiquitous for all species but at least
applies to elephants, horses and humans. Yet interestingly, for all species,
the elbow and knee joints show more narrowly bounded (31–50% and
32–45%, respectively) percent usages of ROM, so the null hypothesis of
there being no difference among joints may roughly apply to some homologous
joints. There is a general trend for maximum possible joint ROM to decline
with size, except for the ankle joint, which is similar in humans and
elephants and strikingly different from other taxa including horses
(Table 3). However, no clear
scaling trend is evident for the percentage of maximal ROM used among
homologous joints. Hence, the second alternative hypothesis is not
supported.
The method used in the present study was limited in that we did not apply massive elephantine loads to the cadaveric material, which would provide an extra amount of flexion and extension. However, we felt that we were reaching reasonable approximations of maximal ROM as the joint ligaments and capsules seemed to be approaching their limits of failure. It is likely that elephants employ larger joint ROM during non-locomotor activities such as lying down, so the behavioral (not to mention mechanical, such as tissue stress and strain) context of ROM usage in animals remains a fertile ground for exploration, and the ROM of the scapula and shoulder/hip joints deserve investigation. We hope that this preliminary investigation of joint ROM inspires researchers to investigate this phenomenon in other species, which may reveal general principles of joint design and control.
Comparisons with previous studies
Our limb motion data (using our mean values for Fr>1.0) concur
with those from previous studies of Asian
(Gambaryan, 1974;
Marey and Pagès, 1887)
and African (Alexander et al.,
1979b; Hildebrand and Hurley,
1985) elephant limb motions during running. However, the lack of
data on marker placement and other methods, speed, size or other parameters
makes comparison difficult; differences of 10–15 deg. are expected and
evident among studies. The closest match among all studies is for thigh
segment motion.
Overall, the results from the present study compare least well with
Hildebrand and Hurley's study (Hildebrand
and Hurley, 1985) and better with others (cited above). The former
study (see also Hildebrand,
1984) showed a less vertical upper arm segment and less extended
wrist, knee and ankle joints, often with disagreement of 30–50 deg. with
measurements observed in other studies. Joint center estimation surely
contributed to some differences, especially for the toe `marker' this was
positioned more laterally at midfoot rather than cranially on digit 3.
Two studies (Gambaryan,
1974; Hildebrand and Hurley,
1985) found much higher wrist ROM compared with our mean values
(152 deg. and 104 deg., respectively, vs 89 deg.; the second value
was observed in some of our faster individuals). Gambaryan's value was clearly
a miscalculation [cf. fig. 113 and table 11 in Gambaryan
(Gambaryan, 1974); total ROM
in the former is 85deg.]. Our angles tended to be the most flexed of
those observed, especially at mid-stance, although the figured angles of Marey
and Pagès (speed unknown) and Alexander et al. (Fr>1.0) are
generally similar (Alexander et al.,
1979b; Marey and Pagès,
1887). Aside from the differences already noted, ROM is generally
larger in Gambaryan's study (we only found a much larger ROM in swing
vs stance phase for the elbow, wrist and knee) but is otherwise
roughly in agreement among studies, as are the typical patterns of flexion and
extension.
We concur with Gambaryan that the more proximal segments of elephants
(upper arm and thigh) switch between flexion and extension (or vice
versa) twice per stride (using angular displacement to identify switches;
Figs 2,
3,
4), whereas more distal joints
typically switch four times (`biphasic' motion)
(Gambaryan, 1974). A
biomechanical or control-based explanation for these patterns remains elusive,
and unfortunately angular velocity data are too noisy
(Fig. 5) to provide deeper
insight. In the present study, we have presented the first data on
segment/joint angular velocities for elephants, although Hildebrand and Hurley
used unspecified values (for an animal traveling at an unsubstantiated speed
of 10 m s–1) to calculate mechanical energies of the limb
segments (Hildebrand and Hurley,
1985). Our finding for the wrist joint's rapid flexion and
extension (>720 deg. s–1) is exceptional but other joints
(even proximal segments) showed in our data peak rotations of >200 deg.
s–1.
Like footfall patterns (Hutchinson et
al., 2006) and center of mass mechanics
(Ren and Hutchinson, 2008),
elephant limb motion changes almost continuously with speed. Differences are
evident at the extremes (Hutchinson et
al., 2003), but those extremes lie on a continuum. Our data show
that the major changes of elephant limb motion with increasing speed are
temporal (i.e. angular velocities; Figs
5,
8,
9). Likewise the slight
increases of joint angular motions with speed we have measured would generate
the observed stride length increases with speed
(Hutchinson et al., 2006). The
elongate limbs of elephants (Alexander et
al., 1979a) allow even small increases of rotational arcs to
contribute substantially to increasing stride length and thus speed. For
example, proximal segments will still contribute the most to increasing step
length – estimated stance phase scapular motion for walking elephants is
15±5 deg., contributing 55% of step length
(Fischer and Blickhan, 2006;
Schwerda, 2003); thigh motion
is 23±1 deg. (our data), contributing up to 82% of step length
(Fischer and Blickhan, 2006;
Schwerda, 2003). However,
measurements of these proximal segment and joint motions will still require
advanced methodology that carefully takes into account the larger relative
skin motions that are expected to occur proximally
(Back et al., 1995a;
Back et al., 1995b;
Leardini et al., 2005;
Reinschmidt et al., 1997;
van Weeren et al., 1988;
van Weeren et al., 1990).
Rather than present flawed kinematic data for these motions, we prefer to
accept the limitations of this study caused by excluding them and await more
accurate measurements.
|
;
Ren and Hutchinson, 2008) and
are, in a sense, `Groucho running'
(McMahon et al., 1987). The
hindlimbs (generally more flexed than the forelimbs) have already been
inferred to involve some bouncing at moderate speeds
(Ren and Hutchinson, 2008).
Yet, as even the forelimbs became more flexed at the fastest speeds we
observed (Fr>1.0), this leaves open the possibility that there are
bouncing forelimb mechanics at such speeds.
Fig. 10 and supplementary
material Movies 1 and 2 summarize the moderate changes of limb motion with
speed. In particular, a flattening of the hip (and shoulder) arc of motion
during stance is evident (Fig.
10), although this motion is less concave in the trial shown than
described for other elephants (Hutchinson
et al., 2003; Hutchinson et
al., 2006).
Although our fastest speed data (4.92 m s–1;
Fr=1.66) are not as speedy as the near-maximal documented velocities
for athletic elephants (6.8 m s–1; Fr>2.5)
(Hutchinson et al., 2006), our
data (e.g. Figs 6,
7,
8,
9) could be extrapolated to
estimate kinematics at greater speeds, with the caveat that at these speeds
stride frequency is probably already at its maximum, and hence increased joint
angular excursions, rather than velocities, should play a larger role in speed
increase (Hutchinson et al.,
2006).
Size and species differences between elephants
We found no major differences between the motions of smaller and larger
elephants, except for the lower angular velocities reached by many segments
and joints in larger elephants (Table
4), and so conclude that there are no biologically significant
size-related differences in elephant limb kinematics. This concurs with
intraspecific data for other animals (e.g.
Pennycuick, 1975), unlike
broad interspecific scaling trends for other species
(Biewener, 1989;
Biewener, 1990; but see
Day and Jayne, 2007).
Likewise, as the differences between African and Asian elephants are so
relatively small (supplementary material Tables S6 and S7), we conclude that
no profound biological differences exist for their locomotor dynamics
(Hutchinson et al., 2006;
Ren and Hutchinson, 2008).
Our conclusions should apply well to other members of the elephantid crown
group such as mammoths, at least where limb proportions
(Pike and Alexander, 2002) and
other aspects of locomotor morphology overlap with extant elephants (e.g.
Christiansen, 2007). Even
small dwarf elephants may not have differed in posture from extant elephants,
although there is tantalizing anatomical evidence that this may not be the
case (Roth, 1992). Yet,
because some elephantids reached masses of >10,000kg
(Christiansen, 2004),
biomechanical constraints might have imposed more severe limits on limb angles
and ROM [see Hutchinson et al. (Hutchinson
et al., 2006) for examples of intraspecific locomotor
scaling].
Comparison with other species
How different are elephant limb motions from bipedal, smaller, less
long-legged or more cursorial animals? As humans have a similar hindlimb
design to elephants (Weissengruber et al.,
2006) (e.g. long femur and short tibia, large functionally
plantigrade foot) despite their bipedalism, it is interesting to compare their
limb motions. These were previously described as quite similar, except for the
damping, compressive and more digitigrade feet of elephants and slightly
greater limb flexion of humans (Marey and
Pagès, 1887). Otherwise, the patterns compare well
(Fig. 11). Elephants also
exhibit similar total limb protraction and retraction angles to humans
[22 deg. (Novacheck,
1998; Schwerda,
2003; Seyfarth et al.,
2003)]. The main differences we find are the smoother motions
[attributed by Gambaryan (Gambaryan,
1974) to fascial sheets] and smaller ROM of elephant limbs. Even
the foot motions of elephants and humans are similar (especially in stance),
despite some anatomical differences, although the more horizontally oriented
feet of humans indicate some differences, at least in relative joint
moments.
Elephant stance phase segmental and joint ranges of motion are also
surprisingly similar to those observed in trotting horses [3–4 m
s–1 (Back et al.,
1995a; Back et al.,
1995b; Dutto et al.,
2006)], although toe joint motions in elephants remain unknown
[cf. Figs 2,
3,
4,
10 and figs
3 and
4 in Dutto et al.
(Dutto et al., 2006)].
Elephants also have more flexed ankles and more extended wrist joints than
horses. This fundamental similarity was recognized long ago
(Marey and Pagès, 1887)
but has since been largely forgotten by comparative analyses. Knee flexion in
elephants exceeds that in trotting horses. Horses can thus be only marginally
less columnar than elephants and even more columnar for some joints (e.g.
ankle, knee). Our in vitro analysis also shows similar maximal joint
ROM for the elbow and knee in horses and elephants, among other similarities
(Table 3). This evidence has
been overlooked by many previous qualitative studies of functional anatomy and
the cursorial–graviportal continuum (e.g.
Bakker, 1986;
Gregory, 1912;
Gambaryan, 1974;
Hildebrand, 1984;
Paul, 1998;
Paul and Christiansen,
2000).
In many ways, limb motion in elephants is typical of walking, and even
running, in smaller quadrupedal mammals. As widely recognized, elephants
generally adopt straighter limbs but their limbs still use poses and ROM that
are similar, or identical, to those of smaller animals, not just humans and
horses. The ROM of elephant limbs (particularly in swing phase but also in
stance) overlaps the ROM used even by many small mammals, although the ROM of
elephants can be smaller than some values in much smaller cursorial taxa (e.g.
Day and Jayne, 2007). For
example, in the stance phase of walking, elephants and many other mammals
retract their thigh segment or extend their hip joint across an arc of
20–40 deg. and flex the knee through 25–50 deg., with similar
values for the forelimb, although elephants flex their elbows and ankles
through smaller arcs during stance (9 deg. and 12 deg. vs 30–40
deg.) (Table 2; Figs
3 and
4)
(Fischer et al., 2002;
Pike and Alexander, 2002). The
rather static stance-phase patterns of the elbow and especially wrist joints
of elephants are also observed in trotting horses [wrist
(Back et al., 1995a)], dogs
[elbow (De Camp et al., 1993)]
and, at least, walking felids [wrist in particular
(Day and Jayne, 2007)]. As
such stasis is less evident in smaller mammals
(Fischer et al., 2002) it is
tempting to speculate that it relates to a more vertical limb orientation,
cursoriality–graviportality and body size.
Like many other mammals, elephants use late swing-phase limb retraction at
all speeds (Day and Jayne,
2007; Fischer et al.,
2002), unlike humans who only use it during running. This allows
limb protraction to be modulated at touch-down in response to locomotor
perturbations, contributing to stability
(Seyfarth et al., 2003).
Hildebrand and Hurley contended that elephants do not exhibit segmental
retraction after lift-off or just before touch-down
(Hildebrand and Hurley, 1985),
but these patterns indeed were evident in our elephants, even at slower speeds
[except where noted in the Results; also see figs 115 and 116 in Gambaryan
(Gambaryan, 1974)].
Elephants change their limb motions moderately with speed, unlike many
smaller mammals (Fischer et al.,
2002). As in other moderately-large mammals
(Gambaryan, 1974), elephants
use somewhat less biphasic distal joint motion (flexion-extension switches;
see above) during stance [cf. fig.
5 in Pike and Alexander (Pike
and Alexander, 2002) for Perissodactyla and Artiodactyla; Day and
Jayne (Day and Jayne, 2007)
for felids] than in smaller mammals (e.g.
Fischer et al., 2002). The
simpler flexion and then 50 deg. extension of the knee during swing most
resembles that observed in some Carnivora
[(Day and Jayne, 2007) see
fig. 5 in Pike and Alexander
(Pike and Alexander,
2002)].
|
CONCLUSIONS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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;
Back et al., 1995b;
Cappozzo et al., 1996;
Filipe et al., 2006;
Fischer et al., 2002;
Leardini et al., 2005;
Reinschmidt et al., 1997;
van Weeren et al., 1988;
van Weeren et al., 1990; but
see Gatesy, 1999;
Rubenson et al., 2007). Here
we have shown that elephant limb motions: (1) do not simply involve a vertical
(or parasagittal) columnar posture [`mobile Doric columns'; p. 214 in Bakker
(Bakker, 1986)] – limb
motion changes gradually with speed, and some joints are quite flexed during
stance, showing potential for elastic energy storage even in the purportedly
inflexible ankle; (2) exhibit appreciable differences within and between limbs
– the wrist has roughly double the range of motion and peak angular
velocities of the ankle, yet the wrist is quite static during stance whereas
the knee is the most mobile joint in the hindlimb; (3) have a general proximal
distal increase in how much of maximal ROM is used at faster speeds (as in, at
least, humans and horses); (4) contribute to speed changes mainly via
angular velocity, which can be quite large (>720 deg s1 in
flexion and extension for the wrist), although an increased ROM occurs for
some segments and joints; (5) reduce their angular velocity as size increases
but otherwise do not change; (6) are essentially identical for African and
Asian species despite some anatomical differences; and finally (7) are more
similar to those of smaller mammals than has previously been acknowledged.
Elephant limbs have a more limited ROM for some segments/joints and are, of
course, more straight-legged on average – but barely so relative to
walking humans or especially to trotting horses. Elephant limb motions fall on
a continuum that does not dichotomize neatly into `flexed, cursorial' and
`columnar, graviportal' categories. To a certain extent, the term columnar
obscures more information about elephant locomotion than it conveys and, as
such, is sometimes not a useful term in light of modern understanding of
locomotor dynamics. The limb motions of running horses are mostly only
trivially less columnar than those of elephants or are actually more columnar
for some distal joints. These data demonstrate that limb configuration and
running ability [top speeds <20 m s–1 in horses, 7 m
s–1 in elephants
(Hutchinson et al., 2006)] are
not as tightly associated as sometimes assumed (e.g.
Bakker, 1986;
Paul, 1998;
Paul and Christiansen, 2000).
The data provided are useful not only in the comparative context emphasized
here, but also for future studies of locomotor mechanics and as baseline data
for clinical gait analysis of elephants. LIST OF ABBREVIATIONS
|
Acknowledgments |
|---|
|
Footnotes |
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|
References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSReferences |
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