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First published online March 31, 2007
Journal of Experimental Biology 210, 1378-1390 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.001545
Mechanics of cutting maneuvers by ostriches (Struthio camelus)
1 Department of Kinesiology, Physical Education Building East 107B, Arizona
State University, Tempe AZ, 85287-0404, USA
2 Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead
Lane, North Mymms, Hatfield, Hertfordshire, AL9 7TA, UK
3 Structure and Motion Laboratory, Institute of Orthopaedics and
Musculoskeletal Sciences, University College London, Royal National Orthopedic
Hospital, Brockley Hill, Stanmore, Middlesex, HA7 4LP, UK
* Author for correspondence (e-mail: devin.jindrich{at}asu.edu)
Accepted 6 February 2007
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Key words: sidestepping, cutting, maneuverability, stability, navigation, locomotion
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). For constant-average-speed terrestrial locomotion, animals
resemble relatively simple `spring-loaded inverted pendulum' (SLIP) systems
(Cavagna et al., 1977;
Farley et al., 1993). Using
simple mathematical models such as the SLIP model to describe movement allows
for design and function to be understood in terms of overall mechanical task
constraints (Full and Koditschek,
1999). For example, considering human legs as linear springs
allowed the joints most responsible for leg stiffness to be identified
(Farley and Morgenroth,
1999).
In contrast to constant-average-speed locomotion, the mechanics of unsteady
locomotion are poorly understood
(Alexander, 2003;
Dickinson et al., 2000;
Greene, 1985). Greene and
McMahon argued that leg force production limits human turning performance
during maximum-effort curve running. Based on the assumption that the ability
to generate force constrains turning performance, they presented a model that
fits the observed relationship between maximum running speed and curve radius
(Greene, 1985;
Greene, 1987). Maximum speed
decreases may primarily be due to limitations of force generation capabilities
of the inside leg (Chang et al., 2001;
Rand and Ohtsuki, 2000).
However, leg force production does not appear to constrain performance for
other animals such as greyhounds
(Usherwood and Wilson,
2005).
By artificially increasing yaw inertia, body shape was shown to limit
maximum turning performance (Carrier et
al., 2001). Moreover, during sidestep (using the leg contralateral
to the turn direction) and crossover (using the leg ipsilateral to the turn
direction) cutting maneuvers, Jindrich et al. argued that for humans, body
shape constrains leg forces during sub-maximal speed turns
(Jindrich et al., 2006).
Specifically, they hypothesized that the braking forces observed during
walking and running turns are required to prevent over-rotation about the
vertical axis, and presented a simple algebraic model capable of predicting
ground reaction forces in several conditions. A variant of this model was also
successful in describing leg force directions used by cockroaches during
turning maneuvers (Jindrich and Full,
1999). However, humans are not ancestrally cursorial
(Schmitt and Lemelin, 2002),
and it is unclear whether the constraints on leg forces observed in humans
apply to other bipeds. Specifically, it is unclear whether braking forces are
required for bipeds of different body shape. Ostriches Struthio
camelus Linnaeus are cursorial bipeds that depend on running to escape
predation, and would be expected to be highly maneuverable. Consequently,
ostriches represent an ideal species with which to test this question and
better understand the mechanics of high-performance bipedal
maneuverability.
Ostriches could use several possible strategies for achieving the
mechanical requirements of changing the movement direction of the center of
mass (COM) (deflection) and rotating the body (rotation) during running turns.
For example, in turns that take place over multiple strides, cockroaches
deflect and rotate in the same stride with body rotation slightly lagging
deflection, whereas mice show the opposite pattern where body orientation
changes lead deflection (Jindrich and
Full, 1999; Walter,
2003). The pattern in mice was attributed to a division of labor
where front legs are primarily responsible for rotation and hindlegs
responsible for deflection, which also takes advantage of the lower rotational
inertia of the body at forelimb-to-hindlimb step transitions. Similar
differences among limb girdles are observed in some primate species
(Demes et al., 2006). Other
such divisions of labor, such as preferentially using one leg to turn, are
also possible. For example, bipeds could preferentially use sidesteps or
crossover cuts.
In addition to the potential for different behavioral maneuvering
strategies there are also many potential motor strategies that could be
employed. Maneuvers could result from substantial changes in muscle force and
joint torque at one or few joints, or alternatively from strategies that
involve modulation and coordination of torque production at many joints.
During smooth curve walking, for example, humans turn by modulating
coordination patterns observed during straight walking
(Courtine and Schieppati,
2004).
The goals of this study were to understand the behavioral and control strategies used by ostriches to turn within the context of the mechanical constraints on legged maneuvers. To this end, we tested the following hypotheses: (1) during anticipated turns, ostriches deflect the trajectory of their COM and rotate their body in the same step; (2) similar to humans, ostriches modulate body rotation during running turns by generating braking forces; (3) turning requires substantial modulation of joint kinematics and torque production for all leg degrees of freedom.
To test these hypotheses, we measured ground reaction forces and joint
kinematics while ostriches executed running turns. We measured
acceleration/braking forces and compared them to the predictions of a simple
turning model to evaluate whether ostriches use these forces to prevent under-
or over-rotation as humans do (Jindrich et
al., 2006). To evaluate the control strategies employed, we used a
quasi-static method to estimate net joint torques during turning and compared
them to straight running trials.
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The ostriches were trained to run along a 23 m rubber trackway with a force platform (model 9287BA, Kistler Instrumente AG, Winterthur, Switzerland) embedded mid-way along the length. Metal fencing constrained the running direction to an approximately 1 m corridor, and prevented turning before the force platform. To elicit turning maneuvers, the area enclosed by the metal fencing immediately around the force platform was enlarged, and a large (approximately 1 m3) cardboard box placed on the trackway behind the platform (Fig. 1A). When confronted with the box, the ostriches executed either sidestep or crossover cuts to the left, which were followed by immediate turns to the right (not analyzed) as the animals continued running around the box. Trials where at least one foot was entirely in contact with the force platform during the stance period were selected for analysis. Following the turning trials, the box barrier was removed, and all animals were induced to run down the same trackway, but not to turn. Depending on whether the ostrich contacted the force platform with the left or right leg, and whether the animal executed a straight run or turn, we grouped trials under four conditions: straight running with the left (SL) and right (SR) legs, crossover turns with the left leg (TL), and sidestep turns with the right leg (TR).
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A coordinate frame for the body was established using the five fixed points on the body: the spine, hip and breast points. During some periods of some trials, one or more body points would become obscured from enough camera views to prevent tracking. As long as three of the five body points were tracked, the positions of the remaining missing points were reconstructed based on the three or more visible points and spatial relationships among the body points established during periods when at least four points (the three tracked points and the missing points) were simultaneously visible.
The position of the COM relative to the body points was established for
each animal by measuring center of pressure (COP) location when the animal was
standing quietly on the force platform, and using the method of zero crossing
(Lafond et al., 2004;
Zatsiorsky and King, 1998).
Given the COM location, the moment of inertia (I) of the animals
about the vertical axis could be determined by enticing the animals to execute
a nearly stationary turn on the force platform, tracking the COM motion and
body rotation, and solving for I using the equations of motion for a
rigid body, the known mass (M), linear and rotational accelerations
(Lee et al., 2001). I
for ostriches was found to be linearly correlated to M5/3,
as predicted for geometrically similar bodies
(Jindrich and Full, 1999). A
least-squares linear fit using M and I values for all
animals yielded I=0.0025M5/3–0.039,
r2=0.85. For more robust estimates of I, we used
this relationship to calculate I from M for each animal.
The global kinematic frame of reference determined by the motion tracking system calibration had a vertical Z-axis, the X-axis approximately aligned with the trackway (and thus approximately aligned with the direction of motion of the animals during straight runs or prior to turning), and the positive Y-axis pointing left in the direction that the animals turned. For each sampled timestep of each trial, the instantaneous COM position was calculated from the body markers. COM positions were then differentiated with respect to time using a fourth-order difference equation, yielding COM velocity. For each step, the initial movement direction (imd) was determined from the instantaneous COM velocity at the beginning of stance. A coordinate frame was established with one axis vertical, one coincident with the projection of the imd on the horizontal plane, and the third axis orthogonal to these axes. Kinematic and force data were then expressed in this coordinate frame.
To test whether forces in the imd were used to control body
rotation, we used a simple, two-dimensional mathematical model that can
predict the ground reaction forces necessary to maintain body rotation aligned
with movement deflection based on few, easily measured parameters
(Jindrich et al., 2006;
Jindrich and Full, 1999). The
model assumes that a biped traveling with velocity V, seeks to
deflect the direction of movement by 
d during a step. At the
beginning of the step, the foot is placed at an anterior extreme position
(PAEP,imd) with respect to the COM parallel to the initial
movement direction, and generates a sinusoidal lateral force for the duration
of stance. If the foot does not remain directly lateral to the COM, generating
the lateral impulse necessary to change the movement direction will result in
a torque that rotates the body by p. The proportion that
body rotation caused by Fp(t) matches movement
deflection can be estimated by a `leg effectiveness number', an indication of
the degree to which maneuvers that maintain body orientation aligned with
movement deflection can be achieved simply by generating the forces
perpendicular to the movement direction necessary for deflection. The leg
effectiveness 
can be calculated using a simple algebraic equation based
on behavioral and morphological parameters:
 | (1) |
is the stance period. Values of close to 1 represent
conditions where little modulation of imd forces is required for body
rotation to match movement deflection at the end of the turn. In the case
where imd forces are required, their magnitude can be predicted using
the equation:
 | (2) |
).
Stance onset and offset were identified as when vertical forces
(Fv) exceeded or dropped below 5% of the maximum forces of
the trial, respectively. For some steps (typically the second step of a
trial), the foot only partially contacted the force platform. Consequently, if
the maximum Fv of an identified stance period did not
exceed 75% of the maximum Fv for the entire trial, the
step was discarded. In addition, some steps showed a `toe' region where low
forces were maintained, and the maximum Fv did not occur
at mid-stance. Consequently, for calculating and
PAEP,imd for Eqn 1 and Eqn 2, stance onset and offset were
normalized to center the maximum Fv at mid-stride. On
average, decreased by 3% and PAEP,imd decreased by
6%.
To characterize the motions of the body and legs, we assumed that the body had 6 degrees of freedom (d.f.) and that the legs could be characterized using five primary rotational d.f. Motions of the body relative to the global coordinate system were described using Euler angles in the order Z-X-Y. Rotation about Z (yaw) and Y (pitch) were calculated from the vectors connecting the spine and mid-breast and mid-hip points, respectively. Rotation about X (roll) was calculated from the hip points.
We modeled the legs as a chain of rigid segments using the points placed
over the joint centers. The orientation of the first segment (the femur) was
expressed relative to the body using Euler angles in the order Y then
Z, to align the X-axis along the long axis of the femur. The
Y angle corresponds to flexion/extension, and the Z angle
approximates ab/adduction of the hip joint relative to the body, although the
correspondence between Euler angles and common clinical definitions of
rotations is not exact (Wu et al.,
2002; Wu et al.,
2005). Each successive segment was related to the proximal segment
using Euler angles in the order Z then Y. Z-rotation
approximates ab/adduction of the distal segment, and Y-rotations
approximate flexion–extension of the joint
(Fig. 1C). These calculations
do not account for potential axial rotations about segmental
X-axes.
Forces and moments were transformed into the kinematic coordinate system
using an empirical calibration derived from measurements of COP location in
the force platform coordinate frame [corrected according to the method
described elsewhere (Bobbert and
Schamhardt, 1990)], using a known weight with position measured
using the motion tracking system. The free moment was calculated using the
forces and moments measured by the force platform
(Holden and Cavanagh, 1991).
Due to the inability to fully account for axial (X) rotations using
the marker set employed, complete inverse-dynamics calculations of joint
torques were not possible. Consequently, quasi-static joint torques (that do
not account for segmental inertias) were calculated from the endpoint forces
and moments and leg configuration angles using an iterative Newton–Euler
algorithm (Craig, 1989).
Quasi-static torques for each joint were expressed in the coordinate system of
the distal segment of the joint (McLean et
al., 2005): Ty represents flexion/extension
torque, Tz varus/valgus torque, and Tx
rotational torque about the segment axis. Torque impulse for each d.f. was
calculated by integrating torque with respect to time. All calculations were
performed using custom analysis routines written in MATLAB (The Math Works,
Inc., Natick, MA, USA).
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). We statistically compared measured parameters using repeated-measures ANOVA, with animal as the repeated measure and maneuver type (SL, SR, TL and TR) as the main effect. Reported means and standard errors (s.e.m.) represent least-squares means from the ANOVA model. We used the JMP 4.0 (SAS Institute, Inc., Cary, NC, USA) software package for statistical calculations.
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2 test; P>0.5),
and the observed body angle changes were not significantly different between
sidesteps and crossover cuts (Table
1). However, ostriches showed significantly greater movement
deflection for sidestep cuts relative to crossovers. The movement deflections
of 14° and 18° corresponded to turning radii of 2.4 m for crossovers
and 1.8 m for sidesteps, respectively.
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d)
during the final turning step showed weak associations with both initial body
angle i and initial body rotational velocity
i, as indicated by correlation coefficients
(r2) of less than 0.1
(Fig. 3A,B). Stance periods
() were 15–20% longer and initial velocity (Vi)
20–25% lower during both sidesteps and crossover cuts, but neither
showed strong correlations to d
(Table 1;
Fig. 3C,D). Anterior extreme
foot placement in the movement direction (PAEP,imd) showed
a significant increase only for sidesteps, but also showed weak correlations
with d (Fig.
3E). In contrast, sidesteps and crossover cuts both showed
significant lateral and medial shifts, respectively, in foot placement
perpendicular to the imd (Ppi) relative to
straight runs (Table 1), and
Ppi also showed a close correlation with
d (Fig.
3F). Both sidesteps and crossover cuts require substantial increases in forces perpendicular to the initial movement direction (Fp; Fig. 4B, Fig. 5A). Turning involved 10- to nearly 100-fold increases in maximum force in the horizontal plane perpendicular to imd (Fpmax) and perpendicular force impulse relative to straight running, and 50-fold increases in net torque impulse about the COM (Table 1). Relative to Fp and net torque impulses, changes to vertical forces and free moment about the vertical axis (FMz) were modest (Fig. 4A, Fig. 5B–D). Differences in FMz that could contribute to modulating body rotation were also small: body rotation due to FMz was 0.2±0.4° for sidesteps and 1.7±0.4° for crossovers.
Acceleratory or braking forces control body rotation during running turns
Although the group differences in acceleratory/braking forces in the
imd among maneuver types were small, ostriches did use forces in the
imd to control body rotation during turning. Only sidestep cuts
showed significant differences in acceleratory/braking forces (ß)
relative to straight-running steps with the same leg
(Table 1). Expected rotations
without acceleratory/braking forces ( wobr) for
sidesteps were twice the body rotation due to total forces (
wbr). Crossover cuts did not show significant
differences in average ß relative to straight runs, and body rotations of
11° without braking forces were not different from the body rotation due
to total forces.
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Significant changes in joint kinematics over the course of turning steps were evident in many joint d.f.s (Fig. 7). However, most of these significant differences were due to increased variability in joint angle trajectories during turning trials. Substantial offsets in joint angle trajectory were only evident in ankle extension during sidesteps.
The substantial increases in Fp during turning did not result in significant alterations of net torque about most joint axes (Table 3; Fig. 8). Only 10 of the 24 joint axes showed significant differences in torque impulse relative to straight runs. The significant increases in axial (X) MTP torque impulses were consistent with significant shifts in the COP relative to the toe of 6 cm medially for sidesteps and 5 cm laterally for crossovers. Increases in MTP X-axis torque impulses represented less than 25% of the total torque impulse experienced by the joint. Ankle extensor (Y) impulses showed significant increases for both sidesteps and crossovers, but the increases were only 17% and 15% for crossovers and sidesteps, respectively. Although turning also resulted in changes in loading about the ankle Z-axis, the absolute changes of 0.84 and 1.04 Nm s were less than 20% of the total torque impulses experienced by the joint. Similar to the ankle, crossover cuts were associated with significantly greater flexor torques at the knee. The knee flexion torques pass through zero, resulting in small net torque impulses during straight-ahead locomotion. Although the increased knee flexor torque impulses during crossovers were twofold those during straight running, the absolute magnitude of the impulses only increased by 10%. Similarly, hip extensor torques during sidesteps also increased over twofold relative to straight-ahead running, but the absolute torque increase was only 44%. Overall, although significant changes in net joint torque impulse were observed, for most joints these changes were small relative to the net torque impulses experienced at each joint.
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Several experimental limitations should be taken into account when interpreting these results. First, due to the structure of the trackway, barrier and motion analysis system the trials could not be randomized. Straight running trials were collected beginning 1 day following data collection from turning trials. This non-random presentation of straight runs may have contributed to the observed asymmetry of some kinematic and dynamic parameters (i.e. Fig. 7). Moreover, the environment around the trackway was not symmetrical, and the presence of computers and experimenters to the left of the force platform could also have contributed to the observed asymmetries. Variability in marker placement was also a source of measurement noise. For example, markers on the breast could move dorso-ventrally relative to the other body markers with each breath. Although these motions could change the calculated COM location vertically, we expect that the fore–aft and medio-lateral noise due to respiration to be small. Finally, in these experiments we elicited turns of modest magnitude, and ostriches can certainly execute turns sharper than the 14–18° turns we studied. Consequently, our findings do not exclude the possibility that ostriches use different strategies during turns of very different magnitudes or speeds.
The three-dimensional nature of maneuvers requires a consideration of the
three-dimensional movements of the body and limbs. To completely characterize
the position or motion of a limb, the segmental (i.e. bone) orientations
should be measured and related to each other using consistent angle
conventions (Grood and Suntay,
1983; Wu et al.,
2002). Determining bone orientations, however, requires multiple
markers on each segment, and was not possible in this study. Consequently, we
chose to affix markers to landmarks near each joint center, and characterize
joint motion using an angle convention that captures the most important
features of movement. However, this characterization was not complete, and
some potential types of movement (such as axial rotation of the segments),
could not be uniquely identified. Moreover, the nature of our kinematic
characterization prevented inverse-dynamic calculations of joint torques that
would account for the contributions of segmental acceleration to ground
reaction forces. However, the impact of these limitations is reduced by the
repeated-measures experimental design, and the small differences in joint
kinematics observed among the four conditions.
Although ostriches did not change their stride to preferentially turn with
one leg, the kinematics of executing crossovers and sidesteps were different.
The greater initial body rotation, and reduced rotation observed during
crossovers, suggest that body rotation is limited. One reason for reduced
rotation during crossovers is the lower leg effectiveness of the inside leg
relative to the outside leg (Table
1). However, with a leg effectiveness of 0.9, the body rotation
caused by Fp would be expected to be 90% of the deflection
magnitude, instead of 36%. This difference is likely caused by other
mechanical factors such as the inertia of the swing leg. During straight
running steps with the left leg, the body rotated on average –4°
(i.e. clockwise; Table 1). The
same mechanical factors are likely to constrain rotation during crossovers.
Relative to straight steps with the left leg, body angles changed 10°
during crossovers, or 83% of the measured deflection. The remaining
discrepancy may be due to the need to swing the right leg in the turn
direction (medially) for correct placement in the subsequent step, similar to
the effects of swing-leg inertia suggested in studies of human maneuvering
(Jindrich et al., 2006). For
crossovers, the body rotation due to this medial movement would act against
the turn direction, and could contribute to the reduced body rotation during
turning steps. The similarity of net torque impulses about the COM during
sidesteps and crossovers supports the possibility that swing-leg inertia
reduces body rotation during crossovers. Although ostriches generated forces
appropriate for body rotation to match movement deflection during crossovers,
body rotation during turning was likely reduced by swing-leg inertia.
Ostriches appeared to compensate for this limitation by beginning crossovers
with increased initial body yaw into the turning direction.
Ostrich morphology is appropriate for effective maneuvers
When humans execute 30° sidestep and crossover cuts, braking forces are
26% of Fpmax compared to 6–11% for ostriches
executing 15–20° turns (Jindrich
et al., 2006). Moreover, whereas humans generated almost
exclusively braking forces during sidesteps and crossovers, 40% of the net
forces observed during turns for ostriches were acceleratory. Although both
ostriches and humans used braking/acceleration forces to control body
rotation, this required almost exclusively braking forces by humans. This can
be explained in part by differences in leg effectiveness. Whereas humans turn
with =2.0–2.5, ostriches operated at of approximately half
these values, 0.9–1.2 (close to 1). Differences in body shape can
account for some of the differences observed between ostriches and humans. In
contrast to the orthograde posture of humans, ostriches have a pronograde
(i.e. more horizontal than vertical) trunk orientation that results in a
larger moment of inertia about the vertical axis. The relationship
M/I for ostriches was 86% of that for humans, and an
ostrich-shaped human would be expected to have =1.2. However, is
most sensitive to the relationship of PAEP,imd to
and Vi (i.e. the multiplicand of Eqn 1). The
fore–aft foot placement (PAEP,imd) for ostriches was
below (76–79%) those used by humans, but this was almost completely
offset by decreases in (ostriches 81–85% of humans), and
Vi (ostriches 87–93% of humans). The multiplicand of
Eqn 1 for ostriches was 95% of human values. Consequently, most of the
differences between ostriches and humans were explained by differences in body
morphology. Ostrich morphology is appropriate for effective maneuvers that
require minimal acceleratory or braking forces.
Turns could be executed with minimal changes in leg kinematics or joint torque production
Ostriches did not substantially alter body or leg kinematics to turn, and
the kinematic changes resulted in few alterations to joint torques relative to
straight-ahead runnning. The lateral shifts in foot placement relative to the
COM (PAEP,ip; Table
1) were caused by increased body roll and increased knee adduction
and abduction for crossovers and sidesteps, respectively
(Table 2). Considering the
height of the COM of 76 cm, an initial body roll of 9° would be expected
to result in a change in PAEP,ip of 12 cm in the absence
of joint angle changes, approximately 60% of the observed shift for sidesteps.
For crossovers, body roll alone without changes in leg kinematics would be
expected to account for 37% of the PAEP,ip shift. The
remainder of PAEP,ip shift can be accounted for by
Z rotation at the knee: increased adduction during crossovers and
abduction during sidesteps, which both serve to shift the foot position
towards the outside of the turn. This rotation is most likely due to axial
thigh rotation, but varus/valgus movements at the knee could also have
contributed to the observed Z-rotation. These observed adjustments at
the knee joint are similar to changes in knee angles observed by guinea fowl
running over rough terrain (Daley and
Biewener, 2006). The only other significant change in initial
angle, MTP Z, contributed to the medial shift in
PAEP,ip during crossovers, but was small in magnitude and
could not account for substantial shifts in foot position given the length of
the foot.
Body roll and leg ab/adduction resulted in transverse leg angles (the angle
of the line connecting the toe and hip) that paralleled changes in transverse
force angle during turns (Table
1). Transverse leg angle increased by 16° during crossovers
and 17° during sidesteps, compared to 14° changes in force angle.
Although medio-lateral shifts in the COP resulted in increased X and
Z torques at the MTP and ankle joints, the alignment of the leg and
force angles prevented significant increases in X and Z
torque impulses at the knee and hip. Surprisingly, Y torques
(extension at the ankle and flexion at the knee) increased during crossovers
despite a significant decrease in the resultant force (Tables
1,
3). This was most likely due to
the increased body yaw at the initiation of the turn during crossovers, which
served to increase the component of Fp directed in the
positive fore–aft direction, relative to the leg. Patterns of
fore–aft and vertical forces relative to the imd were
maintained during crossovers (Fig.
4), even though this resulted in changes in net torque impulses at
distal joints. Overall, considering the large increases in
Fp required for turning, changes in joint loading were
small: less than 25% with the exception of hip extensor torques during
sidesteps. This smooth transition from running to turning is reminiscent of
the smooth transition between grounded and aerial running observed in these
animals (Rubenson et al.,
2004).
These results suggest that, with an appropriately designed morphological
system, maneuvers can be executed with minimal changes to running dynamics.
Although acceleratory and braking forces did serve to control body rotation,
maneuvers did not involve substantial changes to leg kinematics or joint
loading. Consequently, these results suggest that maneuvers in ostriches could
result from minor modifications of the spring-like behavior of legs during
running. Theoretical studies of `Lateral Leg Springs' have shown that
horizontal-plane maneuvers can be executed by spring–mass systems with
minor shifts in COP location (Schmitt and
Holmes, 2000), a proposition experimentally supported in insects
(Jindrich and Full, 1999).
These findings parallel theoretical and experimental studies of saggital-plane
maneuvers, where the spring-like properties of legs can contribute to energy
input in the form of muscle work to result in high performance
(McGowan et al., 2005;
Seyfarth et al., 1999).
Changes in leg placement can contribute to stabilizing movements both through
body dynamics and influencing leg stiffness
(Farley et al., 1998;
Seyfarth et al., 2002;
Seyfarth et al., 2003).
Additional study is required to determine how musculoskeletal dynamics
contributes to satisfying both the translational and rotational stability
requirements during three-dimensional maneuvers.
In summary, ostrich morphology is appropriate for maneuvering without requiring large braking or acceleratory forces. However, ostriches did use forces in the initial movement direction to control body rotation. Ostriches executed maneuvers using a simple control strategy that required minimal changes to leg kinematics or net torque production at individual joints. Body roll and ab/adduction of the leg shifted the foot position away from the turn direction, reducing the braking or acceleration forces required to control body rotation and aligning the leg with the ground reaction force.
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d
p
rwb
rwob
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Acknowledgments |
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References |
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