563
The Journal of Experimental Biology 202, 563–577 (1999)
Printed in Great Britain © The Company of Biologists Limited 1999
JEB1695
RUNNING, BREATHING AND VISCERAL MOTION IN THE DOMESTIC RABBIT
(ORYCTOLAGUS CUNICULUS): TESTING VISCERAL DISPLACEMENT HYPOTHESES
RACHEL S. SIMONS*
Department of Biology, University of Utah, Salt Lake City, UT 84112, USA
*Present address: Department of Biology, Morrill Science Center, University of Massachusetts, Amherst, MA 01003, USA
(e-mail: [email protected])
Accepted 2 December 1998; published on WWW 3 February 1999
Summary
The relative motion of the visceral mass may be
variation in phase angle between the locomotor and
important to ventilation during running. A visceral piston
respiratory periods that is inconsistent with the 1:1 LRC
hypothesis predicts that, during galloping, cranial motion
ratio that has been reported for other galloping mammals;
(2) a tendency towards a 1:1 LRC ratio at higher speeds
of the liver during expiration and caudal motion of the liver
and stride frequencies; and (3) that the relative motion of
during inspiration may characterize efficient quadrupedal
the liver is caudal during expiration and cranial during
mammalian locomotion. Although a theoretical model
based on vibration mechanics casts doubt on this
inspiration, which is inconsistent with the visceral piston
prediction, only limited direct measurements of visceral
hypothesis. The data presented here are generally
consistent with the theoretical vibration mechanics model
mass motion during galloping have been reported. In the
for liver motion and with a pneumatic stabilization
present study, mechanical interactions between running,
hypothesis that the lungs serve an important role in the
breathing and liver oscillations in the domestic rabbit
stabilization of the thorax during locomotion.
are
recorded
using
synchronized
videographic,
cineradiographic and pneumotachographic techniques.
The
analysis
focuses
on
the
variation
in
locomotor–respiratory coupling (LRC) and on the relative
Key words: rabbit, locomotion, locomotor–respiratory coupling,
visceral mass, ventilation, Oryctolagus cuniculus.
position of the liver. Results from running rabbits show (1)
Introduction
The integration of running and breathing is fundamental to
sustained mammalian exercise (Bramble, 1989; Bramble and
Jenkins, 1989; Carrier, 1987). It is generally agreed that
‘locomotory and respiratory interactions may impact upon the
efficiency of either activity’ (Dempsey et al., 1996; p.474).
However, the ways in which this occurs and the relative
importance of the influence of locomotor mechanics on
ventilation during exercise are currently under debate
(Ainsworth et al., 1996; Alexander, 1987, 1989; Banzett et
al., 1992; Bramble, 1989; Bramble and Carrier, 1983;
Bramble and Jenkins, 1993; Dempsey et al., 1996; Young et
al., 1992a,b). Galloping may influence ventilation by several
possible mechanisms: (1) sagittal flexion of the body, (2)
loading forces on the limbs transmitted to the chest walls
changing pressures inside the thorax, and (3) accelerational
changes of the trunk and subsequent inertial movement of the
loosely tethered visceral mass (see references above).
Bramble and Carrier (1983) and Bramble (1989) proposed
that, during galloping, cranial motion of the visceral mass
upon landing would facilitate expiration and that caudal
motion of the visceral mass during the forelimb flight phase
would facilitate inspiration. This ‘visceral piston’ (VP) model
predicts that motion of the liver during galloping will directly
influence the respiratory cycle because the liver alternately
slides forward, thereby decreasing thoracic volume, and
rearward, pulling on the diaphragm and therefore expanding
the volume of the thorax. However, on the basis of modeling
the position of the liver in a galloping horse, using vibration
mechanics, Young et al. (1992a) cast doubt upon the VP
hypothesis and suggested that inertially driven displacements
of the liver are unlikely to drive lung ventilation. Their
vibration mechanics (VM) model predicted that, if inertial
forces alone were responsible for the motion of the liver, then
the liver should oscillate almost 180 ° out of phase with that
required by the VP model.
Simons (1996) introduced a third hypothesis regarding liver
position and its relationship to locomotor and respiratory
mechanics. On the basis of data from running rabbits, Simons
(1996) proposed the pneumatic stabilization (PS) model in
which simultaneous expiration and forelimb support results in
positive pressure in the pleural cavity, which drives the visceral
mass caudally and provides some mechanical stability for the
heart. Motion of the visceral mass in the PS model is consistent
with visceral mass motion predicted in the VM model, but is
564
R. S. SIMONS
inconsistent with visceral mass motion predicted in the VP
model.
Although all three models are largely untested, the VP
hypothesis remains appealing despite the lack of compelling
supportive evidence. If visceral oscillation facilitates the
volume changes of the thorax that accompany breathing
movements, running mammals might avoid possible
antagonistic actions of the thorax and visceral mass, thereby
saving metabolic energy and contributing to the efficiency of
aerobic locomotion (Bramble and Carrier, 1983; Bramble,
1989). The hypothesis that locomotor–respiratory coupling
reduces the work of breathing (Bramble, 1989; Alexander,
1989; Funk et al., 1997) is supported by experimental evidence
in birds (Funk et al., 1997; Boggs, 1997), but no studies have
directly addressed this hypothesis in mammals. Only very
limited direct measurements of the position of the visceral
mass during galloping have previously been available
(Bramble et al., 1994; Simons, 1996). The detailed analysis of
rabbit locomotor, respiratory and visceral mass interactions
presented in the present study provides the first substantial test
of the visceral displacement hypotheses based on direct
observations of the visceral motion in a rapidly running
mammal.
Most of the mammals that have been investigated so far,
such as horses, dogs, wallabies, white rhinoceros and gerbils,
show a consistent 1:1 breath-to-stride ratio during galloping or
hopping such that expiration occurs during forelimb support
and inspiration occurs during the flight phase (Alexander,
1987; Attenburrow, 1983; Bramble and Carrier, 1983;
Bramble, 1989; Hornicke et al., 1983; Lafortuna et al., 1991;
Young et al., 1992a,b; Baudinette et al., 1987). The consistent
nature of the 1:1 locomotor–respiratory coupling (LRC) and
phase relationship at the gallop suggests the functional
importance of LRC to exercising mammals (Bramble and
Carrier, 1983) and underlies the VP, VM and PS hypotheses.
However, the lack of variation of the phase relationship or ratio
of LRC at the gallop also makes it difficult to identify a
possible cause-and-effect relationship between locomotor
mechanics and ventilation. Thus, ironically, the constancy of
locomotor–respiratory coupling at the gallop for the most
extensively examined mammalian species, horses and dogs,
obscures our understanding of the functional significance of
LRC.
Analysis of the locomotor–respiratory interactions of the
domestic rabbit may be useful because, in addition to the
expected 1:1 coupling between gait and breathing, variation
in the LRC ratio has also been observed (Simons, 1996).
Thus, the analysis of the variation of rabbit
locomotor–respiratory coupling presented in this study may
help to establish those conditions under which coupling
occurs and thereby elucidate why it occurs. Furthermore,
although the species investigated in previous studies are
phylogenetically diverse, they are nonetheless relatively
homogeneous in their locomotor behavior in being primarily
endurance runners. In contrast, the domestic rabbit
(Oryctolagus cuniculus) is a sprinter (Gambaryan, 1974;
Howell, 1944). Accordingly, LRC data from rabbits offer a
comparative perspective that may broaden our understanding
of general mammalian locomotor–respiratory interactions.
The rabbit is an interesting subject for an additional reason.
Unlike most quadrupeds that change gait with speed (i.e. walk,
trot, gallop), rabbits use only a single asymmetrical gait, the
half-bound, at all speeds. This gait is much like a gallop in that
the forelimbs strike the ground sequentially, but it differs in
that the hindlimbs move together in the half-bound and
sequentially in the gallop. By studying a mammal that uses
only a single gait, LRC may be examined without the potential
complication of a gait change.
This study examines the locomotor performance of domestic
rabbits using a combination of videographic, cineradiographic
and pneumotachographic techniques to document running and
breathing patterns as well as the position of the visceral mass
during the rabbit half-bound over a range of speeds. Together,
these data allow a test of the visceral displacement hypotheses
(Bramble and Carrier, 1983; Bramble, 1989; Young et al.,
1992a; Simons, 1996), which rely on a fixed phase relationship
between running and breathing and on the position of the
visceral mass during galloping.
Materials and methods
Data acquisition
Three domestic rabbits (Oryctolagus cuniculus) were trained
for 4 months to run on a variable-speed treadmill. Running
performance was recorded using cineradiographic films and
video while the rabbits ran for short bursts (15–45 s) on a
treadmill at speeds of 6–18 km h−1. During both training and
experiments, the rabbits ran inside a large, uncovered Plexiglas
box (1.27 m×0.60 m×0.38 m). Synchronized respiratory flow
(pneumotachographic) data were obtained for both video and
film recordings while the rabbits were wearing custom-fitted
face masks. For a subset of trials, no masks were used and the
animals ran entirely unencumbered.
To assess the internal kinematics of the thorax,
cineradiographic films were taken in both lateral and
dorsoventral projections. Radiographic films were obtained
using a Seimens X-ray apparatus with a high-speed 16 mm
Photosonics cine camera mounted on the image intensifier and
operated at 100 frames s−1. Two metallic markers (2.5 mm
diameter) placed 10 cm apart on the image intensifier provided
fixed reference points and indicated the gravitational
horizontal. A metal disk (1.79 cm) was temporarily affixed to
the midline of the animal as a scale reference. In addition, postexperiment dissections enabled a comparison of actual
vertebral dimensions with those in the films, and scaling and
magnification factors were calculated for each running
sequence. Generally, scapulae, brachia and antibrachia were
visible in radiographic films, and during several representative
trials a second Photosonics camera, with conventional light
film (100 frames s−1), was used to capture simultaneous
anterolateral images of the whole animal to verify limb
placement and body position on the treadmill.
Running, breathing and visceral motion in rabbits
Cable to computer
Pressure
transducer
Polyethylene
tubing:
1.19 mm i.d.;
70 mm length
45 mm
12 mm
14 mm i.d.
Fig. 1. Schematic diagram of a rabbit wearing a face mask. A
differential pressure transducer is mounted on the plastic mask. The
distance from the nares of the rabbit to the screen was 45 mm, and
from the screen to the open end of the mask was 12 mm. The inner
diameter (i.d.) of the breathing tube was 14 mm. See text for details.
Relative respiratory flow (uncalibrated) was measured using
a mask-mounted screen pneumotach. Pressure changes across
the screen were recorded using a differential pressure
transducer (Omega Engineering; 0–17.5 cm of water) mounted
on the mask. The total mass of the mask and the transducer
was 35 g, and the rabbits appeared to have no difficulty
carrying this weight. Two polyethylene tubes (inner diameter
1.19 mm; length 7 cm) connected the pressure ports to metal
tubes (18 gauge hypodermic needles) on either side of a mesh
screen disk (radius 0.7 cm; 36 % open; Small Parts Inc. CMN350) (Fig. 1). The mask fitted comfortably over the muzzle of
the animals and created a dead space of approximately 20 ml
(<8 % of tidal volume (Vt) where Vt is conservatively estimated
from resting rabbits, according to the equation of SchmidtNielson (1990):
Vt = 0.0062Mb1.01 ,
(1)
where Mb is body mass (in kg) and Vt is tidal volume (in l).
Pressure measurements describing flow at the mouth
(sampled at 300 Hz) were synchronized with the
cineradiographic films using a TEAC CT-90 II tape recorder.
Electrical impulses marked the pneumotach tape recording,
the cine film and the light film simultaneously, providing a
synchronization signal (one spike per frame). These data
were also displayed as hard copy records by replaying the
tape on a strip-chart recorder (Western Graphic Inc. Mark 1001 Thermal Arraycorder).
Dorsoventral films confirmed the lateral skeletal and visceral
motions as well as anatomical details observed in lateral view.
The dorsoventral films were analyzed using similar methods,
although the vertebral positional information was gathered
from the fourth thoracic spinous process rather than the
vertebra. The position of the leading edge of the liver was
565
recorded, but no sternal measurements were possible in
dorsoventral aspect.
The cineradiographic data consist of 20 film sequences in
lateral view for which the kinematics are examined in detail.
These sequences represent the running performances of two
rabbits over a speed range of 8.76–18.0 km h−1. The segments
of film in which the animal is in position in front of the X-ray
tube (i.e. anterior portion of the thorax, vertebral column,
sternum and leading edge of the diaphragm all simultaneously
visible) constitute a random sample of running. The range of
consecutive ‘in position’ strides is 3–10. Rabbits wore masks
in 15 of these 25 cine sequences. The cine data were
supplemented with video data (for methods, see Simons, 1996)
from the same two rabbits plus a third rabbit. The rabbits wore
masks in 12 video sequences. Video recordings were taken in
lateral view over a range of speeds from 6 to 10 km h−1. The
range of consecutive strides is 6–21; 1.5–7.5 s of running per
sequence.
Panting as a control for galloping
As a control for the effects of locomotion on breathing,
cineradiographic data were also collected from two of these
rabbits while they were motionless on the treadmill.
Immediately following treadmill exercise, rabbits were
filmed in both lateral and dorsoventral views, and
pneumotachographic data were collected while the rabbits
remained in resting postures. Ten panting sequences were
analyzed in detail. The radiographs and post-experiment
dissections confirmed the relative positions of the heart, lungs,
costal and crural diaphragm and viscera as well as the relative
positions of skeletal elements (sternum, ribs, vertebrae).
Identification and familiarity with thoracic anatomy in the
panting animal assisted in the identification of these same
organs in the radiographic images of half-bounding rabbits.
Data analysis
Video data were analyzed using the Peak Performance
System (for methods, see Simons, 1996). Film analysis was
performed using a Vanguard motion analyzer and Macintosh
computer as well as a Visual Instruments Corporation motion
analyzer (model 1214A) from which data were downloaded
directly into a PowerMac 7100/80AV computer.
The positions and displacements (x and y coordinates) of a
number of skeletal elements and viscera were digitized in
lateral view (Fig. 2). Vertebral column position was digitized
by following the anteroventral corner of the eighth thoracic
vertebra. A more anterior vertebra (fourth thoracic) was also
followed, and a comparison of the two vertebral positions
verified that the anterior segment of the vertebral column is
relatively inflexible. Sternal motion was quantified by
digitizing the posterior edge of the manubrium. The method of
Hall-Craggs (1965) was used to identify the center of mass in
two frozen specimens. The eighth thoracic vertebra is an
approximation of the center of mass of the rabbit and was used
to identify the position of the rabbit (as distinct from the
position of the visceral mass of the rabbit).
566
R. S. SIMONS
Visceral mass
To document the position of the visceral mass, I digitized a
single point on the leading edge of the liver during each
running sequence. The liver and costal diaphragm are
connected via the posterior vena cava at the central tendon, and
in these films, no distinction between the leading edge of the
liver and the costal diaphragm was visible. In most films, no
outstanding natural landmark (i.e. vessel) was reliably present,
and only the horizontal component (the leading edge) of the
liver was therefore followed (Fig. 2). In several sequences,
however, the film resolution did permit visualization of a
discrete landmark on the liver for which both horizontal and
vertical coordinates were digitized. The horizontal
displacement data obtained from the ‘leading edge’ of the liver
were indistinguishable from the horizontal component data
gathered from following a discrete point on the liver. Further
verification of the position of the liver was established by
digitizing the entire leading edge of the liver for a subset of
running trials. Again, the displacement of the ‘whole leading
edge’ of the liver was generally indistinguishable from that of
the single-point leading edge.
Detrending and finding residuals
As an animal runs on the treadmill, some anteroposterior
drifting in position occurs (2–5 cm) when the animal fails to
match the treadmill belt speed perfectly. This is true even
during relatively steady running (3–5 Hz) in which the rabbit
appears to hold its position in front of the camera. This drift
(<0.5 Hz) may be filtered out to isolate and quantify the
amplitudes of the higher-frequency oscillations that describe
each locomotor cycle. Both the horizontal and vertical
digitized displacements are detrended using a non-parametric
regression technique (Hardle, 1990). The larger trend or drift
was identified and approximated by an iterative smoothing of
the raw data using a 21-point moving average. This trend was
then subtracted from the total, raw displacement. Because the
frequency of drift is low, relative to the higher frequency of
running, the locomotor data are not meaningfully attenuated.
The resulting residual data describe the oscillation of the rabbit
about a central or neutral point, enabling mean peak amplitudes
to be ascertained.
Relative motion
Motion relative to the vertebral column was calculated for
the sternum and liver by subtracting vertebral motion from the
skeletal or visceral element. All data were scaled and
detrended. Residual data were used to find relative motion and
excursion amplitudes at all speeds.
Radiodensity volume
In addition to the direct pneumotachographic recording of
respiratory flow at the mouth, a measure of the relative volume
of the lungs was recorded using the radiodensity volume
technique (RDV; Bramble and Jenkins, 1993). Using X-ray
positive film, radiodensity increases during expiration and
decreases during inspiration. The RDV data were recorded
Fig. 2. The square shows the approximate location of the
radiographic image superimposed on a tracing from a single frame of
a running rabbit. A schematic diagram of an X-ray image shows ribs
5–8 cut away to reveal the leading edge of the diaphragm (arrow)
and the liver (shaded). The filled circles indicate digitized points:
(1) anteroventral corner of the eighth thoracic vertebra, (2)
posteroventral corner of the fourth thoracic vertebra and (3)
posteroventral corner of the first element of the sternum
(manubrium).
from the intercostal spaces between ribs 7, 8 and 9. Only
posterior sites were sampled since a forelimb shadow
periodically obscured radiodensity readings from the anterior
regions of the lung field. The RDV measurements were
obtained during both panting and half-bounding. During
locomotion, RDV generally corresponded well to respiratory
flow as measured by the pneumotachograph. For six recorded
sequences in which the rabbits ran without face masks, RDV
measurements were used as a surrogate for relative respiratory
flow.
Thoracic pitch
Sagittal pitching of the trunk during running is one possible
cause of relative position changes of the visceral mass. I
therefore examined the relationship between thoracic pitch and
the relative displacement of the liver and other locomotor
variables. Pitching of the trunk was measured for
representative sequences at all speeds. The angle formed by the
rigid anterior portion of the thoracic vertebral column relative
to the gravitational horizontal was measured during treadmill
Running, breathing and visceral motion in rabbits
running. To identify more directly the effect of pitch change,
kinematic data were also collected from representative sections
of films in which the vertebral column was maintained in the
horizontal plane, thus eliminating any influence of pitch. These
measurements, when compared with data obtained from the
same sequence without correction for thoracic pitching,
revealed differences in timing of relative skeletal and visceral
oscillations ranging from 0 to 30 ms (a phase angle change of
0–54 °). Although this difference in timing could be important
in some calculations (e.g. in determining the onset of driving
forces), it does not influence the relative position profiles of
the sternum and liver and hence does not change any of the
conclusions reached in this paper.
Locomotor and respiratory cycles
The breath-to-stride ratio is the number of respiratory cycles
(or fraction thereof) divided by the number of complete
locomotor cycles. For all running sequences (3–10 strides), the
breath-to-stride ratio was calculated and then tested for
statistically significant correlations with three variables (using
analysis of variance, ANOVA, and analysis of covariance,
ANCOVA): treadmill speed, stride frequency and relative liver
excursion. Each breath within the same run may not be
‘independent’ of the preceding or following breath. By
averaging the number of breaths per stride, the problems
associated with the likely interdependence of breathing within
a single run may be minimized. However, some information is
lost by averaging, and a stride-by-stride analysis was therefore
also conducted. Six phase variables were measured for each
complete stride cycle: the onset and peak of inspiration and
expiration, and the maximum caudal and cranial excursions of
the visceral mass. For each stride cycle (100 %), the phase
angle was measured in increments of 10 ms and then multiplied
by 3.6 to give the phase angle in degrees. The precision of the
phase angle measurement is ±10 ms, which is the duration of
each film frame. To calculate means and standard deviations
of cyclical data (0–360 ° per stride cycle), the data were arctantransformed (Batschelet, 1981). This analysis results in angular
567
standard deviation and takes into account that, for example,
360 ° is the same as 0 °.
Results
Panting
Respiratory cycles, liver motion, vertebral motion and
radiodensity volume (RDV) are plotted for a representative
episode of post-exercise panting in Fig. 3. Caudal
displacements of the liver were associated with inspiration, and
cranial displacements were associated with expiration. Caudal
movement of the liver was also coincident with an increase in
the radiodensity reading, and cranial motion was associated
with decreases in RDV although a 30 ms delay was observed
between the onset of liver movement and the reversal of the
RDV curve (Fig. 3). These relationships remained constant
over breathing frequencies ranging from 3 to 6 Hz. Liver
excursion (mean value 1 cm) was approximately ten times that
of the vertebral column, ribs, sternum, elbow and scapulae.
Kinematic and RDV profiles obtained from the dorsoventral
aspect are consistent with the same parameters observed in
lateral views.
Magnitude of thoracic pitch
As the rabbits ran, the trunk underwent sagittal pitching
during each stride cycle. The rabbits were approximately
horizontal (0 °) for less than 10 ms during the forelimb flight
phase of the stride and achieved a maximum downward pitch
angle of 35–45 ° during the initial 150–200 ms of forelimb
support. The mean maximum pitch angle was not significantly
correlated with treadmill speed (ANOVA, N=7 trials at speeds
ranging from 8 to 15 km h−1; P=0.53), stride frequency
(P=0.45), stride length (P=0.57) or mean peak horizontal or
vertical liver displacement (P=0.69).
Locomotor, respiratory and liver displacement cycles
During treadmill exercise, in 11 out of 25 trials (i.e. 44 % of
the time), rabbits showed a 1:1 correlation between locomotor
Fig. 3. Example of representative posti
exercise panting. The respiratory rate
e
is approximately 5 Hz. Horizontal Cr
oscillations of the leading edge of the
liver (indistinguishable from that of
the diaphragm) and of the eighth
thoracic vertebra are shown. The Ca
pneumotachographic trace indicates
respiratory phase. The radiodensity
F
volume trace (RDV) indicates the
degree of filling (F) and emptying (E)
of a local region of the posterior lungs
E
between ribs 8 and 9. Caudal motion
of the liver is coincident with
inspiration and an increase in
0
500
radiodensity reading. Cranial motion
Time (ms)
of the liver is coincident with
expiration and a decreasing RDV. Ca, caudal; Cr, cranial; e, expiration; i, inspiration. Scale bar, 1 cm.
Relative
respiratory
flow
Vertebra
Liver
RDV
1000
568
R. S. SIMONS
Table 1. Stride-by-stride analysis of the mean phase angles of respiratory and liver cycles relative to the locomotor cycle
Mean φ
for all LRCs1
S.D.3
(degrees)
(degrees)
Onset of inspiration
Onset of expiration
Peak inspiration
Peak expiration
Maximum cranial
liver position
Maximum caudal
liver position
Speed (km h−1)5
(24)2
N4
Mean φ
for LRC≠1:1
S.D.
(degrees)
(degrees)
N
Mean φ
for LRC=1:1
S.D.
(degrees)
(degrees)
N
141.6
337.2 (23)
189.6 (20)
29.3 (30)
334.7 (7)
59
58
56
63
38
57
58
54
59
88
116.6 (37)
308.0 (34)
172.7 (32)
355.3 (27)
350.0 (27)
64
62
61
60
55
37
36
34
39
27
163.9 (20)
1.99 (18)
206.7 (22)
79.5 (26)
330.6 (8)
40
38
42
47
27
20
22
20
21
53
171.7 (10)
40
91
175.7 (26)
54
30
172.6 (14)
43
53
12.2±2.7; N=206
10.6±2.4; N=10
13.9±1.9; N=10
Forelimb support, 0 to 198±22 °; hindlimb support, 198–360 °.
peak angle φ in degrees (acrophase; Batschelet, 1981); LRC, locomotor–respiratory coupling.
295 % confidence interval for circular data; α=0.05.
3Mean angular standard deviation in degrees.
4Number of strides from two rabbits, cineradiographic data only.
5Mean speed ± S.D.
6Number of sequences.
1Mean
and breathing frequencies (1:1 includes ratios ⭐1.05). The
ratio of breaths to strides was averaged for each run (3–10
strides; mean number of strides 6). For all runs combined,
where the speed was 12.2±2.7 km h−1 (mean ± S.D.), the mean
ratio of breaths to strides was 1.17±0.18. At speeds below
12 km h−1 (10.1±1.4 km h−1; N=10 trials), the breath-to-stride
ratio was 1.27±0.2. At speeds greater than or equal to
12 km h−1 (14.4±1.8 km h−1; N=10 trials), the breath-to-stride
ratio was 1.06±0.8. Only trials with perfectly coupled
respiration and locomotion had a breath-to-stride ratio of 1.0.
Ratios of 1.1–1.2 generally reflect runs in which the rabbit,
although running ‘primarily’ at a breath-to-stride ration of 1:1,
took a partial breath in addition to the full respiratory cycle
Fig. 4. Tracings from high-speed video
recordings showing body position during a
typical stride cycle (360 °), where 0–198 °
defines the forelimb ground-contact and
198–360 ° equals hindlimb ground-contact.
Filled arrows indicate the direction of liver
motion. Open arrows indicate the direction of
air flow at the mouth. Mean peak respiratory
flow (open squares) and mean maximal
horizontal liver displacement (filled circles)
are plotted over a single stride cycle. Data
points (squares, circles) are mean values from
all stride-by-stride analyses, i.e. 1:1 LRC
(locomotor–respiratory coupling) and non-1:1
LRC. 95 % confidence intervals are shown.
See values in Table 1. Pk. Insp., mean peak
inspiratory flow; Pk. Exp., mean peak
expiratory flow; Pk. Cranial, mean horizontal
peak cranial liver position; Pk. Caudal, mean
horizontal peak caudal liver position.
during one stride. Stride frequency never exceeded the
breathing rate, and a ratio of less than 1.0 was therefore never
observed.
Stride-by-stride analysis of cineradiographic data showed
that approximately 40 % of the strides represent locomotor
cycles associated with single respiratory cycles (i.e.=1:1); 60 %
of all the strides were not associated with a single respiratory
cycle (i.e. ≠1:1) (Table 1). A stride-by-stride analysis revealed
the mean phase angle, with respect to the stride cycle, for six
events: the onset of inspiration, the onset of expiration, peak
inspiration, peak expiration and the caudal and cranial peak
positions of the visceral mass (Table 1).
During an average stride, rabbits began inspiration during
270°
Pk.
Cranial
Pk. Insp.
0°
180°
Pk.
Caudal
Pk. Exp.
90°
Running, breathing and visceral motion in rabbits
the second half of forelimb support, while they brought the
non-lead limb forward (142±59 °). Peak inspiration occurred
soon after the forelimbs left the ground while the rabbit thrust
forward with the hindlimbs (190±56 °). Expiration began
before the forelimbs touched down (337±58 °), and peak
expiration occurred during the first part of forelimb support
(29.3±63 °) (Table 1; Fig. 4). Coincident with the onset of
expiration, the liver reached its cranial-most position
(335±38 °) and then reversed its trajectory, moving caudally
during expiration. Soon after the onset of inspiration, the liver
reached its maximum caudal position (172±40 °) as the rabbit
was flexing sagittally with one forelimb swinging forward (Fig.
4). The liver then moved cranially during much of inspiration,
as the hindlimbs of the rabbit contacted the ground and pushed
it up and forwards (Fig. 4). Fig. 4 shows the relationships
between peak respiratory flow, maximal horizontal liver
displacement and body position during an average stride.
However, it was also possible for the onset and peaks of the
respiratory phases and the peak positions of the liver
8
Fig. 5. Histograms showing the
relationship
between
peak
respiratory flow and stride cycle
(360 °). For ‘coupled’ 1:1
locomotor–respiratory coupling
(LRC) running trials, the upper
panels show the timing of peak
inspiration
(A)
and
peak
expiration (B) with respect to the
stride cycle. The lower panels
show the timing of peak
inspiration
(C)
and
peak
expiration (D) for ‘uncoupled’
non-1:1 LRC running trials. 0 °
defines ground-contact of the first
forelimb.
Forelimb
support
continues
through
198±22 °.
Hindlimb support begins at the
dotted vertical line and continues
through the remainder of the
cycle. Bin width 20 °; N=sample
size. See Table 1 for mean phase
angles,
angular
standard
deviations and 95 % confidence
intervals.
Number of respiratory cycles
6
A
oscillations to occur at almost any time during the stride cycle
(Table 1; Figs 5, 6).
The cineradiographic data were divided into two groups for
further analysis: strides that show 1:1 coupling ratio and those
that show a coupling ratio that is not equal to 1:1 (Table 1).
The mean speed for the 1:1 LRC data was 13.9±1.9 km h−1
(mean ± S.D.). The mean speed for the non-1:1 LRC data was
10.6±2.4 km h−1. These speeds were significantly different
from one another (t-test, P<0.01). Furthermore, the two groups
differed in the timing of respiratory flow with respect to the
stride cycle (Fig. 5) and in the timing of liver displacement
with respect to the stride cycle (Fig. 6). During 1:1 LRC
(Fig. 5A), the relationship between the respiratory cycle and
stride cycle was less variable (more predictable) than during
non-1:1 LRC (Fig. 5B). Similarly, during 1:1 LRC, the
relationship between the liver displacement cycle and the stride
cycle (Fig. 6A) was less variable than during non-1:1 LRC
(Fig. 6B).
Comparison of LRC and non-LRC running trials also
8
1:1 LRC
Peak
inspiration
N=20
6
4
4
2
2
0
6
B
1:1 LRC
Peak
expiration
N=21
0
0 40 80 120 160 200 240 280 320 360
8
569
C
0 40 80 120 160 200 240 280 320 360
8
Non-1:1 LRC
Peak
inspiration
N=34
6
4
4
2
2
0
D
Non-1:1 LRC
Peak
expiration
N=39
0
0 40 80 120 160 200 240 280 320 360
0 40 80 120 160 200 240 280 320 360
Phase of stride cycle (degrees)
570
R. S. SIMONS
20
Fig. 6. Histograms showing the
relationship between maximum
horizontal liver displacement and
stride cycle (360 °). For ‘coupled’
1:1 locomotor–respiratory coupling
(LRC) running trials, the upper
panels show the timing of
maximum caudal displacement (A)
and maximum cranial displacement
(B) with respect to the stride cycle.
The lower panels show the timing
of maximum caudal (C) and cranial
(D) horizontal liver displacements
for ‘uncoupled’ non-1:1 LRC
running trials. For further details,
see legend for Fig. 5.
Number of liver displacement cycles
15
A
20
1:1 LRC
Max. caudal
position
N=53
15
10
10
5
5
0
15
1:1 LRC
Max. cranial
position
N=53
0
0 40 80 120 160 200 240 280 320 360
20
B
C
Non-1:1 LRC
Max. caudal
position
N=30
0 40 80 120 160 200 240 280 320 360
20
D
15
10
10
5
5
0
Non-1:1 LRC
Max. cranial
position
N=27
0
0 40 80 120 160 200 240 280 320 360
revealed that the onset of expiration as well as both peak
inspiration and peak expiration occurred earlier during non-1:1
LRC half-bounding than during 1:1 LRC half-bounding (nonoverlapping 95 % confidence intervals; Table 1). The onset of
inspiration also occurred earlier in the stride cycle during non1:1 LRC versus 1:1 LRC half-bounding (not significantly
different at P<0.05; overlapping confidence intervals;
Table 1). The mean peak caudal and cranial liver phase angles
were not significantly different during 1:1 versus non-1:1 halfbounding, although the angular standard deviation of the liver
position was almost twice as large for the non-1:1 LRC strides
as for the 1:1 LRC strides (Table 1). Even within the non-1:1
LRC data set, the phase angles of respiratory and visceral
variables were not randomly distributed (Rayleigh test for
randomness, P<0.05).
Relative timing of thoracic pitch and visceral mass oscillation
Sagittal pitching of the trunk was closely associated with
limb placement and liver motion (Fig. 7). As speed increased,
0 40 80 120 160 200 240 280 320 360
Phase of stride cycle (degrees)
no change occurred in the relative timing of pitch angle and
the forelimb support phase. Indeed, the two were tightly linked
because forelimb contact caused an increase in pitch since
forelimb contact decelerated the body of the rabbit. But the
relative timing between liver position and forelimb support (or
pitch) was variable, particularly during non-1:1 LRC running
(Table 1). At 9.8 km h−1, the liver moved caudally during
maximum pitch (=second half of the second forelimb support
phase). In contrast, at 15 km h−1, the liver moved cranially
during this same locomotor phase (Fig. 7; shaded area).
Although the onset of cranial (and dorsal) motion of the liver
was variable, the onset of caudal (and ventral) motion of the
liver was consistently associated with an increase (a reversal)
in the pitch angle of the trunk (Fig. 7; dashed vertical lines).
Variations in liver motion and pitch angle could be correlated
with speed. However, variation also occurred in the
relationship between liver position and locomotor cycle even
within a continuous running sequence at a relatively constant
speed. The data set was too small to demonstrate any
Running, breathing and visceral motion in rabbits
4
Pitch angle
(degrees)
9.8 km h-1
A
Fx
A
40
3
2
1
5
Ex
11
2
1
11
1
1 cm
1
1
1
1
2
2
22
2
1
2
2
2
2
1
2
1
22
2
22
2
Ca
0
0
500
15 km
B
Ex
0
1 cm
Cr
Ca
0
500
Time (ms)
10
12
14
Treadmill speed (km
h-1
Fx
Pitch angle
(degrees)
40
8
1000
Maximum relative liver displacement (cm)
Relative liver
motion
Cr
1
1
2
1
Relative liver
motion
571
B
2.5
2
1.5
1
0.5
0
1
1.1
1.2 1.3 1.4
1.5
Breath-to-stride ratio
1.6
2.5
C
2
1.5
1
0.5
0
0.6
statistically significant relationship between speed and relative
liver position.
The relationships between the mean peak horizontal and
vertical amplitudes of the relative liver displacement and
treadmill speed were not significantly different from zero
(ANOVA, horizontal, P=0.46; vertical, P=0.61). Nor were the
horizontal and vertical amplitudes of liver motion significantly
correlated with breath-to-stride ratio (horizontal P=0.43;
18
3
1000
Fig. 7. Liver motion (solid line) and pitch of the trunk (dotted line)
plotted for 1 s of treadmill running at a slow speed (A; 9.8 km h−1)
and at a faster (B; 15 km h−1) speed. The shaded area in A highlights
the maximum pitch angle occurring during caudal liver motion. In
contrast, the shaded area in B shows the maximum pitch occurring
during cranial motion of the liver. Regardless of speed, however, the
cranial-to-caudal turn-around of the liver is coincident with the
minimum pitch angle (vertical dashed lines). Tracings from highspeed films (top right) illustrate the angle formed by the gravitational
horizontal and the vertebral column when the rabbit is flexed (Fx)
and extended (Ex). Horizontal position is defined as 0 °. The
direction of liver oscillation is indicated by a double-ended arrow:
Ca, caudal; Cr, cranial. Black bars, first and second forelimb support.
The scale bar for liver motion represents 1 cm.
16
h -1)
0.7
0.8
0.9
1
Mean peak respiratory flow
(relative units)
1.1
Fig. 8. Mean peak relative liver displacements (horizontal, open
squares; vertical, filled circles) plotted against treadmill speed (A),
breath-to-stride ratio (B) and mean peak-to-peak respiratory flow (C)
from running trials. Bars show the standard error of the mean
(S.E.M.). The numbers 1 or 2 indicate rabbit 1 or 2. There were no
significant relationships: P values from ANOVA for (A) horizontal,
P=0.46; vertical, P=0.61; (B) horizontal, P=0.43; vertical, P=0.93;
(C) horizontal, P=0.11; vertical, P=0.36.
572
R. S. SIMONS
1.6
A
1.5
1.4
1.3
1.2
Breath-to-stride ratio
1.1
1
6
8
10
12
14
16
Treadmill speed (km h -1)
18
1.6
B
1.5
1.4
1.3
1.2
1.1
1
2.8
3.2
3.6
Stride frequency (Hz)
4.0
Fig. 9. Breath-to-stride ratio plotted against treadmill speed (A) and
stride frequency (B). Each point represents an individual running
trial of 3–10 strides. The breath-to-stride ratio was acquired using the
the pneumotachograph (filled circles) or radiodensity (open circles)
methods. The dashed line indicates the calculated value for the
preferred galloping speed (A) and for stride frequency at the
trot–gallop transition (B) based on established scaling relationships
(Heglund and Taylor, 1988). Results of ANCOVA show a significant
relationship between speed (and stride frequency) and breath-tostride ratio (P<0.001), with no significant relationship between the
individual rabbit and speed (P=0.26) and no significant interaction
between rabbit and speed (P=0.27).
vertical P=0.93) or with the mean peak-to-peak flow at the
mouth as recorded by the pneumotach (horizontal P=0.11,
vertical P=0.36) (Fig. 8).
Breath-to-stride ratio
Variation in the breath-to-stride ratio was observed for both
rabbits (Fig. 9). However, this variation decreased with
increasing treadmill speed and stride frequency (Fig. 9). As
speed increased, stride frequency also tended to increase. As
these two variables increased, rabbits tended to shift towards
a breath-to-stride ratio of 1.0 (Fig. 9).
Discussion
Three general results have emerged from this
cineradiographic study of rabbit locomotor, respiratory and
visceral interaction. First, rabbits do not show the expected,
consistent 1:1 locomotor–respiratory coupling (LRC) observed
in other mammals using asymmetrical gaits (Bramble and
Carrier, 1983; Alexander, 1989; Young et al., 1992a). Second,
rabbits show a significantly reduced variation of LRC and a
tendency towards a 1:1 LRC ratio at higher running speeds.
Similarly, the phase angle of visceral oscillation was most
predictable at faster speeds. Third, during running, in contrast
to the predictions of the visceral piston hypothesis, the general
pattern of relative motion of the liver is caudal during
expiration and cranial during the first half of inspiration. The
first two results will be discussed together and followed by a
discussion of the third result. In combination, these findings
provide a basis for understanding patterns of running and
breathing in rabbits and contribute to our understanding of
general locomotor–respiratory patterns in mammals. Three
hypotheses of visceral displacement during galloping (VP, VM
and PS; see Introduction) are also discussed and may help to
explain the observed patterns of liver motion and respiratory
flow in rabbits.
Is locomotor–respiratory coupling correlated with speed?
Previous studies of running and breathing in mammals
report consistent 1:1 locomotor–respiratory coupling during
galloping (Bramble and Carrier, 1983; Hornicke et al., 1983;
Gillespie et al., 1974; Bramble, 1989; Lafortuna et al., 1991;
Young et al., 1992a,b; Ainsworth et al., 1996) and hopping
(Alexander, 1987, 1989; Baudinette et al., 1987). Therefore,
because the rabbit uses only an asymmetrical gait, the halfbound, the expectation for locomotor–respiratory coupling is a
consistent ratio of 1:1. The results from this study, however,
show that domestic rabbits exhibit some variability in LRC,
but tend to increase the occurrence of coupled breathing and
running (with a ratio of 1:1) at higher speeds.
Further analysis of when 1:1 LRC occurs may help
elucidate why it occurs. On the basis of empirical data, a
‘physiological equivalent speed’ has been identified at the
trot–gallop transition and has been used to make comparisons
among running performances of mammals differing widely
in phylogenetic affinity and body size (Heglund et al., 1974).
Above this transition speed (i.e. during galloping), horses,
dogs, rhinoceros and gerbils all use only 1:1 coupling ratio
(Bramble and Carrier, 1983; Young et al., 1992a). Below the
trot–gallop transition speed, LRC ratios were variable
(Bramble and Carrier, 1983; Hornicke et al., 1983; Lafortuna
et al., 1991). Observations on dogs show that they
occasionally uncouple their running and breathing at the
gallop but that this uncoupling occurs only for brief periods
and is correlated with locomotor performance during the first
few minutes of treadmill running, when the animal is not
‘warmed up’ (D. Carrier and D. Bramble, personal
communication). For the rabbits in this study, however, the
episodes of uncoupled running and breathing showed no such
Running, breathing and visceral motion in rabbits
correlation with ‘warm-up’ periods. Although rabbits do not
change gaits, they may experience a similar speed threshold.
For rabbits, the stride frequency at the preferred galloping
speed (Fp) and at the predicted trot–gallop transition (Fs)
were calculated according to the general scaling equations of
Heglund and Taylor (1988):
Fs = 4.19Mb −0.15 ,
(3)
where Mb is body mass (kg). The treadmill speed associated
with Fp (determined by linear regression of stride frequency
against treadmill speed) is approximately 13.5 km h−1. This
estimated speed is indicated by the dashed line in Fig. 9A, and
the calculated stride frequency at the trot–gallop transition is
shown in Fig. 9B. Like other mammals, rabbits most
commonly used the 1:1 breath-to-stride coupling pattern at
speeds above the trot–gallop transition. Consistent with this
finding for domestic rabbits are data from jack rabbits showing
variable breath-to-stride patterns (2:1, 3:2 and 1:1) at slow
speeds but only a 1:1 ratio at faster speeds and stride
frequencies (>4 Hz) (Bramble and Carrier, 1983).
In addition to LRC ratio, the relationship between running
and breathing is also described by the phase angle between the
locomotor and respiratory cycles. Available data for horses and
dogs galloping with an LRC ratio of 1:1 show a phase angle
between the locomotor and respiratory cycles such that
expiration begins at forelimb contact and is maintained until
the forelimbs leave the ground; inspiration then begins before
the hindlimbs contact the ground and continues through
hindlimb support (Bramble and Carrier, 1983; Bramble, 1989;
Young et al., 1992a,b). The results from the preseent study
show that rabbits, when running and breathing at an LRC ratio
of 1:1, generally demonstrated a similar phase relationship
between breathing and gait cycles. This relationship was also
most commonly observed at higher speeds. However, rabbits
also varied the phase angle between the phase variables slightly
while still maintaining a coupling ratio of 1:1. This ‘flexibility’
of both phase angle and LRC may reflect the relatively less
cursorial nature of the domestic rabbit compared with other
mammals so far examined and is discussed below.
Liver displacement in rabbits
During post-exercise standing panting (3–6 Hz) in rabbits,
relative liver motion coincided with breathing cycles as
expected such that the liver moved caudally during inspiration
and cranially during expiration (Fig. 10A). This relationship
between liver motion and respiratory cycle was also observed
during quiet breathing (2 Hz). During locomotion, the
relationship between liver motion and the respiratory cycle
appeared to be more complex and variable presumably because
muscular forces (e.g. costal and crural diaphragm, rectus
abdominus, obliques) as well as locomotor-induced
accelerational forces are acting on the visceral mass.
Two general patterns of liver motion in the rabbit during the
locomotor cycle are evident (Fig. 7) and are represented
Cranial
Inspiration
A
B
Relative liver motion
(2)
Caudal
C
D
E
F
3
Forelimb
support
G
Cr, Do
Sternum
Fp = 4.44Mb−0.16 ,
Respiratory
flow
Expiration
573
Ca, Ve
H
4
1
2
Hindlimb
support
Horizontal
relative sternum
displacement
Vertical
relative sternum
displacement
Fig. 10. Generalized schematic model showing respiratory flow
(shaded), horizontal liver motion (A,E,F), limb support (horizontal
black bars) and sternal motion (G,H) from rabbits in this study (solid
lines). Relative horizontal liver motion is shown for rabbits standing
(A), using slow to moderate half-bounding (E) and using fast halfbounding (F). The four phases (1–4) are discussed in the text.
Predicted liver displacements (dashed lines) are shown for the
visceral piston model (B; Bramble and Carrier, 1983; Bramble,
1989), for the vibration mechanics model for galloping horses (C;
Young et al., 1992a) and for the pneumatic stabilization model (D;
Simons, 1996). Phase 1 for B and C has been interpolated. Ca,
caudal; Cr, cranial; Do, dorsal; Ve, ventral.
schematically in Fig. 10E,F. Although some variation exists,
the motion of the liver at slower speeds may be characterized
by Fig. 10E and at faster speeds by Fig. 10F. For all speeds,
the first part of inspiration (phase 1) occurs while the second
forelimb is still in contact with the ground. At slow to moderate
speeds, during phase 1, the direction of liver motion is caudal
(Fig. 10E). The second part of inspiration (phase 2) is achieved
574
R. S. SIMONS
while both forelimbs reach cranially (and presumably the ribs
rotate), thereby expanding the anterior thorax and enabling
inspiration. During this phase, the liver moves cranially
(Fig. 10E), possibly due to abdominal muscle contraction (e.g.
external obliques, as observed in galloping dogs; Tokuriki,
1974), while the hindlimbs push against the ground. The liver
may also be drawn or ‘sucked’ cranially by the negative
pressure of inspiration due to ‘bucket handle’ rotation of the
ribs and the craniodorsal motion of the sternum (Fig. 10G,H).
Expiration begins (phase 3) with forelimb support, and the
sternum is driven posteroventrally upon landing (Fig. 10). The
liver moves caudally and ventrally during this phase. The liver
may be pushed caudally by the positive pressures in the thorax
generated upon landing (Simons, 1996). Expiration continues
(phase 4) as the hindlimbs are drawn up underneath the body
and the rabbit is supported by both forelimbs. At this point,
during slow-speed running, the liver continues to move
caudally (Fig. 10E).
At faster speeds, relative liver position varies somewhat
from the relative positions observed at slower speeds. During
phase 1, liver displacement may be cranial (Fig. 10F), possibly
because of the pronounced sagittal flexion of the body
associated with the extended crossed flight of higher speeds.
During the last third of inspiration (end of phase 2), the liver
moves caudally, possibly driven by diaphragmatic contraction
or by inertia as predicted by both the VP and VM models.
During the second half of expiration (phase 4), the liver is
pushed forward (Fig. 10F), possibly as a result of sagittal
flexion of the trunk. The relative liver motion in the halfbounding rabbit is in general agreement with preliminary data
for the liver motion of small, galloping dogs (Bramble et al.,
1994).
Visceral piston and vibration mechanics
The visceral piston (VP) hypothesis (Bramble and Carrier,
1983; Bramble, 1989) predicts that the visceral mass,
represented by the liver, should move rearward within the body
as the animal pushes upwards and forwards from the
hindlimbs, and that the liver should move forward as the
animal decelerates upon landing on its forelimbs (Fig. 10B).
However, the results from this study showed that no single
pattern of LRC or liver motion characterizes all rabbit running
but, instead, variation was possible. This variability is
inconsistent with the expectation that visceral motions in
galloping mammals should be consistently correlated with
inspiration and expiration (Bramble and Carrier, 1983;
Bramble, 1989). Moreover, the direction of liver motion during
an average rabbit stride does not generally meet the expectation
of the visceral piston model. At slow speeds, the motion of the
rabbit liver is almost directly out of phase with the predictions
for the visceral piston. At faster speeds, however, VP
predictions are met during the second half of expiration (phase
4) as cranial motion of the liver may contribute to expiratory
flow and caudal liver motion during the last part of inspiration
may contribute to inspiratory flow.
A theoretical model based on vibration mechanics (VM;
Young et al., 1992a) predicts that inertially driven motions of
the liver in a galloping horse would result in cranial motion
during inspiration and caudal motion during expiration, almost
180 ° out of phase with VP predictions (Fig. 10C). For most
fast running sequences, liver motion in the rabbit is partially
represented by the vibration mechanics model. The VM model
may more accurately reflect visceral motion at higher speeds
because it assumes elastic properties of the trunk and visceral
mass. Such elastic behavior is more likely to occur at faster
running speeds at which many mammals, including rabbits,
show increased body stiffness (Farley et al., 1993; Simons,
1997). If inertial motion drives the liver oscillations as
predicted for the horse by Young et al. (1992a), then where
rabbit liver motion departs from the prediction, forces other
than inertia, such as muscular action, may be at work. For
example, diaphragmatic contraction during phase 1, or sagittal
flexion of the trunk during phase 4, may be responsible for the
liver motion that diverges from the inertial motion prediction.
Mechanical drivers of the respiratory cycle
Even during 1:1 LRC, where expiration coincides with
forelimb support and inspiration is coincident with hindlimb
thrust, liver motion in rabbits cannot be responsible for driving
the respiratory cycles entirely as predicted by the VP
hypothesis because the liver moves cranially during much of
inspiration and caudally during much of expiration. Instead,
other drivers, such as rib rotation, sternal motion and
diaphragmatic contraction, which have not been investigated
here, must be primarily responsible for ventilation during
locomotion.
Although no rib-motion data were collected and only limited
analyses of horizontal and vertical displacements of the
sternum were performed, some speculation is possible
regarding the influence of rib, sternum and forelimb
interactions on ventilation during galloping. Inspiration
coincides with the forward swing of the forelimbs despite the
cranial position of the liver. The first part of inspiration may
be driven by possible ‘bucket-handle’ rotation of the ribs as the
sternum remains in a ventral position (Fig. 10H), resulting in
rapid inspiratory flow while the second forelimb remains in
contact with the ground. Momentarily, as the rabbit ‘breaks
over’ this supporting limb, inspiratory air flow may be reduced
(Fig. 10) as the ribs, sternum and forelimb must provide some
rigidity to support the body weight. Inspiration continues as
both forelimbs reach up and forward and the sternum swings
cranially (Fig. 10G), possibly expanding the volume of the
anterior thoracic cavity due to the ‘pump-handle’ action of the
sternum. During exhalation, the sternal and rib motions,
combined with the locomotor-induced loading forces on the
forelimbs, may enable expiration to occur while the viscera
move caudally. As the thorax comes under load upon landing,
the sternum is driven rearward and the ribs may rotate medially
(reverse ‘bucket handle’), decreasing the total volume of the
anterior thorax and aiding expiration. The idea that sternal
motion may have an important role in ventilation, particularly
during inspiration, is consistent with the observations of
Running, breathing and visceral motion in rabbits
Carrier (1996) that, in trotting dogs, parasternal muscle activity
is associated with inspiration, during both coupled and
uncoupled running and breathing.
Diaphragm contraction may drive the last third of inspiration
during fast galloping. However, during 1:1 LRC, particularly
at slower speeds, forward motion of the visceral mass during
much of inspiration suggests that the diaphragm is not
shortening at this time. A pattern of craniodorsal diaphragm
motion during part of inspiration was observed in trotting dogs
(Bramble and Jenkins, 1993) and may be somewhat analogous
to the situation in rabbits. It is possible that a complex
diaphragmatic shape change (Bramble and Jenkins, 1993)
could result in inspiratory flow while the liver moves forward.
Clearly, direct measurements of the electrical activity and
length changes of the costal and crural diaphragm during
galloping are needed.
Pneumatic stabilization
Although liver motion in rabbits cannot drive ventilation as
predicted by the VP model, the more fundamental idea that
visceral motion may play an important role in
locomotor–respiratory mechanics (Bramble and Carrier, 1983;
Bramble, 1989) is valuable in understanding the integration of
running and breathing. The position of the visceral mass during
locomotion may be related to another aspect of locomotor
mechanics: thoracic stabilization. The relationship between
liver position and thoracic stabilization is explained by a
hypothesis of pneumatic stabilization (Simons, 1996). The
pneumatic stabilization (PS) hypothesis proposes that, if the
breath-to-stride ratio is 1:1 and the phase angle is ‘correct’
(expiration occurring during the forelimb support stage), then
the inflated lungs may play a role in load transfer between the
forelimbs, anterior thorax and visceral mass and/or in
stabilization of the chest walls, heart, anterior thorax. The PS
hypothesis requires that the liver position be cranial (forward)
just before forelimb support so that, upon forelimb contact with
the ground, the thoracic compartment will become positively
pressurized as it is compressed simultaneously by the liver and
by the forelimbs (Fig. 11).
Conditions for pneumatic stabilization include a
locomotor–respiratory coupling ratio of 1:1, a consistent phase
angle between locomotor and respiratory cycles and the cranial
position of the liver before forelimb contact, and a subsequent
positive pressure in thoracic compartment upon landing. For
rabbits, these conditions were only met during faster running.
For rabbits (and possibly other small-bodied mammals), PS
may only be biologically relevant at higher speeds when the
loading forces are large. Moreover, PS may only be possible
when stride length and extension of the trunk are maximal at
high speeds (Simons, 1997), enabling a large volume of air to
be inspired and subsequently used to stabilize the thorax during
the next forelimb support phase.
Cursoriality and consistent locomotor–respiratory coupling
Cursoriality and the rigidity of locomotor–respiratory
coupling appear to be positively correlated among those
575
2
++
3
1
Fig. 11. Schematic representation of the pneumatic stabilization
model (Simons, 1996). The tracing shows a running rabbit during the
forelimb support phase. Ground reaction forces pass through the
limbs to the chest walls and underlying lungs (1). Simultaneously,
liver position is maximally cranial (2) as the hindlimbs swing
forward under the rabbit (3). As a result, positive pressure is likely to
be generated in the thoracic cavity (++).
species so far examined. Horses, dogs and kangaroos, all
specialized for running, consistently couple running and
breathing at a ratio of 1:1 while employing asymmetrical gaits.
In contrast, rabbits, which are only moderate cursors, exhibit
LRC variability at all speeds but tend towards 1:1 LRC at faster
speeds. Pigs, also moderate cursors, show a lesser degree of
coupling during galloping than observed in dogs and horses (F.
Jenkins,
personal
communication).
An
infrequent
locomotor–respiratory coupling is predicted for non-cursors
such as skunks, armadillos or other mammals that are
specialized for non-running behavior. Gerbils, although they
are not a particularly cursorial species, show 1:1 coupling
during galloping (Young et al., 1992b). This finding does not
contradict the hypothesis that cursoriality and LRC are
positively correlated because, like rabbits, other mammals may
well be capable of LRC at any speed. The important
information for refuting or supporting this prediction is the
consistency of coupling. For example, this hypothesis could be
falsified if the asymmetrical gaits of a non-cursorial mammal
were consistently associated with an LRC ratio of 1:1.
From an evolutionary perspective, efficient mechanical and
neurological integration between the respiratory and
locomotory systems is expected in mammals specialized for
sustained aerobic exercise (Bramble and Carrier, 1983;
Bramble, 1986; Bramble and Jenkins, 1989; Johnson et al.,
1993). If consistent locomotor–respiratory coupling facilitates
energetically efficient locomotion, then LRC may be
associated with sustained, aerobic exercise and, therefore, be
more commonly observed in cursorial species. Conversely,
species that are specialized for anaerobic locomotion may not
elicit (or require) LRC. Rabbits are specialized for sprinting
and less specialized for sustained running, particularly in their
thoracic morphology (Simons, 1996). However, LRC does
occur at the highest speeds and stride frequencies, suggesting
that, under increased locomotor-induced loading regimes (i.e.
increased gravitational loads associated with increased
speeds), rabbits may benefit from 1:1 LRC. An approximately
1:1 LRC may be the primitive condition for mammals and
precise sustained 1:1 coupling may be a derived condition,
576
R. S. SIMONS
possibly associated with locomotor efficiency, for highly
cursorial species. The constancy of 1:1 LRC may also reflect
body size. Smaller-bodied mammals may be less constrained
to a 1:1 synchronization between gait and breathing cycles
since loading forces during locomotion for smaller animals are
lower.
In summary, rabbits show variable phase angles between
liver displacement and locomotor cycles as well as
considerable variability in locomotor–respiratory coupling
ratios. Relative liver displacements in running rabbits do not
generally support the predictions of the visceral piston
hypothesis (Bramble and Carrier, 1983; Bramble, 1989).
However, the observed phase angles of the liver position
combined with the tendency towards 1:1 LRC at higher speeds
generally support the pneumatic stabilization (Simons, 1996)
and vibration mechanics (Young et al., 1992) hypotheses. The
pneumatic stabilization hypothesis allows for simultaneous
rearward motion of the visceral mass and visceral-assisted
expiration. The motion of the sternum and forelimbs, together
with rotation of the ribs, may be important in driving
ventilation during locomotion. For rabbits, as well as other
mammalian
species,
the
occurrence
of
1:1
locomotor–respiratory coupling increases with speed of
running. Among mammals generally, increased consistency of
1:1 LRC is observed in more cursorial species. The variation
between locomotor and respiratory cycles reported here for
rabbits may be rare for more cursorial mammals that depend
on a high degree of running efficiency for survival because
rigidity of coupling may be associated with increased
locomotor or respiratory efficiency.
I thank D. Bramble for supporting this work, D. Bramble
and D. Carrier for valuable discussions and comments on the
manuscript, and E. Brainerd, N. Kley, S. Emerson, R. Meyers
and two anonymous reviewers for comments on the
manuscript. F. Jenkins Jr generously made available
cineradiographic equipment. F. Jenkins Jr. A. Glazer, J.
Maughan and A. Purgue provided assistance with the rabbit
treadmill experiments. This research was supported by NSF
Dissertation Improvement Grant IBN-9321638 to R. Simons
and D. Bramble and NSF IBN-9318610 to D. Bramble and F.
Jenkins Jr.
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