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1 Department of Anatomy and Physiology and 2 Department of Clinical Sciences, Kansas State University, Manhattan, Kansas 66506
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Thoroughbred (TB) and draft horses (DH) have
long been selected for tasks of very different intensities and
force-speed relationships. To study their adaptations, we measured
O2 consumption and related variables in three TB and four DH during progressive exercise tests on
a level treadmill. The horses exerted a draft force of 0, 5, 10, 15, or
20% of their body weight at speeds that increased by 2 m/s every 3 min
until they could not maintain that speed. We found that TB could exert
the same draft forces as DH and, at each force, TB achieved about twice
the speed, twice the external power, and twice the
O2 consumption as DH; thus the two
breeds had the same gross efficiencies. We also found maximal
O2 consumption of TB to be about
twice that of DH (134 vs. 72 ml · kg
1 · min1,
respectively), suggesting adaptations to high-intensity exercise. Peak
efficiency was reached at lower speeds in DH than in TB, suggesting
adaptations to high-force, low-speed exercise. These differences
between TB and DH in force-speed and aerobic capacities and in speed
for peak efficiency likely reflect different contraction velocities in
locomotor muscles.
equine; exercise; force-speed relationships; external power; gross efficiency
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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SELECTIVE BREEDING of animals can elicit highly specialized characteristics. This practice provides an opportunity to study adaptive capacities, including the nature, rate, and extent of the structural and physiological changes that can be achieved. We thought that comparison within a species of breeds selected for different kinds of exercise might be a useful approach to study their adaptations.
Thoroughbred (TB) and draft horses (DH) have long been bred for different tasks. TB have been selected for bursts of near-maximal exercise at high speed with low external forces, whereas DH have been selected for long-sustained submaximal exercise at low speeds against high external forces. These tasks differ in two basic ways: they involve very different exercise intensities, and they lie near opposite ends of what can be thought of as external force-speed curves analogous to muscle force-velocity curves (14).
To the extent that TB and DH have adapted to these different activities, we can expect differences in their structure and function; that is, they can serve as an "allometric pair" within a single species. Regarding the musculoskeletal system, we know that TB can run faster than DH, and we expected DH to be able to exert greater draft forces (expressed as a fraction of body weight). We also suspected that maximal power and gross efficiency, as well as the force-speed combinations at which they are reached, might differ between these breeds. We expected TB to have greater aerobic capacity than DH, with associated differences in their gas exchange and transport systems.
To test these expectations, we measured
O2 consumption
(
O2) and related variables
and calculated external power and gross efficiency in TB and DH during
progressive treadmill exercise in a series of experiments covering the
widest feasible matrix of speed and draft force. We also measured
maximal O2 consumption (O2 max) of the two
breeds, the first such measurements in DH as far as we know. The
experimental protocol and procedures were approved by the Animal Care
and Use Committee of Kansas State University.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Animals.
We studied four healthy DH geldings (ranging in age from 4 to 13 yr;
two Percherons, averaging 805 ± 16 kg body wt; and two Belgians,
averaging 750 ± 7 kg in body wt) and three TB (ranging in age from
6 to 7 yr and averaging 553 ± 19 kg in body wt). (Note: we refer to Belgians and Percherons as a single "breed," i.e., DH). The Belgians were allowed to canter, but, at the owner's request,
the Percherons were not. The horses were kept in good nutritional
status on suitable pasture or in pens with shelters and were weighed
weekly; they gained an average of 16 kg during the course of the
experiment. They were given routine hoof and medical care. For
conditioning and learning of the new tasks required for the study, TB
worked in harness and DH exercised on the treadmill twice weekly for 5 mo. Then, for 3 mo before the experiments, both groups practiced the
planned experimental protocols at least twice weekly on the treadmill,
covering the full range of force and speed combinations. During this
time, preliminary measurements were made to determine appropriate bias
flow rates through the O2-measurement system (Fig.
1). This period also served to further habituate the horses to the experimental tasks and conditions and to
establish similar states of fitness in the two groups. To maintain this
state, all horses were exercised at least weekly throughout the
remainder of the study, either in experiments or in training sessions.
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Exercise system. The horses exercised on a level treadmill (SATO, Uppsala, Sweden) in an air-conditioned laboratory equipped with cooling fans; they wore a safety harness attached to an overhead treadmill cutoff switch. They also wore suitable collars and harness attached to a cable running over pulleys to support a basket that was free to move vertically in a steel frame behind the treadmill (Fig. 1). By keeping the basket (and any added weight) suspended, horses could exert a known constant draft force while running at selected speeds. Horses lifted the basket by moving forward on the treadmill; an exercise bout ended when, despite urging, they no longer maintained that forward position.
Protocol. The experiments covered the widest feasible matrix of force and speed. For an experimental run, a draft force was selected equivalent to 0, 5, 10, 15, or 20% of the animal's body weight. Two sets of measurements were then made with the horse standing quietly on the treadmill: first, the horse exerted no force, and then, having moved to the front of the treadmill, it maintained the selected force for 3 min. The horse then exerted the selected force at speeds that increased by 2 m/s every 3 min until it no longer maintained position on the treadmill. Thus each individual's peak speed at a given force was determined as the highest speed sustained for 3 min at that force. To obtain measurements during steady-state conditions of gas exchange, we collected data during the third minute at each speed (20). Postexercise measurements were taken 1, 3, 5, and 10 min after the treadmill was stopped and the weights were released.
Gas exchange measurements.
O2 and
CO2 production
(CO2) were
measured with an open-flow system (Fig. 1). Pumps drew a bias flow of
room air through the loosely fitting face mask and thence through the
system, at flow rates that varied with exercise intensity (5-167
l/s for TB, 6-123 l/s for DH) and were selected as follows. In
preliminary experiments, we measured
O2 of some horses in the
experimental protocol, estimated total ventilation (assuming a
ventilatory equivalent for O2 of
25) (15), and then estimated peak expiratory flow rates, assuming
sinusoidal flow. Bias flows were set at about twice the estimated peak
flow to ensure complete collection of expired gas, which was verified
by the absence of CO2 in gas
continuously sampled anywhere at the inlet of the face mask with a
respiratory mass spectrometer (model 1100, Perkin-Elmer, Pomona, CA).
Expired gas thus collected was led via conduits (15 cm diameter,
totaling 7.5 m in length) through a baffled mixing chamber consisting
of one 200-liter chamber for measurements in standing animals or six
such chambers connected in parallel for measurements during exercise.
Adequate gas mixing was observed at all speeds, as judged by the
absence of fluctuations in the composition of mixed expired gas
(O2 and
CO2 fractions). The compositions
of room air and mixed expired gas were measured with the respiratory
mass spectrometer, which was calibrated before and after each
experiment with gases of known composition provided by precision
gas-mixing pumps (model 301a-F, Woesthoff, Bochum, Germany).
Temperature and relative humidity of the mixed expired gas were
determined with a digital thermohygrometer (model 880 with model 882 sensor, General Eastern Instruments, Watertown, MA) inserted in the
system near the gas-sampling site. Total gas flow was calculated from
the pressure drop across Fleisch-type pneumotachometers
("flow-straightener" elements, Meriam Instruments, Cleveland, OH;
15 cm diameter for preexercise and 25 cm diameter for exercise
measurements), measured with a suitable differential pressure
transducer (model MP-45, Validyne, Northridge, CA). The pressure
transducer was calibrated before and after each experiment with a
precision water manometer (model A7A micromanometer, Meriam
Instruments). The flow-measuring system was calibrated by using a
nitrogen dilution technique (10).
Plasma La.
Blood samples were taken during the last minute of exercise at each
speed from a catheter placed in the pulmonary artery, and they were
stored on ice in tubes containing EDTA. Within 10 min after each
experiment, the plasma was removed and frozen. Plasma
La concentration,
[La], was
measured within 14 days by using an automated lactate analyzer (model
23L, Yellow Springs Instruments, Yellow Springs, OH).
Stride frequency. Stride frequency was measured from a record of footfall-related treadmill vibrations sensed by a pressure transducer (P23Db, Gould, Cleveland, OH) clamped to the treadmill frame. Footfall and gait also were noted manually.
Data recording and processing. Footfall signals were amplified (model 8811A, Hewlett-Packard, Waltham, MA) and recorded on a multichannel pen recorder (model 481, Gould). Data obtained from the mass spectrometer and pressure signals from the pneumotachometer were recorded and processed on-line with a computer (MTech, Microtech, Lawrence, KS) and a commercial software package (CODAS 5.51, DATAQ, Akron, OH).
Calculations.
Stride length was determined as the ratio of treadmill speed to stride
frequency. O2 and
CO2
(STPD) were calculated from inspired
and mixed expired O2 and
CO2 fractions and total gas flow,
after appropriate corrections for temperature, barometric pressure, and
water vapor pressure (26, 27), and divided by body weight to obtain the
mass-specific values used here. Respiratory exchange ratio was
calculated as the
CO2-to-O2
ratio. Mass-specific external power (
) was
calculated as (force · speed)/body weight. Gross
efficiency was then derived as the
-to-O2
ratio by using appropriate transformations and a caloric equivalent for
O2 of 5.0397 kcal/l. Because
[La] increases linearly
as a function of time during constant high-intensity exercise (after a
delay of up to 30 s) (2, 21),
La accumulation rate
(d[La]/dt)
was calculated as the difference in
[La] between
two speeds divided by the time interval (33).
O2 max.
O2 max was determined
in separate experiments. After a 3-min warm-up at 3 m/s, horses exerted
a force equal to 10% of their body weight in a series of exercise
bouts lasting 3 min or less at each of the following speeds: 4, 5, 5.5, or 6 m/s for DH and 8, 9, 10, or 11 m/s for TB. At this draft force, DH
could not complete 3 min at 5.5 and 6 m/s and TB could not complete 3 min at 10 and 11 m/s. To see if steady-state conditions of gas exchange were reached, O2 was
measured at 30, 75, 120, and 165 s during each exercise bout.
[La] was
measured in jugular venous blood sampled from a 14-gauge intravenous
catheter (Delmed A-cath, Dupond, Canton, MA) before exercise, just
before cessation of exercise at the designated speed, and at timed
intervals after the exercise ceased.
Statistical analyses. A repeated-measures analysis of variance was performed by using commercial software (SAS, SAS Institute, Cary, NC), partitioning between breeds, horses within breeds, force, and the interaction between breed and force. Two-tailed tests of a multiple-comparison method (general linear model and procedure of least square means) were also used to compare results.
Two within-breed comparisons were made. 1) At a given force, d[La]/dt
as a function of speed was compared to see whether a significant rise
occurred; and 2) between forces,
peak O2
(O2 peak),
[La],
d[La]/dt,
, gross efficiency, and regression lines of stride
frequency and O2 as
functions of speed were compared to see whether they varied with force.
Two between-breed comparisons also were made. 1) At a given force, peak
, gross efficiency,
[La],
d[La]/dt,
and regression lines of stride frequency and
O2 as functions of speed
were compared to see whether they varied with breed; and
2) preexercise
O2 and
O2 peak, averaged for
all force-speed relationships, were compared to see whether they
differed between breeds. Means that differed with a
P 
0.05 were considered to be
significantly different.
For analysis of
O2 max, we used the
highest O2 observed and the
[La] obtained
just before cessation of exercise at each speed. The highest
O2 values at each speed
were averaged for all horses within a breed; differences among speeds
were tested with Fisher's least significant difference test (SAS);
means differing with P 0.05 were
considered significantly different.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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O2.
At each force, O2 increased
linearly with speed (Fig. 2), the slope of
the relationship being greater for higher forces. The rate of change of
O2 with speed at the various
draft forces did not differ between TB and DH, except at a force of
15% of body weight. TB reached the same
O2 peak regardless of
the force-speed combination used, and DH nearly did so (Table
1).
O2 peak for TB was
twice that of DH (Fig. 3), with the average
O2 peak over all draft
forces being 117 ± 3 and 59 ± 4 ml · min1 · kg1,
respectively.
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O2 max of TB was about
twice that of DH (135 ± 8 ml · min1 · kg1
at 10 m/s vs. 72 ± 3 ml · min1 · kg1
at 5.5 m/s, respectively; Fig. 4, Table
2). Acceptable plateaus of
O2 were found at 5-5.5
m/s in DH and at 9-10 m/s in TB (Fig. 4), except for one animal.
Horses could maintain the highest of our test speeds for only
45-120 s. Peak
d[La]/dt
and respiratory exchange ratios were high in both TB and DH (Table 2).
Heart rates were also high (Table 2), reaching the plateau values
reported by Birks et al. (2) in horses exercising at
O2 max. By these
criteria and the observed plateaus of
O2, we believe the horses had
achieved O2 max.
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La.
At any given force, TB reached a higher speed than DH before
[La] began to
rise. With increasing force, the rise in
[La] began at
lower speeds in both breeds (Fig. 2). TB reached the same peak
d[La]/dt
regardless of the force-speed combination used, and DH nearly did so
(Table 1). At the peak speed, neither
d[La]/dt
(except at a draft force equal to 20% of body wt) nor
[La] differed
between TB and DH at any force (Table 1).
Force, speed, and power. Draft forces ranged up to 20% of the animals' body weight for both TB and DH. As speed increased, maximum sustainable force decreased (connected data points in Fig. 5). At each force, including 0% body weight, TB reached peak speeds about twice those of DH; thus the force-speed curves converge as force increases, and the force intercepts may not differ.
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(max) of TB
was about twice that of DH, 7.9 vs. 4 W/kg (or 10.7 and
5.44 · 103
horsepower · kg1),
respectively (isopleths in Fig. 5).
max was achieved
at several force-speed combinations (plateaus in Fig.
6). At
max, force was
10-20% of body weight in both breeds, with TB achieving twice the
max at twice the
speed of DH. Data from Procter et al. (28) suggest that at very
low speeds and high forces,
max of DH
diminishes (Fig. 6); indeed, for both breeds must
approach zero as speed approaches zero.
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Gross efficiency.
The differences in external power achieved by TB and DH were matched by
differences in O2,
so both breeds reached maximal gross efficiencies of ~20% (Fig.
7). For TB, external power and gross
efficiency both plateaued over the same range of force-speed combinations (Figs. 6 and 7). For DH, we found no plateau of gross efficiency; instead, it reached a maximum at the lowest speed our
treadmill permitted, in good agreement with data from Procter et al.
(28) (Fig. 7).
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Stride length, stride frequency, and gait. Our protocol did not establish the exact speeds at which gait changes occurred. The walk-trot transition was ~2 m/s in both breeds. The transition from trot to canter (gallop) was between 4 and 6 m/s for TB and between 6 and 8 m/s in DH, compared with allometric predictions of 5.9 and 6.4 m/s for TB and DH, respectively (13). Changing draft force did not cause a change in gait at any speed.
Stride patterns varied with speed in known ways (6) that did not differ between breeds: both TB and DH tended to increase speed by increasing stride length more than stride frequency (Fig. 8). Stride frequency appeared to be greater in TB than in DH during the canter (for example, mean stride frequency of 1.84 and 1.68 Hz, respectively, at 8 m/s with no draft force). At a given speed, increasing force was associated with increased stride frequency and decreased stride length, as observed by others (19). At 2-6 m/s, slopes of stride frequency as a function of speed differed between breeds only at a draft force equal to 10% of body weight. Thus the locomotor (stride) patterns for increasing speed at no draft force and when changing force at constant speed were similar in TB and DH.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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We first discuss major findings under three headings: aerobic capacities, force-speed relationships, and power and efficiency.
Aerobic capacities.
Mean O2 max of 134 ml · kg1 · min1
in our TB can be compared with reported values ranging from
129 and 163 ml · kg1 · min1
for detrained and conditioned TB, respectively, to an extreme of 198 ml · kg1 · min1
(7-9, 16). Mean
O2 max for our DH was
72 ml · min1 · kg1.
We believe these are the first measurements of
O2 max in DH. Procter
et al. (28) reported O2
up to 43 ml · min1 · kg1
in one DH. Using Nadaljak's (25) data, we estimate that his horses may
have reached a O2 of ~84
ml · kg1 · min1
during near-maximal draft work (speed = 1.6 m/s, force = 32% body wt);
however, his group of horses included non-draft-type animals.
O2 than are DH.
We think the twofold difference reflects adaptation of TB to higher
exercise intensities, presumably influenced by task-specific selective
breeding and genetic differences rather than conditioning. Our TB and
DH were conditioned similarly, and, furthermore, training increases
O2 max of TB
by a factor of only ~1.2-1.3 (7).
Genetic differences underlying the different aerobic capacities of TB
and DH might exist in their gas exchange and transport systems (not
addressed in this report) and in their
O2 sinks, i.e., muscles,
which account for most of the
O2 during heavy exercise
(38). Muscle differences of several kinds may help explain the greater
mass-specific aerobic capacity of TB. Muscle constitutes 53% of the
body weight in adult TB but only 44% in DH foals and, perhaps, in
adult DH (12). Also, during peak exercise in our tests, TB and DH may
have used their muscles differently; for example, TB may have recruited
a greater fraction of their muscle mass. We think it likely that
intrinsic muscle properties (force-velocity characteristics) also
differ between the two breeds and underlie their differing aerobic
capacities. That is, locomotor muscles of TB may achieve higher
contraction velocities at similar forces, which are associated with
higher rates of ATP hydrolysis and higher
O2. We would expect
associated differences in muscle mitochondrial densities.
We found no differences in these experiments between breeds in the peak
rates or extents of rise in plasma
[La],
suggesting similar dependence on anaerobic metabolism. However, our
study was not designed to see whether TB and DH had different capacities or tolerances for anaerobic metabolism, and we do not know
whether DH ever reach the extraordinarily high
[La] (35 mmol/l) reported for TB during maximal exercise (34).
Aerobic capacities of both breeds exceeded those predicted by
allometric equations for mammals their size (37); that of TB is 3.1 times and that of DH 1.8 times the predicted mass-specific value. Even
the latter departure is greater than that of elite human athletes who
achieve a O2 max of
~80
ml · kg1 · min1
(31). Aerobic capacities of ancestral equids may have been even greater
than those of modern DH; that is, perhaps selective breeding for
long-sustained submaximal exercise tasks has actually reduced the
aerobic capacity of DH below that of ancestral equids.
Such breed differences in aerobic capacity exist within at least one
other species. Greyhounds can reach
O2 of 182 ml · min1 · kg1
(R. L. Pieschl, personal communication), 1.2-1.6 times the
O2 max of mongrel dogs
of similar weight and of unconditioned and conditioned foxhounds (113 and 146 ml · min1 · kg1,
respectively) (24). We know of no such great differences in aerobic
capacity among breeds of mammals within a single species except where
selective breeding for high exercise capacity by humans has been a
major influence. Thus we suggest that the differences are results of
the selective breeding and that they, and the anatomic and
physiological differences that support them, reflect the nature, rate,
and extent of the adaptations so achieved.
Force-speed relationships. We expected, and found, that TB could run faster than DH. We were surprised, however, to find that they could do so while exerting the same range of draft forces as DH. We were able to study draft forces only up to 20% of body weight; this limit was set in DH because our treadmill did not run at the necessary low speeds (<2 m/s) and in TB because they became agitated if asked to exert greater force. DH are capable of exerting draft forces of 100% of their body weight at very low speeds (5); we do not know whether TB could do the same. The force-speed curves of the two breeds appear to converge as force increases, so perhaps their force intercepts are not very different.
These whole animal, maximal external force-speed curves (Fig. 5) for TB and DH resemble maximal force-velocity curves of fast and slow skeletal muscles having different maximal contraction velocities and similar maximal isometric tensions (4, 39). Musculoskeletal adaptations of TB and DH to exercise with different force-speed requirements may consist largely of differences in the intrinsic properties of their locomotor muscles, although gross anatomic differences yielding different mechanical advantages (18), or different muscle masses or recruitment patterns, also may exist in the two breeds.Power and efficiency.
We mention three salient points here. First,
max of TB was
twice that of DH (Fig. 5). Second, the associated
O2 was also about twice as
great in TB, so maximum gross efficiencies were similar in the two
breeds, reaching ~20% (Fig. 7) and approaching the maximum of
~25% that is achievable by mammals. Third, maximum power and
efficiency were reached at lower speeds in DH than in TB (Figs. 6
and 7). Again, these differences could arise from different
intrinsic properties of the skeletal muscles in these breeds. In
muscles with similar maximal isometric tensions, those with lower
maximum contraction velocities necessarily have lower maximum power and
reach it at lower speeds. Furthermore, maximum efficiencies will tend
to occur at lower speeds in the slower muscle.
O2 might be seen in at least some of the force-speed combinations we tested. We found,
however, that the O2 cost of
locomotion (i.e., the slope of mass-specific O2 as a function of speed)
(32) was not different for the two breeds (Fig. 3). In addition, stride
frequencies were the same in the two breeds during trot, with draft
forces of 0 and 5% of body weight. Taken together, these similarities
suggest similar elastic energy storage and recovery in the two breeds under these circumstances.
On the basis of their body weights and an assumption of elastic
similarity (22) in TB and DH, one would predict that stride frequencies
of TB during the gallop would be 5% greater than those of DH; instead,
we found a difference of 9.5% at 8 m/s. This discrepancy is not
surprising; there is little reason to suggest that the breeds are
elastically similar, and presumably the lighter structures of TB have
higher natural frequencies than those of DH, allowing higher stride
frequencies (36) and constituting an adaptation for higher running
speeds.
In summary, we found that TB can run about twice as fast as DH while
exerting the same range of draft forces and achieving about twice the
O2 (and
O2 max) and external
power (all scaled to body weight), with the same maximum gross
efficiencies in draft work, and that maximum power and efficiency are
reached at lower speeds in DH than in TB.
We suggest that these differences are largely explainable by different
force-velocity characteristics in the locomotor muscles of the two
breeds, attributable to genetic differences resulting from
task-specific selective breeding, in TB for high-intensity exercise in
high-speed tasks with low external forces and in DH for low-intensity
exercise in low-speed tasks with high external forces. Associated
adaptations of gas-exchange and -transport mechanisms can be expected.
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ACKNOWLEDGEMENTS |
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We are very much indebted to B. Burnett Registered Percherons for lending us draft horses; to E. Szczurek-Raub, J. Peterson, and L. Rall for superb technical help; to volunteer students, particularly P. Hickman, for help with the experiments; and to Dr. D. E. Johnson for advice on statistical analysis.
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FOOTNOTES |
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U. S. B. Potard was supported by a scholarship from Justus Liebig-Universität, Giessen, Germany.
Present address of U. S. B. Potard: McKinsey & Company, Inc., St.-79 Avenue des Champs-Elysees, 75008 Paris, France.
Address for reprint requests: D. E. Leith, 5025 Lakewood Dr., Manhattan, KS 66503 (E-mail: leith{at}ksu.edu).
Received 27 June 1997; accepted in final form 18 February 1998.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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1.
Armstrong, R. B.,
and
M. H. Laughlin.
Metabolic indicators of fibre recruitment in mammalian muscles during locomotion.
J. Exp. Biol.
115:
201-213,
1985
2.
Birks, E. K.,
J. H. Jones,
L. J. Vandervort,
A. K. Priest,
and
J. D. Berry.
Plasma lactate kinetics during exercise.
In: Equine Exercise Physiology 3, edited by S. G. B. Persson,
A. Lindholm,
and L. B. Jeffcott. Davis, CA: ICEEP Publications, 1991, p. 179-187.
3.
Cavagna, G. A.,
N. C. Heglund,
and
C. R. Taylor.
Mechanical work in terrestrial locomotion: two basic mechanisms for minimizing energy expenditure.
Am. J. Physiol.
233 (Regulatory Integrative Comp. Physiol. 2):
R243-R261,
1977
4.
Close, R. I.
Dynamic properties of mammalian skeletal muscle.
Physiol. Rev.
52:
129-196,
1972
5.
Collins, E. V., and A. B. Caine. Testing
draft horses. Iowa Agric. Exp. Stn. Res.
Bull. 240, 1926.
6.
Dusek, J.,
H. J. Ehrlein,
W. V. Englehardt,
and
H. Hörnicke.
Beziehungen zwischen Trittlänge, Trittfrequenz und Geschwindigkeit bei Pferden.
Z. Tierzüchtg. Züchtg. Biol.
87:
177-188,
1970.
7.
Evans, D. L.,
and
R. J. Rose.
Maximum oxygen uptake in racehorses: changes with training state and prediction from submaximal cardiorespiratory measurements.
In: Equine Exercise Physiology 2, edited by J. R. Gillespie,
and N. E. Robinson. Davis, CA: ICEEP Publications, 1987, p. 52-67.
8.
Evans, D. L.,
and
R. J. Rose.
Cardiovascular and respiratory responses in Thoroughbred horses during treadmill exercise.
J. Exp. Biol.
134:
397-408,
1988
9.
Evans, D. L.,
and
R. J. Rose.
Cardiovascular and respiratory responses to submaximal exercise training in the Thoroughbred horse.
Pflügers Arch.
411:
316-321,
1988[Medline].
10.
Fedak, M. A.,
L. Rome,
and
H. J. Seeherman.
One-step N2-dilution technique for calibrating open-circuit O2 measuring systems.
J. Appl. Physiol.
51:
772-776,
1981
11.
Gunn, H. M.
Differences in the histochemical properties of skeletal muscles of different breeds of horses and dogs.
J. Anat.
127:
615-634,
1978[Medline].
12.
Gunn, H. M.
Muscle, bone and fat proportions and muscle distribution of Thoroughbreds and other horses.
In: Equine Exercise Physiology 2, edited by J. R. Gillespie,
and N. E. Robinson. Davis, CA: ICEEP Publications, 1987, p. 253-264.
13.
Heglund, N. C.,
and
C. R. Taylor.
Speed, stride frequency and energetic cost per stride: how do they change with body size and gait?
J. Exp. Biol.
138:
301-318,
1988
14.
Hill, A. V.
The heat of shortening and the dynamic constants of muscle.
Proc. R. Soc. Lond. B Biol. Sci.
126:
136-193,
1938.
15.
Hörnicke, H.,
M. Weber,
and
W. Schweiker.
Pulmonary ventilation in Thoroughbred horses at maximum performance.
In: Equine Exercise Physiology 2, edited by J. R. Gillespie,
and N. E. Robinson. Davis, CA: ICEEP Publications, 1987, p. 216-224.
16.
Knight, P. K.,
A. K. Sinha,
and
R. J. Rose.
Effect of training intensity on maximum oxygen uptake.
In: Equine Exercise Physiology 3, edited by S. G. B. Persson,
A. Lindholm,
and L. B. Jeffcott. Davis, CA: ICEEP Publications, 1991, p. 77-82.
17.
Kram, R.,
and
C. R. Taylor.
Energetics of running: a new perspective.
Nature
346:
265-267,
1990[Medline].
18.
Kronacher, C.,
and
A. Ogrizek.
Exterieur und Leistungsfähigkeit des Pferdes mit besonderer Berücksichtigung der Gliedmassenwinkelung und Schrittlängenverhältnisse.
Z. Tierzüchtg. Züchtg. Biol.
23:
183-228,
1932.
19.
Krüger, L.
Die Bestimmung der Arbeitsfähigkeit bei Pferd und Rind durch Leistungsprüfungen, physiologische und psychologische Messwerte und durch die Exterieurbeurteilung.
Z. Tierzüchtg. Züchtg. Biol.
70:
1-20,
1957.
20.
Landgren, G. L.
Ventilation, Gas Transport, and Responses to CO2 Breathing in Horses at Rest and During Heavy Exercise (PhD dissertation). Manhattan, KS: Kansas State University, 1989.
21.
Margaria, R.,
P. Cerretelli,
P. E. Diprampero,
C. Massari,
and
G. Torelli.
Kinetics and mechanisms of oxygen debt contraction in man.
J. Appl. Physiol.
18:
371-377,
1963
22.
McMahon, T. A.
Using body size to understand the structural design of animals: quadrupedal locomotion.
J. Appl. Physiol.
39:
619-627,
1975
23.
McMahon, T. A.
Muscles, Reflexes, and Locomotion. Princeton, NJ: Princeton Univ. Press, 1984.
24.
Musch, T. I.,
G. C. Haidet,
G. A. Ordway,
J. C. Longhurst,
and
J. H. Mitchell.
Dynamic exercise training in foxhounds. I. Oxygen consumption and hemodynamic responses.
J. Appl. Physiol.
59:
183-189,
1985
25.
Nadaljak, E. A.
Gas interchange in horses used in transport work at walk and trot with different draftpower and speed of movement.
Trudy vses. Inst. Konevodstvo
23:
228-245,
1961.
26.
Otis, A. B.
Quantitative relationships in steady-state gas exchange.
In: Handbook of Physiology. Bethesda, MD: Am. Physiol. Soc., 1964, sect. 3, vol. I, chapt. 27, p. 681-698.
27.
Pelletier, N.,
and
D. E. Leith.
Cardiac output but not high pulmonary artery pressure varies with FIO2 in exercising horses.
Respir. Physiol.
91:
83-97,
1993[Medline].
28.
Procter, R. C., S. Brody, M. M. Jones, and
D. W. Chittenden. Efficiency of work horses of different
ages and body weights. Univ. Mo. Agr. Exp. Sta. Res.
Bull. 209: 1934.
29.
Roberts, T. J.,
R. L. Marsh,
P. G. Weyand,
and
C. R. Taylor.
Muscular force in running turkeys: the economy of minimizing work.
Science
275:
1113-1115,
1997
30.
Rome, L. C.,
R. P. Funke,
R. M. Alexander,
G. Lutz,
H. Aldrige,
F. Scott,
and
M. Freadman.
Why animals have different muscle fibre types.
Nature
335:
824-827,
1988[Medline].
31.
Saltin, B.,
and
P. O. Åstrand.
Maximal oxygen uptake in athletes.
J. Appl. Physiol.
23:
353-358,
1967
32.
Schmidt-Nielsen, K.
Locomotion: energy cost of swimming, flying, and running.
Science
177:
222-227,
1972
33.
Seeherman, H. J.,
C. R. Taylor,
G. M. O. Maloiy,
and
R. B. Armstrong.
Design of the mammalian respiratory system. II. Measuring maximum aerobic capacity.
Respir. Physiol.
44:
11-23,
1981[Medline].
34.
Snow, D. H.,
R. C. Harris,
and
S. P. Gash.
Metabolic response of equine muscle to intermittent maximal exercise.
J. Appl. Physiol.
58:
1689-1697,
1985
35.
Stull, C. L.,
and
W. W. Albert.
Comparison of muscle fiber types from 2-year-old fillies of the Belgian, Standardbred, Thoroughbred, Quarter horse and Welsh breeds.
J. Anim. Sci.
51:
340-343,
1981.
36.
Taylor, C. R.
Force development during sustained locomotion: a determinant of gait, speed and metabolic power.
J. Exp. Biol.
115:
253-262,
1985
37.
Taylor, C. R.,
K. E. Longworth,
and
H. Hoppeler.
Matching O2 delivery to O2 demand in muscle. II. Allometric variation in energy demand.
In: Oxygen Transfer from Atmosphere to Tissues, edited by N. C. Gonzalez,
and M. R. Fedde. New York: Plenum, 1988, p. 171-181.
38.
Weibel, E. R.
The Pathway for Oxygen. Cambridge, MA: Harvard Univ. Press, 1984.
39.
Wells, J. B.
Comparison of mechanical properties between slow and fast mammalian muscles.
J. Physiol. (Lond.)
178:
252-269,
1965.
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) and DH (
) as a function of speed and draft force.
) and DH (
). 
, data obtained by Procter et al. (
