THE EFECTS OF COMBINED RIDER AND TACK WEIGHT ON THE LACTIC
ACID PRODUCTION IN THE HORSE ( Equus caballus) AT THREE
DIFFERENT GAITS: WALK, TROT, AND CANTER.
Anahita A. Ariarad and Allison S. Lindsay
Department of Biological Science
Saddleback College
Mission Viejo, CA 92692
During glycolysis, L-Lactate is produced from pyruvate via the enzyme lactate
dehydrogenase (LDH) during normal metabolism and exercise. Due to increased
recruitment of skeletal muscle during exercise, it was predicted that lactate levels in
the horses would increase when the combined weight of the rider and tack was
added. To test this, blood lactate levels of five horses were taken before exercise and
again after walking, trotting, and cantering for various lengths of time. There was
found to be a 37.5% increase/decrease difference between the values of the canter
when comparing unmounted versus mounted blood lactate levels (p = 0.026, Tukey
Correction). This data partially supports our hypothesis, but further investigation is
needed.
Introduction
Lactic acid is produced in minute amounts during rest but in greater quantities
during intense exercise. It is naturally present in humans as well as animals. When the
oxygen level in the body is normal, carbohydrate breaks down into water and carbon
dioxide. When the oxygen level is low, carbohydrate breaks down for energy and makes
lactic acid. Lactic acid is formed from glycogen by muscle cells when the oxygen supply
is inadequate to support energy production (Wickler and Gleeson 1993). Buildup of lactic
acid in the muscle occurs only in short bouts of exercise due to high intensity and it is
usually correlated to fatigue, exhaustion and muscle soreness. During aerobic exercise,
the heart and lungs supply adequate amounts of oxygen to the body for energy.
Anaerobic exercise forces the body to demand more oxygen than the lungs and heart can
supply. This shows that the energy supply is less and thus causes a high lactic acid level
in the blood. Typically anaerobic exercise forces a person to slow down because lactic
acid build up causes moderate to severe muscle throbbing and rigidity. In human athletes,
ATP production in Glycolysis may be as high as 3mMol/g body weight x s (Newsholme
& Leech 1983), and although similar estimations from equine muscle are unavailable, the
comparison of the activities of key glycolytic enzymes in equine muscles to those in
human muscles (Essen-Gustavsson et. al. 1984, Henriksson et. al. 1986, Roneus et. al.
1991, Cutmore et. al. 1993) indicates that anaerobic ATP production may be equally
important in horses. Lactic acid gathers in the muscles when the supply of oxygen is
scarce for the oxidative processes and quickly diffuses out into the blood stream. As
lactic acid diffuses out of the muscles and other tissues, it appears in the blood as lactate.
Blood lactate can be useful for evaluation performance. Lactate, which is produced by the
body all day long, is re-synthesized by the liver from glucose that provides the body with
energy.
In most mammals, lactate formed during exercise is oxidized to carbon dioxide
and water (Wickler and Gleeson 1993). Under extreme conditions, horses contracting
muscles are fueled by aerobic and anaerobic metabolic processes. When a horse performs
or exercises, they use their muscles to accomplish tasks. As lactic acid is produced in the
muscles it leaks out into the blood and is then carried around the body. If this condition
continues, the functioning of the body can then become impaired and the muscles can
fatigue very rapidly. When oxygen becomes available, the lactic acid is converted to
pyruvic acid and then into carbon dioxide, water and ATP (Kobayashi 2007). Horses
performing low intensity exercise for long periods lose large amounts of sweat. Lower
intensity exercise uses oxygen to provide energy, and is known as aerobic exercise.
Aerobic exercise does not produce high levels of lactic acid. Muscles in horses secrete Dlactate as a by product of energy production during anaerobic exercise. In this study, the
effects of exercise and the cause it has on the increase of blood lactate levels in horses
was tested. The objective was to determine if lactic acid levels in the horse (Equus
caballus) increase between mounted and unmounted at each gait. It was expected that
there would be a difference amid the blood lactate production between mounted and
unmounted after all three gaits.
Materials and Methods
Five horses were used in this study to determine if the combined weight of the
rider and tack has a significant affect on blood lactate levels after walk, trot, and canter.
All testing took place at the J.F. Shea Center for Therapeutic Riding in San Juan
Capistrano, California under the supervision of a board-certified vet, Dr. Richard Markel.
The horses were chosen for testing based on size, temperament, and soundness. This
information was available in their medical/ health records and known by staff. Horses
were removed from their stalls one at a time and a fresh, sterile needle was inserted into
the jugular vein. The first two drops of blood were discarded before the base level sample
was taken and read by the Lactate Scout (Sports Resource Group, Inc.).
Over a period of a week, all five horses were lunged on a line by a staff member
in a twenty meter diameter round pen. Each horse walked and trotted for fifteen minutes
at each gait, and cantered for six minutes. Blood was taken in the same manner as before
and analyzed by the Lactate Scout immediately after each gait.
From 8 November to 10 November 2009, all horses were ridden in the same tack
and by the same rider to reduce variability in weight. Horses were ridden for the same
amount of time at the walk, trot, and canter as in the unmounted tests. Blood was taken
and analyzed in the same manner as before. All horses were then weighed using Blue
Seal Horse and Pony Height and Weight Tape (Blue Seal Feeds Inc., Lawrence,
Massachusetts). All data were transferred to MS Excel (Microsoft Corporation,
Redmond, Washington) where data were analyzed by ANOVA and Tukey Correction (if
p ≤ 0.05).
Results
The mean unmounted blood lactate level at the walk was 0.0005 ± 1.13 x 10-04
mM•L-1•kg-1 (±SEM, N=5), at the trot was 0.0004 ± 8.9941 x10-05 mM•L-1•kg-1 (±SEM,
N=5), and at the canter was 0.0008 ± 9.853 x 10-05 mM•L-1•kg-1 (±SEM, N=5). The
mounted blood lactate level at the walk was 0.0003 ± 4.17 x 10-05 mM•L-1•kg-1 (±SEM,
N=5), at the trot was 0.0003 ± 2.1 x10-05 mM•L-1•kg-1 (±SEM, N=5), and at the canter
was 0.0005 ± 8.25 x 10-05 mM•L-1•kg-1 (±SEM, N=5). As seen in Figure 1, between the
groups, no difference was found when comparing mounted and unmounted values in the
walk and trot (p = 0.164 and p = 0.348 respectively). In the canter group a difference was
found (p = 0.026). Within the groups, a difference was found between the unmounted
values for trot and canter (p = 0.007), and the values for walk and canter (p = 0.044).
Similarly there was a difference found between the mounted values for trot and canter (p
= 0.006) and walk and canter (p = 0.028).
0.001
0.0009
mM • L-1 • kg-1
0.0008
0.0007
0.0006
Unmounted
0.0005
Mounted
0.0004
0.0003
0.0002
0.0001
0
Walk
Trot
Canter
Figure 1. Mean combined lactate levels at walk, trot and canter unmounted versus mounted. No difference
was found between unmounted and mounted blood lactate levels in the walk and trot groups. A difference
was found between unmounted and mounted blood lactate levels in the canter group (p = 0.026, Tukey
Correction).
Discussion
Lactic acid is capable of releasing energy to re-synthesize adenosine triphosphate
(ATP) without the involvement of oxygen. Lactic acid is produced from pyruvate in the
glycolysis cycle via the enzyme lactate dehydrogenase (LDH) during normal metabolism
and exercise. The amount of lactate present after exercise can be a helpful tool in
determining performance because it is an estimation of aerobic capacity (Poso, 2002).
Within the unmounted group, no difference was found between the walk and the
trot values. However, the trot to canter and walk to canter comparisons showed a
significant difference in blood lactate values.
Similarly within the mounted group the walk to trot comparison revealed no
difference, where the trot to canter and walk to canter assessments discovered a
significant difference.
The hypothesis for this project stated that blood lactate levels would be higher at
all three gaits while mounted versus unmounted. However, collected data showed that
blood lactate levels were higher in the unmounted group. No statistical difference was
found when comparing the mounted and unmounted levels in both the walk and the trot.
This is most likely because the animals were not pushed into a state of anaerobic
respiration at these gaits. For this same reason, there was a difference found in the canter
values between the groups. Though there are several variables that can be taken into
account when examining blood lactate levels, the researchers believe that the results in
this experiment could be explained by looking at the level of energy applied when
comparing exercise by lunging versus riding. During the unmounted testing, horses
showed a higher energy exertion at the trot and canter when compared to the mounted
testing. This is evidenced by the amount of forward momentum at each gait while
lunging versus under saddle. Though the researchers did not measure the velocity of each
animal, obvious differences were observed while testing. Though it is possible to push a
horse into a higher energy level while mounted, our rider did not attempt to do this.
Though this experiment did find a significant difference, the relationship between
blood lactate production and increase in load was opposite to what was originally
hypothesized. In a study to determine lactate minimum speed (LMS), the individual
lactate production and removal rates, in horses, Gondim et al. (2007) found no difference
in blood lactate concentration at rest and at LMS, despite an increase in heart rate. The
data found in both these studies is inconsistent with the majority of information available
on blood lactate. LMS has previously been tested in basketball players and runners
(Tegtbur et al., 1993), in swimmers (Ribeiro et al., 2003) and rats (Voltarelli et al., 2002)
as well. In all the above studies lactate levels at LMS were significantly higher than those
at rest (Gondim et al., 2007).
All of the above experiments indicate that there exist one or more key differences
in the processing of post-exercise lactate in humans and equines. There are several factors
that can affect the lactate concentration in blood and these need to be accounted for when
blood lactate measurement serves as a marker of performance. The rate of lactate
production in exercising muscle is influenced by oxidative capacity and thus training,
which is often accompanied with an increase in the number of mitochondria, may reduce
lactate production (Poso, 2002). On top of this, horses already have a marked increase in
oxygen consumption with a maximal oxygen uptake of about 160 ml/kg body weight ×
min (Evans & Rose 1988, Rose et al. 1988). This is more than twice the uptake in human
elite athletes (Poso, 2002). Training can also increase the monocarboxylate transport
proteins in the sarcolemma (Poso, 2002, Hashimoto et al., 2008, Brooks et al., 1999).
This allocates for a faster rate of facilitated diffusion and therefore would add to the
lactate concentration. The quantity of lactic acid that is permitted to build up is
determined by the effort that is needed to increase the lactate concentration to levels
above its resting value. This occurs when anaerobic glycolysis produces lactate at a
greater rate than the animal’s capacity to remove it (Gondim et al., 2007). In skeletal
muscle, the fast-twitch glycolitic fibers are mainly producers of lactate while the slow
oxidative fibers act as consumers; and therefore these two fibers, along with other factors,
are responsible for creating the net change in lactate levels (Hashimoto et al., 2008).
Acknowledgements
We would like to thanks the J.F. Shea Therapeutic Riding Center for allowing us
the use of their facilities, horses, medical supplies and time of their staff, with a special
thanks to Richard Markel, DVM. We would also like to thanks the following people for
their assistance and input to this project: Professor Steve Teh, Aaron Ko, and Amir Zand.
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