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Kentucky Derby: Race Horse Physiology Is
Model For Speed
Equine
athlete has natural supplemental oxygen supply ready for race; blood
thickens 50% with infusion of oxygenated red blood cells
BETHESDA, MD (April 27, 2006) – When the elite horses
of the Kentucky Derby jump from the gate on May 6, will the physiologists
who study them be able to predict the likely winner?
Exercise physiologists
have used horses for research for hundreds of years because the equine
athlete’s blood vessels are large, they love to exercise, and they are
domesticated, noted Eric K. Birks, assistant professor of exercise
physiology at the University of Pennsylvania School of Veterinary Medicine.
Important discoveries about heart rate, blood clotting,
blood pressure and the role of oxygen in the blood have resulted from
research with horses, said Kenneth H. McKeever, associate professor and
associate director of the Equine Science Center at Rutgers University, New
Brunswick, NJ, and an advisor to the New York state racing board’s
medication advisory committee.
And the research continues. McKeever, for instance, is
currently doing research funded by the U.S. Army, looking at the
anti-inflammatory properties of several foods. Soldiers, who run and walk
with lots of heavy equipment, often develop inflammation, which can lead to
injuries, he said. His team found that extracts of cranberry, black tea and
orange peel reduced inflammation in horses, but ginger did not. This finding
could help soldiers reduce inflammation by eating these foods.
Born to run
Physiologists use treadmills to identify how the
horse’s physiology contributes to maximum performance and how it limits it.
Getting a 1,200 pound thoroughbred to run on a treadmill has its challenges,
but part of the reason the horse has become a favorite of exercise
physiologists is that they love to run, McKeever said.
“That’s part of the lure of using horses rather than
rats or monkeys -- they give it their all,” said McKeever. Horses in his lab
paw at the belt when they get on the treadmill, trying to get it to move, he
said.
“If they had opposable thumbs, they would open their
stalls and go down to the treadmill themselves,” said Birks. “Left to their
own devices, the horses would probably run more than what we allow them to
run in races.”
And what have physiologists learned about horse
physiology?
Fun physiological facts
about horses
Horses
-
can breathe only through their noses
-
can only breathe in synch with their stride
-
have outsized spleens that release oxygen-rich red blood
cells into the blood stream when they run
-
have hearts that can handle blood that thickens with the 50%
increase in red blood cells
-
are the only animals, other than humans, that sweat through
their skin
A horse’s body is like a huge bellows, McKeever
explained. Its breathing is dictated by the movement of its body and is in
synchrony with its stride. Horses can inhale only when their front hooves
are striding outward, they exhale only when all four legs come together --
the in and out of the bellows.
Unlike human runners who
can take a deep breath independent of their leg movements, horses cannot
take that extra-deep gulp of air when in full gallop, said Lawrence R. Soma,
professor of anesthesia and clinical pharmacology at the University of
Pennsylvania School of Veterinary Medicine. That means a horse with a longer
stride has more time to inhale and exhale, allowing the horse time to
breathe more deeply. Horses cannot increase their breathing rate without
running faster or shortening their strides.
The horse with the longer stride has an advantage,
because it has more breathing time, Birks said. Triple Crown winner
Secretariat had one of longest stride ever recorded, allowing him to take
bigger breaths, he said. So, the longer the stride, the more likely to win,
right?
Thoroughbreds share same
30 ancestors
Unfortunately, it’s not so simple. Some very successful
horses have had shorter strides, including Kentucky Derby and Preakness
winner Smarty Jones. Racing success is built on multiple factors, including
the horse’s psychological makeup, not to mention the jockey’s, Soma said.
The horses that run the Derby run approximately the
same speed, Soma noted. One horse length is equal to one-fifth of a second,
meaning the horse that finishes five lengths back in a two-mile race is only
one second behind the winner. So even small things can make a difference.
“It doesn’t look like breeding makes all that much
difference,” Soma said. Modern race horses do not have much genetic
variability, because they trace their ancestry to the same 30 or so horses.
Since the 1700s, horses have become a little faster, but not much, he said.
Like standing on one
finger
Horses pack great speed for their size, a skill that
comes in handy when you’re sharing the open plain with hungry predators. But
their thin legs and relatively small hooves support a much greater
proportion of weight than the human foot, which is relatively larger.
“Anatomically speaking, they run on their toes,” Soma
said. “That makes them very fragile.” A human being would have to put his
weight on his middle finger to duplicate the proportion of weight the horse
hoof supports as it hits the ground.
The strain of a two-minute race can cause problems for
horses, McKeever said. They may be unable to get enough oxygen to their
tissues and carbon dioxide may build up in their blood. Blood vessels in
their lungs sometimes burst from the strain and if enough burst, the horse
may bleed from its nose.
Physiologists are still trying to understand why horses
are susceptible to this bleeding but inhaling dust into the lungs may play a
role, as may the shock produced as the horse’s 1,200 pound body lands on its
legs in full gallop.
Oxygen consumption is key
Horses need a lot of oxygen when they run. They carry
their own emergency supply in their spleens.
“The horse spleen is a giant bag of extra blood,”
McKeever said. When the horse runs, its spleen contracts and forces a fresh
supply of oxygen-rich red blood cells into its blood stream, effectively
pumping extra oxygen into its cardiovascular system virtually on demand.
Human athletes, on the other hand, must train at high altitude to produce a
greater supply of red blood cells.
When at rest, about 35% of the total blood volume in
humans and horses is comprised of red blood cells. Humans maintain that
proportion even during exercise. Horses increase their red blood cell
numbers to more than 65% of blood volume during a race. This greatly
increases the horse’s ability to carry needed oxygen, but also makes the
blood thicker. Fortunately, the horse’s heart is able to overcome this
viscosity.
Horses also have a tremendous ability to use the oxygen
it produces. An elite human athlete uses 70-90 milliliters of oxygen per
kilogram of weight per minute, McKeever said. Thoroughbreds use more than
150 milliliters per kilogram per minute.
Any predictions?
So, should you consult your favorite physiologist
before placing your bet on the Derby, or any horse race?
“We can predict the horses that will be in the top 5%
of thoroughbred racing, but every horse at the racetrack is in that 5%”
Birks said.” There are too many variables beyond physiology: training, diet,
the jockey, horse psychology and more; and too little difference in
performance between the first- and last-place finishers to be able to
predict the outcome.
“We choose not to gamble,” Birks said with a laugh.
* * *
The
American Physiological Society was founded in 1887 to foster basic and
applied bioscience. The Bethesda, Maryland-based society has more than
10,000 members and publishes 14 peer-reviewed journals containing almost
4,000 articles annually.
* * *
APS
provides a wide range of research, educational and career support and
programming to further the contributions of physiology to understanding the
mechanisms of diseased and healthy states. In May 2004, APS received
the Presidential Award for Excellence in Science,
Mathematics and Engineering Mentoring (PAESMEM).
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