麻豆传媒

Once again, records tumbled at the Olympics and one could be forgiven for
thinking that the human body鈥檚 capabilities are limitless. But although Olympic
athletes of the human kind break records with regular monotony, horses do
not鈥攊n spite of the promises that came with the introduction of equine
sports medicine in the 1980s.

In the past 50 years, the world record in the (human) 1500 metres has been
pushed down from 3 minutes 50 seconds to under 3 minutes 28 seconds, and
marathon runners have shaved the record from 2 hours 35 minutes to below 2 hours
7 minutes. Virtually every track and field record on the books has been set
since 1980, many since 1990.

But horses are another story. Northern Dancer won the Kentucky Derby in 2
minutes in 1964 and the only horse to do better in the years since was
Secretariat in 1973, at 1 minute 59 seconds. Average winning times in the
British Derby are about the same today as they were in the 1930s, and in the St
Leger they are about 3 seconds worse. And while winning times in 1-mile American
harness races have dropped by about 5 seconds in the past three decades, the
standardbred horses (an American breed developed principally by crossing
Thoroughbreds and Morgan horses) that compete in these events still have shown
nowhere near the improvement of human athletes over the same period.

Human athletes鈥� continued assault on the record books was given a tremendous
boost in the late 1970s and 1980s by the advent of sports medicine.
Biomechanical analyses of technique, studies of how training affects aerobic
fitness, research into the effect of diet on performance鈥攁ll contributed
to dramatic, measurable gains on the athletics track. That success encouraged
the belief that equine sports medicine could do much the same for horses. Now,
the animal scientists are beginning to understand why the high hopes of equine
sports medicine have been dashed.

For starters, it turns out that horses are naturally built to operate at very
high metabolic rates for their size when exercising at peak capacity鈥攔ates
that may already bump up against the very limits of what lungs and biochemistry
can do. If you plot the maximum metabolic rate of a range of mammals against
their size, the horse is way off the curve, with a maximum metabolic rate 3.75
times as high as expected for an animal of its size. The reason horses can鈥檛 get
much better with exercise, in other words, is that nature has already endowed
them with a near-perfect balance between lungs, heart and muscle. Unlike human
athletes, horses simply don鈥檛 have much room for improvement. A programme of
long, slow work-outs, or short, fast work-outs, or any of the other fashionable
training strategies that have come and gone, cannot alter this fact.

Nor can any exercise programme alter the anatomical peculiarity that limits a
galloping horse to taking only one breath per stride, thus setting an upper
limit on how much oxygen even the fittest horse can deliver to its muscle cells.
And though human athletes under intensive training can significantly boost the
ability of their muscle cells to generate energy by cranking up a biochemical
pathway that does not require oxygen, the same capacity for improving anaerobic
performance has not been found in horses (for reasons not entirely clear). A
horse galloping in a sprint may rely almost entirely on this anaerobic metabolic
pathway, and in longer races total energy output is determined by the sum of
aerobic and anaerobic metabolism. What is certain is that both these
factors鈥攁erobic and anaerobic respiration鈥攁re affected only
marginally by exercise.

Fitness factor

The studies of human athletes that first inspired the equine sports docs
found that the limiting factor for many humans was peak metabolic output, or
鈥渇itness鈥濃�攖he ability of the lungs to supply oxygen to the blood, the
heart to pump the blood to the muscles, and the muscles to generate energy
efficiently. Exercise programmes designed to selectively stress the various
elements of this chain have produced dramatic results. A trained marathon
runner鈥檚 heart, for example, can pump 33 litres of blood a minute, compared with
19 litres a minute for the average person.

But a 1990 study shows that enormous differences in exercise have remarkably
little impact on equine fitness, at least among the horsy
elite鈥擳horoughbred and standardbred racehorses. Thoroughbreds typically
compete only once every two weeks, galloping distances of 戮 to 1陆 miles;
between races they are put through their paces at racing speeds only once or
twice, and only over a short distance, perhaps half a mile. Standardbreds, on
the other hand, face far more demanding race schedules, competing once a week
over a distance of a mile, and at the much more energetically tiring gait of the
trot or pace鈥攁nd they work out almost every day between races, and at full
race speeds on three of those days.

When animal physiologist Howard Seeherman and his colleagues at Tufts
University in Grafton, Massachusetts, compared the fitness of 26 standardbreds
and 16 Thoroughbreds as they ran on a treadmill at varying speeds, they found
that despite their radically different training schedules the standardbreds were
no fitter for the extra effort than the Thoroughbreds. For example, at top
speeds of about 45 kilometres per hour, both breeds had average heart rates of
230 beats per minute and consumed 160 millilitres of oxygen per minute per
kilogram of bodyweight.

A number of studies since the 1980s have examined the limitations that the
peculiar mechanics of the equine respiratory system impose on
performance鈥攍imitations that all the training in the world cannot alter.
The discovery that galloping horses (and dogs) take just one breath per stride
goes back to pioneering work at the University of Utah in Salt Lake City by
Dennis Bramble and David Carrier. Horses, unlike humans, have no collar bone.
Bramble and Carrier, and subsequent researchers, showed that instead, in horses,
the motion of the forelimbs is tied directly by powerful muscles to the ribs,
and thus to the lungs. As the forelimbs strike the ground at the gallop, the
impact is transmitted through the legs and forces the ribs upward, squeezing air
out of the lungs like a bellows. At the same time, the horse is lowering its
head and neck, which presses the rib cage backward, and the front of the body is
decelerating, so that the internal organs slosh forward, giving the lungs a
further squeeze. As the head and neck are raised and the load is lifted off the
forelimbs, the effects are reversed and the lungs expand.

Racing gallop

This mechanical coupling forces the horse to take exactly one breath per
stride. Up to a point, oxygen supply is perfectly matched to oxygen demand. But
as horses approach the top range of the racing gallop, they come up against the
upper limit of stride frequency, which is fundamentally a matter of mechanics
(determined by the length of an animal鈥檚 legs). So at the highest speeds, horses
increase speed primarily by taking longer strides at the same frequency, which
means they are unable to compensate for the extra effort by taking more breaths.
A horse that is beginning to tire for lack of oxygen is stuck.

The problem of improving performance through training is compounded by the
fact that a racing horse draws heavily on metabolic pathways that don鈥檛 even
involve oxygen, whose efficiency cannot be improved through exercise. When a
sudden demand is placed on a muscle for a burst of energy, its first recourse is
to throw more fuel into the metabolic furnace鈥攆uel that is nearby and that
ignites with a bang. Muscles keep a small load of such a fuel at hand in the
form of phosphocreatine (PC). A single, quick chemical reaction turns PC into
ATP, the molecule that powers the contraction of the muscle fibres. Muscle cells
also keep a very small reserve of ATP ready to go. Together these reserves can
provide the horse with 10 to 15 seconds of peak muscular activity. A
Thoroughbred accelerating out of the starting gate, a jumper pushing off over a
fence, or a roping horse is drawing almost entirely on these rapidly
deployed鈥攁nd rapidly depleted鈥攔eserves.

Within about 10 to 20 seconds, another mechanism kicks in and glycogen in the
muscles is converted to ATP. Like the PC reaction, this process of anaerobic
glycolysis helps to bridge the inevitable gap between the start of the race and
the time when the heart and lungs are able to step up their delivery of
oxygenated blood to the muscles. Anaerobic glycolysis has two disadvantages.
First, it is inefficient (liberating only about one-thirteenth of the energy
contained in the glucose molecules from which glycogen is made), and second, one
of its byproducts is lactic acid, which triggers fatigue. If a horse had to rely
solely on anaerobic glycolysis it would come to a screeching halt after just 60
seconds of maximum muscular effort.

Before that happens, however, oxygen-dependent energy production finally gets
in on the act, in all but the shortest races (for example, quarter horse races,
which are traditionally 录 mile, and Thoroughbred races under 戮 miles). But
even then anaerobic glycolysis continues to run in parallel鈥攃alculations
by Seeherman and others show that the energy demands of a horse in mid-race
exceed what could be provided by aerobic glycolysis alone. In the final sprint
to the post, the ability to draw upon additional anaerobic glycolysis plays a
crucial part.

Studies of muscle cells in horses suggest that anaerobic capacity is largely
determined genetically. In sprinters such as quarter horses, the muscles that
propel the hindquarters have been found to consist of as much as 100 per cent
鈥渇ast-twitch鈥� fibres, which have a very high anaerobic capacity. In successful
endurance horses, which compete in events over as much as 180 kilometres at much
slower speeds, the proportion of fast-twitch fibres is considerably less, around
50 per cent.

Magic formula

So the problem is not that trainers have yet to hit on the magic formula for
realising the horse鈥檚 potential, but that any reasonably strenuous programme of
exercise will already have achieved this. Moving beyond these limits would
require a horse that can breathe faster than it runs, or a horse that can run
without breathing at all. Neither is possible.

Seeherman suggests that in the end the real benefits of rigorous training may
actually be more psychological than physiological. He postulates that the
particularly gruelling training regimen for standardbreds, for example, may help
those horses to develop the 鈥渕ental toughness鈥� needed to withstand the much
greater physical demands of a race at a pounding trot or pace pulling a sulky, a
small two-wheeled buggy.

As with human athletes, the ultimate test of the equine athlete may not be
the physical ability to avoid exhaustion, but the mental ability to ignore
it.