It is what is on the inside that really counts ahead of Dubai World Cup

Once the 'huge engines' of thoroughbreds switch on and horsepower kicks in, the heart takes over the matter. Audio interview

The hearts and lungs of thoroughbreds allow them to maintain speeds, and some barely break a sweat, in running a mile and half.

When Michael Johnson blazed through 400 metres in 43.18 seconds at the 1999 World Championships, he put up such an incredible athletic performance that his time has yet to be bettered by any other human.

Compared to the million-dollar thoroughbred racehorses that will be on display for Saturday's Dubai World Cup meeting, however, Johnson's time is positively pedestrian.

The horses will hit speeds of 40mph (64kph) in three to five strides.

In this season's Meydan Sprint on March 10, Inxile, who runs on Saturday in the Dubai Golden Shaheen, covered the first 400 metres in 24.76 seconds; Johnson would barely be halfway.

The horses will be paraded the crowd at Meydan before each of the eight thoroughbred races on Saturday, and much like a bodybuilding show, this provides racegoers with a useful arena in which to judge the condition of the participants.

Anyone can see with the naked eye how well-built and athletic racehorses are, but it is only when you go under the bonnet that you see what incredible athletes they are.

The three main physiological aspects that make thoroughbreds one of the fastest land animals on earth is the capacity of their lungs and heart, combined with the ability of their legs to propel and store energy as well as cushion their immense weight.

Standing in their box before entering the parade ring a racehorse will have a resting lung capacity intake of around 150 litres of air a minute, dwarfing that of any human.

Given racehorses are eight times the size of their jockeys that does not come as such a surprise, but once the engine is switched on, and the horsepower kicks in, it becomes a different matter entirely.

In a landmark article in Ride Cycling Review magazine a few years ago, Dr David Martin examined the myth that the seven Tour de France victories of cyclist Lance Armstrong were down to a freakish physiological make up.

As a doctor at the Australian Institute of Sport, he ran tests on Cadel Evans, another cyclist, and compared his scores to Armstrong's results in another test performed by a different doctor. The outcome: Evans was actually the better physical specimen.

As an indicator to fitness, Evans recorded a figure of 87 millilitres of oxygen used in one minute per kilogram of body weight in comparison to Armstrong, who in 2005 registered 81.

Both were shy of the 88 posted in a different test by the five-time Tour winner Miguel Indurain and all three were left trailing by the record 96 set in 2005 by Espen Harald Bjerke, a Norwegian cross country skier.

The result achieved by Evans means he took in around six litres of air per minute. A racehorse in full flight it breathes in 40 litres every second.

Observers often refer to the size of an athlete's heart after a particularly gutsy showing, but in the case of a racehorse it is a crucial factor in athletic output.

Phar Lap, the great Australian racehorse that won 37 of his 51 races, had a heart that weighed 6kg, double that of a non-thoroughbred horse.

Secretariat, who in 1973 became the first American horse for 25 years to win the Triple Crown, never had his heart weighed.

The heart of Sham, Secretariat's great rival in all three Triple Crown races, was found to be 8.18kg, and after an autopsy on Secretariat, Dr Thomas Swerczek, head pathologist at the University of Kentucky, estimated the great horse's heart to be 10kg - the average weight of three newborn babies.

"We just stood there in stunned silence. We couldn't believe it. The heart was perfect. It was just this huge engine," Swerczek said. "I have done thousands of autopsies, and I had noticed differences in heart size in horses before but he was completely out of everybody else's league."

The horse's heartbeat ratchets up from 35 beats per minute at rest to a maximum of 240, as the heart pumps 260 litres of blood to provide enough oxygen to the muscles.

By comparison, Armstrong matched a thoroughbred with a resting heart rate of around 35 beats per minute during his prime, but the maximum rate he recorded after exercise was 207, which dropped to 200 after he had testicular cancer.

David Sykes, the Emirates Racing Authority veterinary officer, is quick to highlight, however, that horses' hearts work in a slightly different way to those of humans.

"I'm sure Lance Armstrong has a huge cardiac muscle and in humans resting heart rates is a good indicator of fitness, but that measurement in horses is not," said Sykes, who qualified as a vet in 1978 and has worked at Meydan since the racecourse's inception in 2010.

"A better indicator is recovery from exercise. A horse might have a post-race exercise heart rate of 200 and you time it out for something like every two minutes to see how long it takes them to recover.

"Cadel Evans has a cardiovascular system that is way in front of most athletes. So the question is should we breed horses with big hearts? No. Secretariat's heart helped him have a larger cardiac output than other horses but it wasn't the only reason why he was a champion."

The heart and lungs of thoroughbreds allow them to maintain their speeds for long distances, as well. Yeats, the only horse to have won the 4,023-metre Gold Cup at Royal Ascot four times in the race's 203-year history, could run a mile and a half and barely break sweat, said Aidan O'Brien, his trainer.

"We have never had a horse with as big a pair of lungs or as big a heart as he has," O'Brien, who saddles six horses at Meydan, said.

"With most horses when you go a mile and a half they are at the end but this fella after going a mile and a half his heart is only just getting going up to 180 beats, which is unbelievable."

So let the thunder of hoofs on Saturday remind you of the amazing physiological make-up of these animals.

"There are two main animal types, fight or flight," Sykes said.

"As soon as horses start to gallop their circulation goes up 75 per cent. The blood is then preferably distributed to cardiac, pulmonary arteries and muscle.

"With legs and lungs, that is all they need to run away and remain alive. They always can eat afterwards."

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& Geoffrey Riddle