The centuries-old practice of whipping racehorses to encourage them to run faster has been called into question by an independent study that found horses are relatively thin-skinned and feel pain when struck.

Dr Lydia Tong, a University of Sydney veterinary pathologist and forensics researcher, investigated the skin thickness of horses and how they sensed pain for the ABC's Catalyst program.

Despite extensive literature research, Dr Tong said she was surprised to learn this had never been done before.

"We've looked closely at human skin and horse skin from the same area, on the flank," she said.

"The really interesting part is that right up in the epidermis, which is the top layer and that's where the pain-sensing C fibres are, in the human specimen that's thicker than the horse's.

"So by the old argument of horse's skin is thicker and they feel it less, actually you could argue human's skin is thicker.

"It was unexpected actually."

Watch the full program on the Catalyst website or catch up on iView

Dr Tong said one difference from humans was that animals, who evolved as prey, were more likely to hide their pain.

"If a prey animal shows its pain very overtly, they are more likely to then be noticed and picked out by a predator," she said.

"So actually often prey animals they kind of shut up and put up. That's recognised."

She said there was no reason to think that something that would hurt a human would not hurt a horse.

"I think we should assume that it's likely to be painful," Dr Tong said.

Rules introduced to restrict whip use

A padded whip was made compulsory in the racing industry in 2009 and was meant to settle the controversy once and for all.

The rules limited the number of strikes depending on how the whip was held.

For forehand strikes there is a limit of five strikes in non-consecutive strides prior to the last 100 metres of a race. In the final 100m, strikes are unlimited.

There are no limits, however, on the number of strikes if the whip is held in a backhanded fashion, with the assumption that horses could not be hit as hard.

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Former vet and Catalyst presenter Dr Jonica Newby decided to try it for herself.

Ten minutes after being struck on the leg, thermal imaging showed the impact clearly, with a one-degree temperature rise indicating some inflammation.

She said it hurt a little less than she would have thought, but then she took a stronger strike simulating a hard forehand hit.

"This hurt more and stung for longer, leaving a bruise where the [unpadded] knot hit," she said.

Jockeys and trainers are of the belief that the newer whips do not hurt and that you can hit a human with them.

They say whips are needed to help the horse focus or be used for guidance.

"What it's there for is to encourage the horse to finish off, because you've got to remember they've got a thicker hide than you or I," Victorian jockey Chris Symons said.

"They're a 500kg animal. If they want to bludge, they'll bludge."

Analysis questions effectiveness of padded whips

While some perhaps express their opinion more colourfully than others, this appears to be the dominant view across the industry.

But a couple of veterinarians — Dr Paul McGreevy and Dr David Evans — are trying to tackle the same issue with science.

Between them, since 2011 they have published a series of studies on the whip in racing, one of which was funded by the RSPCA.

Horse skin (top) with nerve fibres stained red compared with human skin (bottom). ( Supplied )

Perhaps the most compelling contained a frame-by-frame analysis of 15 races at Gosford racetrack in June 2011.

"Eighty-three per cent of the time there was a visible indentation [caused by a whip]," Professor McGreevy said.

"That deformation of the skin cannot be pleasant.

"Seventy per cent of the time the abdomen was struck, the flank was struck."

This evidence also showed the unpadded shaft of the whip making contact in 64 per cent of cases.

"Now that's important because the section between the padded and unpadded sections has a hard knot, so we know that 64 per cent of the time, that knot is hitting the horse," Professor McGreevy said.

To test the assumption backhands were softer, Professor McGreevy recruited six jockeys and asked them to hit a pressure-detecting pad as if they were finishing a race.

"So we found when they were using their dominant hand, we were very surprised to find that this system picked up more force with the backhand which isn't policed," Professor McGreevy said.

No such thing as a standard whip strike

But a study — partly funded by the racing industry — by equine scientist Dr Glenys Noble and racetrack veterinarian Dr Peter Knight, found the opposite result: that the forehand was stronger.

"And so the conclusion was that the rules as they stand that put a limit on the use of the whip with the forehand action are actually justified and that they are looking after the welfare of the horses," Dr Knight said.

The studies by Professor McGreevy and his colleagues have all been published in peer-reviewed journals. The Noble-Knight study, at this point, has not.

Dr Noble agreed there was no such thing as a standard whip strike.

"[There's a] huge variation, which is why, likely there are some whip strikes that do hurt a horse but there are probably a lot that don't," he said.

Australian Racing Board chief executive Peter McGauran said he was more than comfortably satisfied that on all of the available veterinary advice, if jockeys kept to the limit of strikes allowed for under the rules there was no cruelty to a horse.

But he said the industry would further restrict use of the whip or do away with it completely if there was evidence that repeated whipping inflicted pain.

"You would further restrict it or abolish it," he said.

"You can't say you believe it does not inflict pain and then, if evidence was to the contrary, you continue."