Serious concerns have been raised about riding equipment to be used at this year's Rio Olympics, with scientists claiming nosebands and double bridles could cause unnecessary pain and suffering to horses during equestrian events.

Key points: Researchers founds nosebands causing stress response in horses

Researchers founds nosebands causing stress response in horses They say the findings could be the "tip of the iceberg"

They say the findings could be the "tip of the iceberg" RSPCA wants the equipment banned from dressage

A study from the University of Sydney's Faculty of Veterinary Science found the use of restrictive nosebands — most common in dressage — can bind together the jaws of a horse, preventing yawning, tongue movement and potentially swallowing.

"This study is the first to show that a very common practice and a very common piece of gear is eliciting a stress response in horses," said veterinarian and senior author on the study, Professor Paul McGreevy.

"It's worth noting that this response emerged in horses without riders and without rein tension, so it's possibly just the tip of an iceberg."

The crank noseband is a leather strap which goes around the horses' jaw. It can be tightened by the rider using a system of pulleys.

The tighter it is the greater the contact between the animal's mouth and the metal bits of the bridle, giving riders greater control of the horse.

The stress response — indicative of pain and suffering — was measured by monitoring horse's eye temperature and heart rates when subjected to nosebands being tightened to levels used in competition.

In some instances resting heart rates went from 34bpm to 100bpm after noseband tightening — a significant result according to lead author on the study, Kate Fenner.

"It was really quite alarming to me to see just how high the heart rates went," she said.

Eye thermometers were used to measure stress levels in horses. ( ABC News: James Thomas )

Ms Fenner said the heart rate and temperature analysis of the horses were crucial to the study.

"Everything told us the horse was stressed physiologically. But, looking at the horse, the only change we could see was that it wasn't chewing as much."

Professor McGreevy said that was because a tight noseband could prevent the outward display of stress in a horse such as yawning, mouth opening or a thrashing tongue — all traditionally signs that a horse is being ridden too aggressively.

"By having a tight noseband you eliminate all those ugly behaviours that would normally attract penalties (in dressage) and you also get more of the horse's attention," Professor McGreevy said.

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To avoid such penalties, Professor McGreevy said many riders cranked the jaws together with a system of leather pulleys known as a crank noseband.

"Essentially, restrictive nosebands deny the horse a voice," he said.

'No evidence' nosebands cause serious injury

It is not a view shared by Equestrian Australia's veterinarian Nathan Anthony.

"We certainly don't see any evidence that crank nosebands are causing any serious injury to horses," he said.

Dr Anthony said the study had limitations.

"A very small number of horses were tested and looked at."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 8 minutes 29 seconds 8 m New research links common equestrian gear to stress response in horses ( James Thomas )

Dr Anthony added: "These horses were unconditioned to the use of... a noseband. They'd never had that on before. So, as you'd expect, these horses are likely to have a greater stress response just by virtue of the fact that this is new equipment."

In response to the criticism, lead author Kate Fenner said: "I am not sure that they would ever habituate to something that stops them chewing, licking and pretty much swallowing and can do tissue and nerve damage."

Ms Fenner said Equestrian Australia or the International Equestrian Federation "would have to repeat the study with horses accustomed to wearing them to claim that".

She also defended the sample size as "statistically valid".

Crank nosebands are permitted under current competition rules, even though they increase pain and discomfort — a problem which is compounded with the use of the double bridle, researchers said.

"Every dressage horse at the Olympic level must compete with a double bridle which means there are two metal bits in its mouth. One of which is a lever that tightens a metal chain under the chin," said Professor McGreevy.

"What we have discovered is, as you tighten the noseband, those bits are compressed against the tongue and it appears to immobilise the tongue."

Cut to the mouth of a horse caused by external noseband pressure. ( Supplied: Mick Field )

In addition, there is evidence the tight nosebands cause cuts and lacerations to the inner mouth and tongue of horses because of the pressure from the noseband pushing soft tissue against sharp teeth.

"Denying horses normal behaviour, in the name of sport, is quite difficult to defend ethically" said Professor McGreevy.

RSPCA calls for complete ban of crank nosebands

The study, published in the prestigious Public Library of Science, has led 30 animal protection groups from around the world to call for mandatory testing of the nosebands at competitions to ensure they are not too tight.

"It's disturbingly clear from this study that the use of restrictive nosebands is causing horses significant pain, suffering and distress during training and competition," said Bidda Jones, the chief scientist of the RSPCA.

"Yet their use appears to be on the rise and nothing is being done to prevent it."

The RSPCA has called for an end to crank nosebands completely on the grounds they are cruel.

"The rules of dressage and other events must be changed before even more horses are forced to endure this unacceptable practice."

Equestrian Australia's chairwoman Judy Fasher said horses are inspected at every competition to ensure gear is compliant and not harmful.

"They are inspected prior to the beginning of performance. The gear is inspected to make sure it fits the horse correctly," she said.

Equestrian Australia's assurances have done little to convince the chief scientist of the RSPCA.

"Horses shouldn't be made to suffer, in the name of sport. So, it is really clear that these nosebands should be removed from all equestrian competition," Dr Jones said.