Leveraged bits only modestly increase poll pressure in horses – study

Bits designed to give strong poll pressure using simple pulley or lever principles showed a much reduced transfer of the rein tension through the bit to the poll.
Bits designed to give strong poll pressure using simple pulley or lever principles showed a much reduced transfer of the rein tension through the bit to the poll, a British study has found.

The use of leveraged bits does not significantly increase pressure on the poll when rein tension is applied, a study has found. Instead, the tension applied to the reins tended to move the bit backwards in the mouth.

British researchers used an equine bridle tension system, with electronic force gauges in both the reins and the cheek-piece on one side of the horse, to gather the data for their study.

They set out to investigate the dynamic response of the cheek-piece to rein tension in the ridden horse and to quantify the pressure on the poll.

It is generally thought that some horse bits can operate as levers of rein tension, amplifying the forces transferred to the poll or chin of the horse.

“Popular discussions focus on poll pressure and how it might affect the training and behaviour of the horse,” the study team noted. “Some even consider it a topic of horse welfare.”

University of Durham senior physics lecturer Graham Cross and his colleagues Michael Cheung, Thomas Honey, Michael Pau and Kara-Jane Senior found that bits designed to give strong poll pressure using simple pulley or lever principles showed a much reduced transfer of the rein tension through the bit to the poll.

This reduction, they said, was readily understood when the horse’s mouth was recognised as a “floating” fulcrum, effectively degrading the otherwise required fixed pivot point of an ideal lever.

“Furthermore, any use of a curb chain diverts higher rein-induced cheek-piece tension to the chin rather than to the poll,” they reported in a paper published online ahead of print in the Journal of Equine Veterinary Science.

“Unexpectedly, however, a simple loose ring bit is found to give modest poll pressure, transferring rein tension through a pulley-like action.”

The study team also examined the curb bit of a double bridle, finding physical interference between the curb and bridoon mouthpieces.

“Data reveals that the poll pressure produced by the curb bit is predominantly due to tension in the bridoon reins and not the curb reins,” they said. Physical overlaying of the bridoon on the curb mouthpiece may prove worthwhile, they said.

The researchers employed the tack fitted with sensors to gather data using riders with differing levels of training and ability, using horses at the walk, trot and canter. Three styles of bit were used – a loose ring snaffle, a universal (continental gag) bit, and a Weymouth (curb).

The leveraged designs had limited effect on poll pressure because of the floating fulcrum effect of the mouth, they wrote.

“Upon tensioning of the reins, the corners of the mouth and internal tissues are stretched and compressed and do not provide the ideal fixed point of restraint that is required for the lever or pulley.

“Even if the cheek rotates, the result tends to involve the movement of the bit towards the poll, resulting in limited increased pressure on the poll.”

They continued: “Poll relief is a universal property of the action of bits of any type, but particularly relevant to the present study, which serves to counter arguments that lever bits can exert excessive pressure on the poll.”

Application of a Dual Force Sensor System to Characterise the Intrinsic Operation of Horse Bridles and Bits
Graham H. Cross, Michael K.P. Cheung, Thomas J. Honey, Michael K. Pau, Kara-Jane Senior.
DOI: http://dx.doi.org/10.1016/j.jevs.2016.01.017
The abstract can be read here

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