Dr. Hilary Clayton – Before she became the McPhail Dressage chair in Equine Sports Medicine at Michigan State University’s veterinary College, Dr. Clayton taught many of Canada’s equine practitioners at the University of Saskatoon. During her 15 years in Canada, she cultivated a keen interest in equine biomechanics, which led her to become one of the world’s top experts on biomechanics in dressage horses. She has been a featured speaker at many international conferences, including the Global Dressage Forum and the USDF’s FEI Trainers Symposium.

As Dr. Clayton points out in the introduction to her research paper, choosing the right bit for a horse has always been a matter of trial-and-error based on anecdotal evidence. In 2005, Clayton set up an experiment to either prove or debunk some of the most commonly held theories about how different mouthpieces work in the horse’s mouth. Previous studies had used cadaver heads placed on a table, rather than live horses, which limited the value of the results.“Since gravity affects the position of the mouthpiece, bit position is more accurately represented when the horse’s head is oriented as it would be during exercise.”

Clayton’s research also revealed that the horse’s tongue plays a crucial part in bit action, “so measurements made in vivo are preferred to cadaver measurements.”

The Horses and the Bits

Eight horses were used in the study: four Warmbloods and four Thoroughbreds, all of which had basic dressage training. Clayton pointed out in the paper that a significant factor in obtaining meaningful results was that when rein pressure was applied to the subjects, they responded as trained horses would, going ‘on the bit’ and, therefore, revealing how the bits sat as they would in training conditions.

The six bit types helped to underline some of the key differences in how different mouthpieces act in the horse’s mouth. The most common bits were a single jointed loose ring snaffle and a KK Ultra double jointed lozenge bit. Also used in the experiment was the unique Boucher snaffle, which attaches to the headstall with a separate ring at the end of a short shank. Three Myler bits – the Low Port Comfort Snaffle, the Ported Barrell, and the Correctional-ported Barrell – were the three other bits used.

How It Worked

The horses were placed in stocks at the MSU college facility, and were effectively ‘driven’ by their trainers, who stood behind them and held long reins that were run through the top ring of a longing surcingle. Measurements were taken both on a loose rein and with rein pressure. Radiographs taken during the study revealed exact positions of the bits in the horses’ mouths.


At Rest

When she analyzed the data of all six bits at rest, Clayton discovered several key points. The two more common snaffles sat farther away from the cheek teeth than the ported Myler bits. With all six bits, when there was no rein tension there was also no contact between the bit and the bars on the lower jaw. “The tongue appears to act as a cushion, elevating the cannons from the bars,” she writes. Clayton uses the term ‘cannons’ to describe the part of the mouth piece that attaches to the rings and either to each other or to a central piece, as with the KK bit.

The study also revealed that the space between the centre of each bit and the soft palate didn’t vary much from the single jointed bits to the KK and Myler bits, which are promoted as bits that eliminate palate pressure. The Boucher bit sat at a very different angle to the other five bits, due to the fixed position of the bit to the headstall. Clayton equated its position in the mouth to that of a full-cheek snaffle that has keepers attaching the upper cheek to the cheek piece of the bridle.

At Work

All six bits moved away from the palate when rein pressure was applied. When pressure was applied to the Boucher bit, it did not get closer to the cheek teeth; instead, because of its attachment to the bridle, there was upward pressure on the lips, as opposed to backward or downward on the bars.

One of the most surprising discoveries was that the commonly held theory of the palate action of single jointed snaffles is incorrect. Instead, the bit simply presses more deeply into the tongue.“Any tendency of the joint to protrude toward the palate was more than compensated by indentation of the tongue,” Clayton concluded.

Clayton suggests that one reason the KK bit may work for so many horses is the central link, which creates a wide, even pressure on the tongue, compared with a single jointed bit. It is important that the central link not be too wide, she noted; otherwise the joints may be placed directly over the bars. The Myler bits all moved closer to the cheek teeth than the other bits, which Clayton supposed may lead to a quieter mouth from horses that have “excessive tongue movement” with bits that lie lower on the tongue. Clayton also noticed that all three Myler bits were more embedded in the tongue than the others, “perhaps indicating relaxation of the lingual muscles.”

The Opposite Effect

It is a widely held theory that horses which tend to lean on the contact have insensitive mouths. The discovery about palate pressure from the single jointed bits, however, has led Clayton to theorize that something completely different may cause some horses to lean. If a horse has a sensitive soft palate, a single jointed snaffle at rest applies more palate pressure than when contact is applied. “The horse may learn to lean against the bit to relieve this pressure by allowing the mouthpiece to recede into the tongue,” she writes.

Clayton’s research, though academic in nature, yielded results that may explain what riders and trainers have previously experienced but not understood.