Dr. Clayton is a graduate of the University of Glasgow, and currently holds the Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University. Her research interests are in the area of equine sport sciences, especially locomotion and conditioning of equine athletes, and the interaction between horse and rider.
The USEA funded a study on bits and their position and movement in the mouths of different horses.
One of the reasons for the study, was to examine Myler bits which currently are not legal for dressage competitions, along with three legal bits.
Results of the study indicate that the type, style, and size of bit cannot be determined from the outside of the horse, but by measurements of the inside of the mouth.
The part of the mouth between the front teeth and the back teeth is a place without teeth called the diastema. Dr. Clayton relates that the ratio of the size and length of the diastema to the length of the horse's lips, can add an additional bit-fitting element, and feels a horse with short lips can be more challenging to fit.
Inside the horse's mouth, the tongue takes up the internal cavity and lies against the palate. There is no empty space between the tongue and the palate, therefore the bit is displacing either one.
The following are the numbers obtained from measuring a number of horse mouths:
Length of Hard Palate: 71 to 86 millimeters
Height of Hard Palate: 33 to 44 millimeters
Height of the Roof of the Mouth: 20-32 millimeters
They determined that the size of the mouth cannot be determined by the size of the horse, and there is no correlation between palate height and horse height.
Oral cavities can vary a great deal and are unpredictable.
Measurements of tension (T) and non-tension (NT) were taken of six bits, three legal for competition (single jointed-snaffle, KK, and Boucher) along with three Myler bits; Level 1, 2 (port), and 3 (port).
Snaffle 8.6 T / 5.2 NT
KK Ultra 11.1 T / 5.6 NT
Boucher 6.3 T / 4.6 NT
Myler I 9.5 T / 7.6 NT
Myler II 8.3 T / 5.6 NT
Myler III 7.7 T / 4.6 NT
The KK Ultra featured the most distance from bit to palate without tension, while the Myler I featured the most distance with tension.
The distance of the bit from the premolars was measured with and without tension. All the bits moved closer to the teeth and the KK Ultra provided the most space without tension, while the plain snaffle and Boucher provided the most space with tension.
Also measured was the angle of the mouthpiece to the cheekbone: "In the ported bits, there was a marked increase in angle with tension, but it was the Boucher that showed something very different. Because of the way in which the hanging cheek piece functions, it angles up sharply into the mouth, creating negative angles - a very different sort of action than the other bits."
Fluoroscopy was used to capture images of the bits inside the horses' mouths. Further report to follow