Dr. Hilary Clayton, a veterinarian and researcher, is a recognized expert in bit fit and action. She currently holds the McPhail Dressage Chair in Equine Sports Medicine. Now at Michigan State University, Clayton has also worked at the Western College of Veterinary Medicine, University of Saskatch-ewan, Canada, and Glasgow University in Scotland.
While at Glasgow, she performed fluoroscopic studies, which involve an X-ray device that allows the researcher to see the action of the bit within an individual horse’s mouth.
HJ: Tell us about your own interest in horses and riding.
HC: I grew up in England where I was an avid foxhunter and eventer. It was my interest in horses that led me to pursue a career in veterinary medicine. After graduation I worked in a mixed practice for a couple of years, then returned to Glasgow University to complete a Ph.D. I really enjoyed research, especially when it involved sport horses. My current position as the McPhail Chair fits my interests perfectly.
HJ: What prompted your interest in bit fit and action'
HC: While the assumed mechanisms of action of bits are described in classical equitation texts, scientific studies to investigate these suppositions had not been published.
HJ: How does fluoroscopic examination work'
HC: An X-ray machine, placed a short distance from one side of the horse’s head, sends X-rays through the head. The image is received, intensified and recorded on a video receiver/recorder on the other side of the horse.
The images are viewed on a television monitor, and the video recorder produces a permanent motion picture record of the study, which we can play back at regular speed or in slow motion, and we can freeze individual frames for detailed analysis. This study was performed 20 years ago when the technology was not as good as it is now, but it was adequate to see the movements of the bit in relation to the horse’s tongue, teeth and palate.
Radio-dense objects like metal bits, bone and teeth look black on the image, while soft tissues such as the tongue are various shades of gray. Air and open spaces are white.
HJ: How did your study setup mimic riding'
HC: The horse stood in stocks to keep his body in one place while his head and neck were free to move. Manipulations of the bit were achieved by long reins that passed from the bit through rings high on each side of a longeing surcingle to mimic the position of the rider’s hands. The trainer stood behind the horse.
HJ: Which bits and adjustments did you use in your first study'
HC: We used jointed eggbutt snaffle bits for the first study. Our “correct width” bit was slightly wider than the width at the lip corners and our “too wide” bit was 2.5 cm (1”) wider. We considered the “correct” position to be in contact with the corners of the lips with slight wrinkling, and “too low” to be 2 cm lower (each cheek piece was lengthened by one hole).
My guide for correct adjustment is that the bit should be supported by the cheek pieces at all times, especially when the reins are being used. If the cheek pieces become loose when you use the reins, or if the bit bounces up and down within the cheek pieces, it indicates that they are too long.
HJ: Where does the bit lie at rest'
HC: The tongue normally lies against the hard palate (roof of mouth). On its underside, the bit indents the soft tissues (the top of the tongue and the lips). The other side contacts the hard palate.
HJ: Riders are cautioned against the bit touching the roof of the mouth. At rest, the bit is already in contact with the palate.
HC: Yes. When the correct-width bit was adjusted correctly, its joint pointed forward and downward as it rested on the tongue. The downward angulation was more marked and the joint was closer to the tip of the tongue with the wide bit or when the correct width bit was adjusted to hang too low.
Both too-wide bits and too-low adjustments made it easy for the horse to raise the bit between his molars or to get his tongue over the bit, causing a potentially dangerous loss of control and either a training problem or a bad habit. Our subjects, and other horses I’ve observed, seem irritated by having the joint hang too low on the tongue, perhaps because its position stimulates the oral reflexes associated with eating.
HJ: What are your recommendations based on the common practice of using a thicker bit to distribute pressure over a wider area and thus have a milder effect'
HC: That’s fine if the horse’s oral cavity is large enough for the bit. Since the bit displaces the tongue at rest, a thicker mouthpiece in a horse with a small oral cavity may cause more discomfort than a thinner mouthpiece that occupies less space. Under these circumstances, the use of a dropped noseband to prevent the horse opening its mouth could further exacerbate the problem.
HJ: How much is the horse able to move the snaffle bit, and what did you discover about bit width and adjustment related to movement'
HC: By elevating and retracting his tongue, the horse can raise the bit between the cheek teeth (premolars). You can actually hear the teeth crunching on the metal of the bit when this happens. This action is facilitated when the bit is too wide or adjusted too low.
HJ: What bit action was produced by equal pressure on both snaffle reins'
HC: The bit deepened the indentation in the horse’s tongue as it was pulled back by the reins.
HJ: Riders often try to influence/communicate with one side of the horse’s mouth. You looked at the action of one rein, and the action of both reins used unevenly. What did your studies show'
HC: Generally, the effect depended on the relative forces applied to the active and opposing reins. Interestingly, we found that it was not possible to produce a totally independent effect on one side of the horse’s mouth. Instead, rein movements are transmitted through the joint to the opposite side of the bit.
HJ: In the second study, you used a full-cheek snaffle with and without keepers. What difference did the keepers make'
HC: The keepers kept the mouthpiece in a more horizontal position, and reduced the mobility of the bit in the oral cavity, making it harder for the horse to raise the bit between his cheek teeth. I think, in general, the full-cheek bit is good for horses that tend to be fussy with their mouth, and by using the keepers the bit is stabilized even more than without them.
HJ: Tell us about the other bits you studied.
HC: The mullen mouth loose ring snaffle lay higher on the tongue than the jointed bit. It was also less mobile within the oral cavity, so even though it hung higher on the tongue it was difficult for the horse to elevate and bite it.
The Dr. Bristol had very different positions depending on how the central plate was angled. If the plate lay with its front edge angled toward the palate the plate lay flat across the tongue and appeared to have a mild action. Conversely, when the front edge of the plate was angled toward the tongue the plate lay at right angles to the tongue and hard palate, indenting the tongue and pressing against the ridges of the hard palate. In this position, traditionally considered “more severe” or “upside-down,” the bit would be expected to have a more severe action.
The W or double-mouthpiece loose-ring snaffle’s lower mouthpiece hung at approximately the same height as an ordinary jointed snaffle. The upper mouthpiece was considerably higher on the tongue.
Observation of the videotapes showed that the range of movement was strictly limited and the horse was unable to elevate either mouthpiece as high as any single-jointed mouthpiece. Almost continuous movements of the tongue were observed, which were probably an expression of discomfort in our research horses.
The upper part of the cheek of the mullen mouth pelham (where the br idle cheeks attach) was superimposed on the first upper cheek tooth in the video and if the cheeks were too narrow, they could cause pressure abrasions. Use of the upper reins, as expected, moved the mouthpiece backward.
Use of both reins, interestingly, had a very similar effect. Use of the curb reins alone caused rotation of the cheeks about the mouthpiece. When a single pair of reins were used with a bit converter, there was some backward movement and only a slight amount of rotation. In other words, the curb action was slight.
The Hanoverian pelham’s high port lay at an angle of about 30 degrees to the palate, pressing against the palate when the lower reins were used. The subject horses kept their mouths open to minimize the discomfort of this pressure.
The test Kimberwick had a round cheekpiece attachment that let the cheek piece slide when the reins were applied. This minimized poll pressure. It was not possible to tell from the side view whether the small port allowed more tongue room.
Our test double bridle curb had a shallow tongue port and was of the Weymouth variety, allowing a little vertical movement of the mouthpiece on the cheeks. It is usually assumed that the curb bit rests on the tongue while the bradoon, hanging higher in the mouth, continues to lie and act similar to a snaffle used alone.
What we found was that the joint of the bradoon was supported by the mouthpiece of the curb and it pointed toward the palate rather than toward the incisor teeth. The upper cheek/branch of the curb overlay the first cheek tooth and could be seen rotating back and forth as pressure was applied to the curb reins.