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Dr Caroline Benoist, Manager of Research and Education, The Academy at Neue Schule
When it comes to the field of “bits and bitting” – what we like to call “lorinery science” there appears to be a fair amount of misinformation and a general lack of understanding in regards to the action of bits. The current gap in rigorous scientific knowledge within the bitting industry has severe ramifications for equine welfare, ranging from a lack of knowledge that results in the oblivious use of severe bits, to the use of escalating severity in bits in an attempt to cover up training faults.
The bit is a major concern for equine welfare when it comes to tack due to injuries caused by mouthpieces, ill-fitting or inappropriate bits which – when the horse reaches a sufficient pain threshold- can cause the horse to seek evasions such as raising the head which causes the back to extend and creates problems throughout the horse (Le Simple, et al. 2010) and can also cause more localized injuries such as ischemic (blue) tongue, and severe tissue injuries. This holds true for the school horse, through to the FEI level horse. According to a recent study (Horseman, et al. 2016), lack of knowledge is perceived to be a root cause of poor horse welfare. The lorinery industry must therefore bear the responsibility of educating the equestrian industry in regards to bits and bitting in an effort to improve the welfare of the horse.
By laying down the foundations and intricacies of lorinery science we can help not only enlighten the rider to make informed choices when choosing an appropriate bit, we can also educate and inform committees to help them decide on the legality of bits by basing it on peer-reviewed research. This can only begin with a fundamental understanding of the field of lorinery science – where sound science is trickled down to the lay-person through (revised) rule books and popular literature that can give reliable advice. We can begin our education with a persistent misconception, one that is taught in the Pony Club manual. I am referring to the French-link and Dr. Bristol mouth pieces where the received wisdom is that the former is ‘kind’ and the latter, ‘severe’. By looking more closely at the situation it can readily be seen that in fact the situation is reversed. It is the narrow edge of the French-link that presses into the horse’s tongue and not that of the Dr. Bristol.
This becomes clear only when one understands the action of the bit under rein tension and examines the features on the mouthpiece. By simply observing the central plate of the French-link mouthpiece it can be seen that this feature aligns parallel to the bore axis, that is the where the cheek-pieces attach to the mouthpiece – in the case of a loose ring bit this would be the axis of the hole through which the rings slide – therefore we can say that the angle between the two is zero degrees. This means that the plate will rotate in alignment with the bit. By now looking closely at the Dr. Bristol mouthpiece, which when it is correctly designed has the central plate angled at 135 degrees to the bore axis, it should be clear that when the bit rotates under rein tension that the final position of the feature will be very different to that of the French-link. It is in fact the opposite, with a more formal analysis we can calculate the central plate of the Dr. Bristol to lie within 10 degrees of the surface of the tongue – in other words the broad surface of the link lies flat onto the tongue when it is under rein tension, which is exactly what Dr. Bristol intended.
Until this is accepted by governing committees, and it is changed in the Pony Club manual and exams, which continue to force students to mark down the incorrect response, this misconception will remain pervasive to the detriment of our horses. The Dr. Bristol can be fitted two ways. In the instance that the Dr. Bristol is fitted correctly, with the plate lying flat on the tongue, we find that the loops are pressing into the tongue. Whereas an incorrectly fitted Dr. Bristol emulates a French-link bit and the flat edge of the plate presses into the tongue. Neither situation is particularly desirable for the horse and this is why we must learn to educate ourselves on the precise mechanism of action for bits and learn to understand how we can determine what is pressing on the horse’s tongue – particularly if we are going to be tying the mouth shut.
It is important to also understand the relevant structures of the mouth before considering the precise mechanism of action of the bit. The tongue occupies the majority of space in the oral cavity and covers the bars thus protecting them – unless the tongue is retracted in which case the bars will be exposed to the bit. The palatine arch is narrow and the distance between it and the tongue is very small. The distance between the upper and lower jaws is also very small. On average it is 34 mm and this can vary between the left and the right side, it can be as low as 25 mm and if we consider yet another common myth, the one that says a thicker mouthpiece is kinder than a thinner one then we need to appreciate that some mouthpieces have a diameter of 23 mm – only 2 mm less than space allows.
Upon placing a bit inside a horse’s mouth the tongue becomes compressed and has nowhere to go other than to the bony floor of the mouth, so it should not be a surprise to as to why some horses retract their tongues or stick them out. Our response to that is to tie their mouths shut thereby causing further damage inside the mouth. This practice can also result in crush injuries to the blood vessels and nerves that run nearby and in severe cases it can lead to localized ischemia.
Now that it is understood how a bit without rein tension affects the tongue, we can look at what happens when the bit is placed under rein tension. With 2kg of rein tension one can see the bit is translating along the tongue towards the teeth while causing further indentation and slight bulging of the tongue behind the bit. Under 4kg of rein tension the bit is seen to move even closer to the teeth, and indentation and bulging of the tongue are increased. These are typical rein tensions for every day riders however rein tension values well above these values have been observed and therefore we should consider the effects that this might have inside the horse’s mouth, and then reflect on what is happening when nosebands and flash straps are severely over-tightened.
Let’s now look at the shape of a mouthpiece. Simple physics dictates that thicker objects with large radii generate lower pressure by providing a larger weight-bearing surface, while objects with small radii or narrow points will produce high and localized pressure. Designing a mouthpiece with a large weight bearing surface will distribute the pressure more evenly and create very little of it. By applying Young’s Modulus, which measures the extent to which an object compresses based on its elasticity, we can estimate the extent to which a feature on the mouthpiece compresses the tongue. Depending on the thickness and orientation of the cannons, a compression of nearly 50% of the tongue tissue is measured. In the case of flatter and broader cannons, tissue compression can be reduced to a mere 16%. Note that these values were calculated under 500g of rein tension and therefore do not resemble normal riding circumstances during which the full thickness of the tongue tissue is compressed.
One last thought in terms of bitting to consider is poll pressure. Before considering poll pressure however, we need to understand the concept of poll-relief which is what occurs when the hotly debated baucher bit is used. There will always be some amount of poll-pressure when the bit is hanging from the cheek-piece and the cheek-piece is tensioned. This can be set to zero and then any forces through the reins onto the poll can be measured using strain gauges. Because the baucher cheek does not have a lever action – there is no lever – we see a poll-relief effect up to approximately 3kg of rein tension. This is because the bit is lifting – or translating- along the tongue as the mouthpiece rotates in an anticlockwise manner thereby lifting the cheek-pieces and creating the poll-relief effect. At rein tensions beyond 3 kg cheek-piece pre-tension is re-established.
Levered bits do produce poll-pressure, the amount of which depends on the length of the shanks (the longer the shank the more leverage that will be produced). Based on simple mechanical principles we would expect a 7 cm Weymouth to put 1.5 times the rein forces onto the poll; so that with increasing rein tension the poll pressure would increase 1.5 times. Fortunately, we do not see this simply because there is no perfect fulcrum on which the lever can act, and this was first observed in 1907 by Frank Swales, inventor of the Swales bit, when he realized that the lips of the horse stretch. The result is therefore a significant attenuation of the lever effect. Furthermore, we have observed that beyond 1 kg of rein tension no further poll-pressure is produced when a properly fitted curb chain is in use. The curb chain should prevent the Weymouth from rotating further than 45 degrees, which typically comes into effect when 1 kg of rein tension is applied where after any further amount of rein tension will be distributed among the tongue, lips, poll and now chin groove of the horse.
One final thought is the idea of poll pressure without rein tension. We see occasionally see this in the case of double bridles where can clearly see low poll pressure values at low rein tensions. However, with the addition of the bridoon we can see significant poll pressure at very low rein tension values. This is likely caused by the bridoon acting on the curb, which we call trapping, and sends conflicting signals to the horse when the curb rein is not in use.
· Caroline Benoist, Effects of Bits – here