Light Hands, Heavy Consequences: What Rein Tension Data Cannot Tell Us About Horse’s Experience

Professor Nat Waran (NavigateWelfare)

An ancient Military Bit with long shanks and Curb action (Saumur)

In equestrian circles, a light, following contact is widely regarded as the hallmark of a skilled and sympathetic rider. When researchers measure rein tension, lower forces recorded at the hand are broadly assumed to signal less pressure on the horse’s mouth. However as responsible equestrians it’s important to question that assumption, wherever equipment using lever action is in use. The implications extend well beyond elite dressage, touching disciplines as varied as show jumping, polo, western cattle work, and reining. If we are serious about evidence-based equestrianism, it is time to examine what rein tension data can and cannot tell us, and to ask whether equipment whose original purpose was practical survival in high-stakes often in one-handed riding contexts, still belongs at the centre of modern sport.

What the Research Found

A curb bit consists of a mouthpiece, curb chain or strap and a shank, with one ring on each side of the purchase arm of the shank, and one ring on the bottom of the lever arm of the shank. A recent study comparing rein forces across three configurations, namely a snaffle bit, a curb bit, and a double bridle, found no significant difference between the combined rein forces recorded with the double bridle and those measured on a snaffle rein. When the curb and bridoon reins were assessed separately, both registered lower tension than the snaffle alone. On the face of it, this might appear reassuring for riders. The research suggests that the double bridle, compulsory at Grand Prix level dressage under FEI rules, would seem no more demanding on the horse's mouth than a simple snaffle.

But rein tension recordings and the rider's own ‘hand experience’ tells us little about what is actually happening inside the horse's oral cavity, and for the horse. The curb shank introduces mechanical leverage, amplifying the force transmitted to the bars, tongue, and through the curb chain to the chin groove relative to the tension that the rider perceives through the reins. External force measurements do not capture this leverage effect or the resulting intraoral pressure. A low rein tensiometer reading in the presence of curb action may, paradoxically, indicate that the horse is responding to a higher level of oral pressure, precisely because the leverage is doing the work before the rider is even aware of the magnitude of force being applied.

Understanding Leverage: An Everyday Analogy

The physics of leverage bits are straightforward, and they can be illustrated with something familiar from everyday life. Imagine using a spanner to loosen a stubborn bolt. With a short handle, you must apply considerable force to generate enough torque to shift it. Switch to a spanner with a handle three times as long and the same bolt yields with a fraction of the effort. The force experienced at your hand is much lower, but the mechanical effect at the bolt is identical, or greater using the longer lever. You feel less resistance; the bolt experiences the same turning force regardless.

Now imagine that the bolt cannot move freely, and that your ability to sense resistance through the handle is imprecise. You might assume the bolt is barely engaged, while in reality it is being subjected to substantial torque. This is analogous to the situation with a leverage bit. The shank acts as the lever arm and the mouthpiece becomes the fulcrum. The ratio of the upper shank, known as the purchase, to the lower shank, the lever arm, directly determines the multiplication factor. A standard curb bit produces a force ratio of approximately 1:3, meaning that three times as much pressure as applied through the rein, is generated at the chin groove and bars of the mouth. A rider using a long-shanked bit may therefore feel very little in the hand while the horse experiences substantially amplified pressure on highly sensitive oral tissues.

What was the original purpose of these bits?

To understand why leverage bits one needs to understand the contexts in which they were designed and used. Their origin was not competitive sport but functional necessity, often in circumstances where a rider's life, their livelihood, or the lives of others depended on the ability to respond rapidly whilst mounted and via one-handed control of an animal in unpredictable conditions.

Equestrianism in the military cavalry is the clearest example. The mounted soldier needed both hands free, one for a weapon and one for a shield, lance, or firearm. The horse had to be controlled with a single hand responding to the lightest possible signal in the chaos of battle. A leverage bit allows precisely this: a small movement of the wrist can produce a meaningful response without requiring the rider to release their weapon or adjust their grip. The double bridle, is a refinement of this military tool, allowing nuanced communication through two sets of reins by an experienced rider.

Cattle work and ranch riding have similar requirements. The working cowboy needed a free hand for a rope. Roping cattle demands that the horse respond to weight shifts and leg aids as much as rein cues. Western bits, were designed for horses trained so that the rein was carried in a loose loop, barely touching the bit ring. The bit was a communication tool of last resort, not a constant point of contact. A horse ridden in this tradition was expected to work largely from seat, weight, and leg, with the rein reserved for refinement. The severity of the equipment was matched by the expectation of lightness and the depth of training required before such a bit was ever introduced.

Polo also requires this sort of control. The polo player must strike a ball at speed, change direction sharply, and jostle for position against other riders, all while managing a highly reactive, fit pony at full gallop. One hand is occupied by the mallet throughout. The bits used in polo are designed to produce rapid upward head responses and deceleration from a single-handed signal. Again, the context was defined by the demand for quick, one-handed control in high-pressure, unpredictable situations.

The common thread is that: leverage bits were developed for situations where two-handed, nuanced contact was physically impossible, where speed of response was critical, and where the horses involved were working in fast moving and unpredictable situations. These were not tools designed for the controlled environment of a horse arena. They were tools designed for function in the field.

When Context Changes but Equipment Does Not

Modern competitive equestrian sport operates in a fundamentally different context. The dressage arena, the show jumping course, and the reining pen are controlled environments. The rider has two hands available. There is no enemy, no cattle, no ball to strike. The horse is not asked to carry the rider into unpredictable terrain at speed whilst its rider fires a weapon. The entire purpose of training, as judged in competition, is to demonstrate refined, willing and light responsiveness.

Dressage in particular, claims to judge harmony, lightness, and the willing cooperation of the horse. The FEI definition of the ideal in dressage speaks of the horse as appearing to do of its own accord what is required. And yet the double bridle, developed for the military need to control a horse with one hand in the middle of combat, remains compulsory at the sport's highest levels. This seems at odds with the what is being judged. The equipment was invented for conditions of maximum disruption and minimum contact; it is now used in conditions of maximum control and maximum contact, where the rider has both hands, both reins, and is actively judged on lightness of their communication/aids, harmony and lack of tension.

In polo and show jumping, the functional argument is more defensible: both disciplines do involve rapid changes of direction, high-speed decision-making, and, in the case of polo, genuinely one-handed riding. But the escalation of bit complexity and restriction in modern show jumping, driven by the demands of hyper-technical courses, raises questions about whether mechanical force is substituting for training quality, and whether the regulations related to bit choice are keeping pace with the evidence on oral discomfort and injury.

Leverage Bits Across Disciplines: The Evidence on Oral Injury

The research on bit-related oral trauma spans multiple disciplines, and the findings are concerning, regardless of bit type.

Dressage and the double bridle. Researchers have found that oral lesions had a prevalence of 45.5% in competition dressage horses, with researchers concluding that mouth opening during ridden work is a behavioural indicator of oral lesions. A further study found that dressage horses at higher competition levels showed lesions more frequently, suggesting that the demands and equipment of advanced training do not protect horses from oral trauma. The FEI Sport Forum 2025 included vigorous debate about double bridles and whether tongue discolouration should be treated as a welfare concern, with what appeared to be considerable reluctance among attendees to address the issue directly. 

Research examining event horses immediately after a cross-country phase found that 52% had acute oral injuries, with lesions most commonly located on the inner aspect of the oral commissures. The rise of hyper-technical course design in show jumping has coincided with increasing use of combination bits, lever mechanisms, and restrictive nosebands in order to achieve the rapid, precise responses demanded by modern courses.

Whilst bits that operate through lever action pose a significant risk to horse health and welfare, it’s important to note that leverage bits do not hold a monopoly on oral injury risk. Oral lesions have a high prevalence across all disciplines and bit types, and the problem is compounded by the fact that injuries occur inside the mouth where they frequently go undetected.

The Horse's Experience: What the Data Doesn’t Capture

The oral cavity is not a blunt-force structure. The bit exerts pressure on a number of highly sensitive tissues, including the lips (corners), buccal mucosa, tongue, the bars of the lower jaw and, at times the chin. Depending on bit type, pressure may also be transmitted to the hard palate and the base of the second premolars. These are richly innervated tissues, and the tongue in particular plays a protective role by distributing pressure across a broader surface area. When a lever bit engages the curb chain and rotates the port, this protective function is compromised in ways that external rein tension measurements cannot quantify.

Research has demonstrated that behaviours indicating oral discomfort, including mouth opening, tongue retraction, tongue protrusion, head tossing, and resistance, are present in bitted horses and largely absent or less prevalent in horses ridden bit-free. Many equestrians do not recognise these behaviours as indicators of oral pain, they become normalised in some cases, which means that the scale of the problem is routinely underestimated. When horses exhibit these responses and are then managed with tighter nosebands, stronger bits, additional equipment such as tight martingales or stronger schooling rather than investigation of the underlying cause, further potential harm is imposed on the horse.

The role of the trainer/rider is key to the size of the risk to the horse. A 2024 study found that a small number of trainers at a competition accounted for a disproportionate share of horses with oral ulcers, suggesting that training and riding practices, play a significant role.  At the same time, awareness itself appears to be a meaningful intervention: in a cohort of ponies examined in successive years, there was a significant reduction in ulcer prevalence between examinations, suggesting that when riders and trainers are made aware of the problem, they take steps to reduce the causes and the incidence of resulting damage.

Training as the Test, Not Equipment as the Proxy

The original justification for leverage bits in competitive settings has always rested on tradition and convention rather than welfare evidence. The argument that the double bridle demonstrates a horse's training and the rider’s skill, or that the curb bit rewards lightness in western competition, deserves scrutiny in light of the growing research evidence related to conflict and pain related behaviours, responses of horses to bits and nosebands, actual intraoral pressure and injury data, and horse welfare assessment results. If the training is genuinely of the quality that these competitions claim to reward, the key question is - why is a mechanism that relies on exerting significant pressure on the horse’s mouth and head, required to demonstrate that training outcome?

A horse that is truly collected, truly ‘through’, and working in self-carriage should not need a leverage mechanism to maintain these qualities. The ideal, has always been that the equipment should be nearly superfluous: the horse should carry itself, and the rein use should be barely closed. When a leverage bit is regularly engaged in the course of a competition test, or when the horse's responsiveness to it is the product of sensitivity to pain rather than trained understanding, the equipment is not demonstrating quality of training. It is substituting for it.

This is not an argument for the wholesale abolition of certain bits in all contexts rather, it’s for an honest assessment with purpose and context. Equipment designed and then used for the battlefield or the cattle range, carries a specific mechanical and functional legacy, but with the evidence on intraoral pressure, oral tissue injury, and behavioural indicators of pain now sufficiently substantial, continuing to use such equipment in competition arenas and to treat rein tension at the hand as a proxy for the horse's experience, is hard to defend.

Conclusion

The study of rein tension has made a valuable contribution to the evidence base for equine welfare. But it measures only what the rider feels, not what the horse experiences. For leverage bits, the gap between those two things is not a minor methodological nuance: it is the entire mechanism by which the equipment works. A lower rein tension in the presence of curb action or a long shank is not evidence of reduced oral pressure. It may be evidence of the opposite.

The equestrian community has the knowledge, the research infrastructure, and the welfare frameworks to move beyond proxy measurements and into a more complete account of the horse's experience. Doing so requires acknowledging that some of the most normalised and deeply traditional pieces of equipment in the sport carry a specific historical context that no longer matches the setting in which they are used. The arena is not the battlefield. The competition horse is not the cavalry charger. The standards we apply to equipment should reflect the world we are actually in, not the world in which the equipment was invented.

Key References

·         Murray RC et al. Comparison of rein forces and pressure beneath the noseband and headpiece of a snaffle bridle and a double bridle. Study comparing three rein configurations in high-level dressage horses at collected walk, trot, and canter. Published findings referenced in MacKechnie-Guire R, presented at BEVA Congress 2025.

·         Uldahl M, Clayton HM. Lesions associated with the use of bits, nosebands, spurs and whips in Danish competition horses. Equine Veterinary Journal. 2019;51(2):154-162.

·         Christensen JW, Uldahl M. Oral behaviour during riding is associated with oral lesions in dressage horses: a field study. Presented at the 19th International Society for Equitation Science (ISES) Conference, New Zealand, 2024. Reported in: Eurodressage. Oral conflict behaviour is a potential indicator of oral lesions in dressage horses. October 2024.

·         Uldahl M, Clayton HM. Pre-competition oral findings in Danish sport horses and ponies competing at high level. PMC. 2022. doi: 10.3389/fvets.2022.844700.

·         Tell A et al. The prevalence of oral ulceration in Swedish horses when ridden with bit and bridle and when unridden. Equine Veterinary Journal. 2008;40(7):635-641.

·         Tuomola K et al. Oral dimensions related to bit size in adult horses and ponies. Frontiers in Veterinary Science. 2022. PMC9133790.

·         Mellor DJ. Mouth pain in horses: physiological foundations, behavioural indices, welfare implications, and a suggested solution. Animals. 2020;10(4):572. doi: 10.3390/ani10040572.