In recent years, there has been considerable debate regarding the use of the double bridle, particularly relating to the mechanical effect that the lever extending ventrally from the curb mouthpiece has on the forces applied to the intra-oral structures.
A newly published study has shed new light on how the bridle interacts with the horse during riding. Dr David Marlin, Dr Rachel Murray, Dr Russell MacKechnie-Guire and Dr Graham Cross measured the tension between the rider’s hands and the bit, between the bit and the cheekpiece and in the curb chain in a Prix St Georges level dressage horse ridden in a double bridle with a curb bit.
Image shows force sensors between the left upper curb ring and the left curb cheekpiece, the left lower curb ring and left rein and sensor inserted in the middle of a modified curb chain (covered by pink cohesive bandage for comfort)
One of the key findings was that even when the rider had very low contact on the bit via the reins, the horse could generate high tensions in both the curb chain and curb cheekpiece. Increasing tension on the cheekpiece increases pressure on the top of the horse’s head, whilst increasing tension on the curb chain can increase pressure in the mouth via the action on the bit. In the absence of rein tension from the rider, the increased curb and cheekpiece tensions must be generated by the horse’s own movements.
In walk, trot and canter the majority of the variation in curb chain and cheekpiece tensions could not be explained by the riders actions through the reins. This serves to highlight the complex interaction between the bridles, bits, the horse and the rider.
Having this data also allowed the authors to confirm their previous theoretical estimates of the forces acting in the mouth from the bit. The conclusion from both these studies is that forces generated by the curb mouthpiece via the reins are equivalent to the rein tension or some multiple, but not as high as some previous researchers have proposed.
Image shows curb rein, curb chain and curb cheekpiece tensions in Canter-Halt-Rein-Back-Trot transition. Values are in Newtons (N) – 10N is approximately 1kg. Red arrows indicate multiple periods when there are peaks in curb chain and curb cheekpiece tension when the rider has almost no rein tension. [1] Horse in halt – Peak in curb chain tension independent of rein tension; [2] Peaks in cheekpiece tension and curb chain tension independent of rein tension; [3] Rein tension close to zero (0.1kg), but multiple peaks in curb cheekpiece (~1.4kg) and curb chain tension (0.5kg) may be caused by horse mouth movements.
Dr David Marlin said, “This pilot study raises the possibility that even with a correctly fitted bridle, the horse can generate high forces in the mouth and on the poll when the rider has almost no contact. Whilst we would want to see aggressive riding properly dealt with by stakeholders, it’s vital that we base rules and interpretations on fact rather than opinion”
Dr Rachel Murray said, “There is clearly a need for more study in this area as this was a pilot in one horse. But it does illustrate the need for science and evidence as opposed to opinion.
Dr Russell MacKechnie-Guire said, “This pilot study reinforces the importance of careful bit and bridle selection and fitting, as well as the need for rider education and training to ensure horse comfort and performance.
Funding – The study was funded by Animalweb and the authors.
The study was approved by Hartpury University Ethics Committee URN 2021-126.
The full paper is available here: Marlin, D. J., MacKechnie-Guire, R., Cross, G. H., & Murray, R. (2026). A preliminary investigation quantifying curb rein, curb cheekpiece and curb chain tensions in a horse ridden in a double bridle with estimates for intra-oral forces. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology. Advance online publication. https://doi.org/10.1177/17543371261468845
