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

Many lay equestrian texts describe the use of curb bits in horses, but understanding of their mechanism of action is limited with respect to the complex interaction between the curb bit, the horse and the rider. Forces within the rein-bridle system may be generated by the rider and or by motion or movement of the horse. Understanding the interaction between the curb mouthpiece, the reins, the cheekpiece and the curb chain is an important step towards understanding the interaction with the horse. This study aimed to quantify the forces on the reins, the bridle cheekpieces to the horse’s occiput and the curb chain. A high-level dressage horse was ridden in walk, trot and canter, and in transitions between gaits. A double bridle was equipped with load cells in the left curb rein, left cheekpiece, and centre of the curb chain and synchronised with video (100 Hz). In walk, trot and canter, curb rein and curb chain tensions were of similar magnitude, and of lower magnitude than curb cheekpiece tensions. Peaks in curb cheekpiece tension (>10 N) frequently occurred in the absence of increases in rein tension, indicating that the horse can increase cheekpiece tension in the absence of rider-created rein tension. Using the principles of classical static mechanical equilibrium, intra-oral forces were estimated using the tension data for the curb chain, cheekpiece and rein when in canter. These findings show that it is possible for increased cheekpiece tensions to be created by horse interaction with the bit in the absence of rider rein forces.