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Horse Biomechanics

Studies of the biomechanics of the racehorse are not new. People of all nations have been trying to breed and train the best possible racehorse for centuries. As a result, research into the running horse is one of the oldest fields of research known to man. In many ways, however, this is the reason that current research has fallen behind. Traditions, and knowledge have been past down generation to generation for so long that every aspect of horse training, breading, and racing is well established in tradition and resistant to change.

Technological advances such as high speed video analysis, and instrumented horse shoes are starting to reveal that the forces placed on each of the joints in the horses leg are phenomenal. Each hoof experiences >9kN of force applied at a rate >10m/s. Put another way, this means that the force applied to the horses hoof is somewhere around 175% of the animal's own body weight (about 900lbs plus the saddle and jockey), while the force applied to the cannon bone is 3 to 4 times that due to the lever action of the fetlock (Clanton et al. 1991, Pratt and O'Connor 1978). A ballpark estimate, therefore, is that the cannon bone (subject to fracture in catastrophic injuries) experience about 6,000lbs of pressure with every step during a race.The force placed on the cannon bone, however, is dependent both on the mass of the animal and the rate at which this mass is applied and this rate of loading is dependent on the track surface.

As the weight of the horse is placed on the leg, the force increases and the fetlock lowers, acting as a shock absorption system. The amount of flexure in the fetlock depends on how hard the track is, if some of the landing force is cushioned by give in the track then the force absorbed by the fetlock is reduced. The harder the track the more force the leg must absorb. Finally, as the horse pushes off, the load on the hoof is lifted and the stored spring force in the fetlock is returned and incorporated into the stride forward.

Although, the concept of a hard track causing an increase in the jarring of a horse’s leg as it runs is rather intuitive, the impact of the shear strength of the track may not be as obvious. Both the pressure on the front of the hoof applied by the track, causing the hoof to stop, and the pressure exerted on the back of the hood by the track as the horse pushes off is dependent upon the track's shear strength. If the shear strength is low, then the hoof slides to a halt and the stored spring energy has nothing solid to push against. This is what is referred to as a "cuppy" track because rather than pushing forward, the horse’s energy goes into throwing dirt in the air. On the other hand, if the shear force is high, then the forces on both the front and back of the hoof are high causing a substantial horizontal force (or torque) on the leg.

Therefore, both the hardness and shear strength of the track directly effect the forces exerted on the horse as it runs. Understanding these forces and keeping them within an acceptable range is essential to injury prevention.

As a horse runs, the rate of loading of the leg is dependent on three things: how fast the hoof stops, how hard the landing is, and how much resistance is provided as the horse pushes off. To break this down, the initial hoof contact with the track occurs with a relatively low load and experiences a resistive force from the track against the front of the hoof as it slides to a halt.