horse hooves in arena

How farriers trim and shoe horses for their unique biomechanics, discipline, and riding surface

I spent some time the other day cleaning my Quarter Horse’s paddock. I watched him walk from the water trough to his hay pile, paying close attention to where and how each hoof landed on the soft ground and how he raised each heel from the muddy surface. He was diagnosed with podotrochlosis (aka navicular disease, a degenerative condition of the navicular bone and soft tissues in the back of the horse’s foot) via MRI long before I became a veterinarian, so I credit his pathologies for my eye for lameness. A rolled toe and heel elevation help ease his breakover to allow for a more comfortable existence.

How do veterinarians and farriers get and keep horses with and without conformational deformities like this happy and sound? With a simple, yet thorough, understanding of equine biomechanics.

Biomechanics Overview

The word “biomechanics” sounds scary, doesn’t it? As a veterinary school student, I thought so too. Thankfully, Thilo Pfau, PhD, a faculty member of veterinary medicine at the University of Calgary who’s spent much of his career researching performance horse biomechanics, has broken the basic concepts down for us.

Gaits, whether walk, trot, or canter, can be split into stride cycles that get repeated over and over. “For each limb, a stride cycle can be subdivided into the stance phase—the time period when that limb is in contact with the ground—and a swing phase—the time period when that limb is swinging through the air without ground contact,” explains Pfau. “The stance phase is often further subdivided into impact, support phase, and breakover.”

Impact, Support, and Breakover phases of a horse's stride.

The impact phase is when the hoof makes contact with the ground. The support phase follows, when that hoof supports the horse’s weight while the opposite hoof is in its swing phase. “Finally, breakover is initiated by the heels leaving the ground when the toe starts to rotate further into the surface,” he says. “On a hard surface, breakover ends when the toe leaves the ground. On a soft, deformable surface, it ends when the whole solar surface is losing contact with the (ground) surface. Delineating these distinct phases is less straightforward on soft ground.

“Biomechanically, the stance phase is characterized as the period of time where forces are being exerted onto the ground by the horse and following Newton’s laws of motion: Equal and opposite forces are exerted back from the ground onto the horse,” he continues. “These are the ground reaction forces (GRFs). During the swing phase there is no ground contact and, hence, no ground reaction force.”

Hoof biomechanics are fluid, and farriers and veterinarians can manipulate them using podiatry. Take, for example, the point beneath the foot where the GRF originates—the point of interaction between the hoof and the surface. “The position under the hoof changes over the stance phase, typically being rather central over much of the stance phase and then migrating dorsally toward the toe during the breakover period,” says Pfau. “The position, however, can be altered—for example, when using heel wedges or graduated shoes to translate the point of force (toward the heel) and unload the deep digital flexor tendon (DDFT, which passes around the navicular and attaches to the coffin bone). However, since the overall amount of force, which is strongly associated with the body mass of the horse and the speed of locomotion, remains unchanged, other structures such as the superficial digital flexor tendon (SDFT, which lies over the DDFT) and/or the suspensory ligament will have to compensate for the drop in load of the DDFT to keep the horse limb stable.”

For this reason, trimming and shoeing are powerful tools for altering the biomechanics of movement by manipulating the point of force application under the hoof. “However, it is vitally important to always consider the complex interaction between the different structures within the horse limb in relation to the forces acting from beginning to end of stance,” he adds.

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