The biggest variability in riding surfaces is their depth and support. The farrier’s goal is to support the foot but also prevent the coffin bone from sinking and increasing the load on the limb. | iStock

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. Breakover is initiated by the heels leaving the ground when the toe starts to rotate further into the surface. | The Horse

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.

Biomechanics in Practice

So how do we apply biomechanics in practice? Rodney King is a certified journeyman farrier who operates North Canterbury Equine Podiatry, in Amberley, New Zealand. He focuses his practice on sport horses and therapeutic shoeing.

“I feel that very little can be achieved with the trim alone,” says King. “I trim fairly consistently on most of my horses regardless of the foot type or pathologies. I trim the heels down to the widest part of the frog and the toe down to the junction of live sole at the point I can start to feel the sole yield under firm thumb pressure. It’s a bit subjective, but I use my thumbs to feel and can get a good read on feet and sole depth with them. The dorsal wall (the front of the hoof) is dressed to remove flares and dishes. Once that’s done … it all comes down to the shoe.”

As Pfau explained, it’s the application of the shoe that adjusts the GRF and the point of force application. “The shoe will always be tailored to the pathology,” says King. “We can add elevation (at any point that requires it), increase mechanics (reduce breakover with a rocker or roll at any given area of the shoe), or we can alter ground reaction force (change the web width of the shoe so its interaction with the ground results in the desired effect on the foot). Often, the shoe will have more than one of these modifications added to fit the requirements of the pathology.”

He uses laminitis, which occurs when the laminar tissues that suspend the coffin bone within the hoof capsule become inflamed and fail, as an example. With these cases he’d try to reduce DDFT peak strain by elevating the heels of the shoe and reduce breakover by rockering the shoe.

“The shoe will usually have some kind of frog and palmar (rear-facing) support to not only support the (coffin) bone but to also help prevent the heels from sinking into the ground and increasing DDFT strain,” he explains.

Discipline Matters

It’s no surprise that farriers typically trim and shoe a Warmblood that performs high-level dressage differently than a Quarter Horse that runs barrels every weekend. Each discipline requires different movement and momentum from the horse and, to the farrier, that matters.

“In my day-to-day shoeing on straight and sound horses, I will always take into account the discipline of the horse when deciding how to shoe,” says King.

While a good trim can go a long way, King uses the shoe to support the specific discipline. “I will shoe the palmar hoof slightly differently for different disciplines,” he says. “As a rule, I would say as the speed of the discipline increases, the shorter and tighter I will fit (the shoe to) the palmar hoof.”

For example, he says dressage horses can be shod with a wider, flatter shoe and can tolerate a more supportive, fuller fitting shoe. Show jumpers might wear a similar style but without the fullness of a dressage horse’s shoes, as the chances of pulling a shoe increase due to the job.

“Endurance horses can be shod with a lighter shoe with a profile or section that suits the environment they are training in,” he says. “They can be shod with enough cover to support them through a shoeing cycle and are shod at around four-week intervals, as the shoes are generally worn out by this point.”

Not all Surfaces Are Created Equal

Arena and sport surfaces have come a long way, even in my lifetime as an aging millennial. I recall barrel racing growing up, when every arena we frequented was filled with deep, deep sand. Today, arenas can be any mixture of sand, wood products, stone dust, rubber, and synthetics.

“These surfaces offer great support and traction to horses and are far superior to older sand type arenas,” says King.

Eventers and endurance riders don’t compete exclusively in arenas, of course. Many athletes take their competitions to the field or trail. “I find that customers often ask for more traction via stud holes when competing on sand or grass,” he says. “A lot of our endurance horses will use pads and silicone packing leading up to race days, more as a preventive measure against foreign body trauma, but will remove this on race days.”

The biggest variability in these surfaces is their depth and support. The farrier’s goal is to support the foot but also prevent the coffin bone from sinking and increasing the load on the limb.

“I try to use shoes that will allow the foot to maintain a relatively flat stance during loading,” says King. “For example, if the surface is deep and deformable, I might add something that will prevent the foot from sinking at the heels. This might be as simple as adding silicone to the back half of the foot or a bar to the shoe.”

Take-Home Message

Shoes, surfaces, and sport all affect a horse’s movement. Biomechanics is the foundation of therapeutic podiatry. A thorough understanding of how the horse moves is necessary to maximize and manipulate it effectively.