As far back as ancient Greece, horsemen have recognized the importance of the hoof, and the influence of the hoof-ground interface (on the way of going of the horse). Lacking the wings of Pegasos, real horses are subject to the shock of impact with the ground passing through their hooves and lower limbs.

Since almost those early times, humans have sought ways to mitigate the damage and maximize the efficiency of hoof-ground contact. Evidence for removable horseshoes dates to the 4th century B.C.1

Modern researchers have found a variety of factors that influence the interaction of the hoof with the ground and concussion through the limb including the composition of the surface, shoeing materials, speed of impact, angle of impact and hoof/limb shape.

How Does The Foot Interact With The Ground?

Hoof-ground interaction has three phases during which the downward velocity converts to forward momentum.

Impact. As the hoof hits the ground, hoof velocity quickly slows, coming to a stop. This deceleration depends upon the ground surface and the foot velocity.

Mid-stance. In an article in the Equine Veterinary Journal, authors Rebecca Sarah Parkes and Thomas H. Witte describe mid-stance as “a period of hoof stability.”2 They explain that during mid-stance, the limb is loaded distal-proximal (hoof-toward body) and the force is transferred through the muscles and tendons of the limb.

Push-off. More commonly known as breakover, this transition between the stance phase and the next stride occurs as the center of force shifts toward the toe. As the foot rotates, the toe pushes into the ground on a soft surface or slips slightly backward on a hard one. According to Parkes and Witte, “Properties of the foot and surface can have an impact on force in the deep digital flexor tendon and the timing of breakover.”3

How Is The Foot’s Impact On The Ground Measured?

According to University of Guelph biomechanics re­searcher Jeff Thomason, the impact of the foot hitting the ground is measured as shock using a device called an accelerometer. The slowing of the hoof as it hits the ground is called deceleration. That deceleration can be lessened by horizontal foot slip and/or the sinking of the foot into the ground. The magnitude of the deceleration depends upon the impact velocity.4

How Does Shock Spread Within The Foot And Limb?

The vibrations of shock appear to transfer from the hoof wall and sole, through the laminae and up the limb through the metacarpal bones. These vibrations are damped at the laminar junction and by the superficial and deep digital flexor muscles. To some degree, this vibration helps strengthen bone and muscle, but in excess it is implicated in bone and joint injury.5

There are three phases involved in hoof-ground interaction when downward velocity converts to forward momentum: impact, mid-stance and push-off.

How Does Gait Influence The Concussion?

According to Thomason, the decelerations vary significantly between breeds, ranging “from 20-80 [grams] for performance horses (jumpers and dressage), 60-100 g for racing Thoroughbreds, and up to 400 g for North American Standardbreds.”

How Does The Ground Surface Affect Concussion Within The Hoof?

Thomason cites ground surface as having a strong effect on concussion. He isn’t alone in this conclusion. In their article, Parkes and Witte state, “Surfaces vary in their capacity to rapidly damp vibration at impact. Surfaces may damp vibrations structurally by the deformation of elastic particles or by the friction of particles sliding past one another.”6

In a 2013 Veterinary Journal article, Nathalie Crevier-Denoix and co-authors explain that at the same speeds, different surfaces can drastically influence hoof-ground interaction and limb loading.7

Parkes and Witte state, “The firmness or hardness of a surface determines how quickly the hoof is brought to a halt and therefore helps determine the rate of deceleration at impact and the amplitude of the impact force. Stiff surfaces such as asphalt and crushed sand lead to increased amplitude and frequency of impact decelerations, while more compliant surfaces such as synthetic tracks and sawdust have a significantly reduced amplitude and frequency of vibrations at impact. On synthetic surfaces, the braking phase following impact is longer than that on sand, reducing the vibrations generated by the shock of impact.”8

How Do Shoeing Materials Change Impact And Shock Absorption?

We tend to focus on shoe materials when thinking of shock absorption. However, the difference between metals may have less effect than one might think, according to Thomason. He says there are “more significant differences between metal and plastic/epoxy-resin, which is more cushioning but less abrasion resistant.” Thomason also calls the influence of padding on shoes significant in terms of reducing deceleration force.

Simply being shod vs. barefoot can change hoof velocity, according to Parkes and Witte.

“Shoes have an effect on grip, hoof expansion on impact and the inertia of the distal limb. As a result, shod horses experience a higher peak vertical force compared with unshod horses and a higher landing velocity of the foot results in greater force at the moment of impact.”9

They point out that different shoeing materials can alter slip and impact, writing, “Variation in shoeing has the potential to vastly change the interaction between the horse and the ground and therefore alter the way of going and impact upon injury risk. The use of different shoe materials changes the slip distance seen at impact and may have an impact on jarring during foot deceleration. For example, plastic shoes significantly reduce the decelerative force experienced compared with both rubber and steel shoes on concrete.”10

How Does Shoe Weight Affect Concussion?

Weight of the shoe apparatus may influence hoof velocity, though the evidence appears less conclusive than for some other factors.

Shock vibrations transfer from the hoof wall and sole, through the laminae and up the limb through the metacarpal bones.

According to Parkes and Witte, “Small changes in weighting on the distal limb can change kinematics; stride duration and swing angle of the limb are increased and the arc of foot flight is lower. The changes in distal limb weight required to alter gait are relatively small with a 700 g weight or simply shoeing the horse with normal shoes weighing between 400 and 500 g leading to these changes.”11

However, in research done by Thomason and co-authors, published in a 2014 article in Equine Veterinary Journal. “Addition of the ballast [on the hoof] did not have a significant effect (P?0.05) on any of the variables tested.”12

How Do Trimming And Hoof Angle Impact Concussion?

Thomason says while hoof shape has some effect on impact and concussion, “it is likely to be minimal in relation to [ground surface and shoe materials].”

However, Parkes and Witte write, “The hoof represents the interface between the horse and the ground and so by altering the shape of the hoof or the type of shoe we can influence many features of the hoof-ground interaction.”13 They cite a link between flat foot contact and high limb loading and short braking period.

There are “more significant differences between metal and plastic/epoxy-resin” in shock absorption.

Additionally, they reference studies correlating long toe/low heel shape with increased risk of racing injuries. According to their article, “a low heel increases the peak force of the deep digital flexor tendon acting on the navicular bone by up to 20%, which can be reversed with heel wedges [that] move the point of force toward the wedge, mimicking hoof imbalance.” They point out that on certain surfaces, calks, grabs, studs, wide webs and lateral extensions have a similar effect.”14

Thomason and co-authors discuss the importance of hoof angle in the 2014 article in Equine Veterinary Journal. “A well-balanced hoof should land flat at the moment of impact, but toe-first and heel-first landings are other strike possibilities, with heel-first contact being the most prevalent depending on conformation, speed and footing surface. Manipulation of the toe angle of the hoof and the pastern angle can affect the manner of loading.”15

They looked at the tendency of racehorses to be trimmed to encourage a heel-first landing and found that “a heel strike is associated with the highest peak accelerations and peak frequencies. Although it is unknown whether these signals are potentially dangerous, further investigation may find that trimming to encourage this type of impact could have negative implications and contribute to bone injuries.”16

Until horses are fitted with wings, there are multiple factors influencing hoof-ground interaction to take into account. Hoof shape and angle, point of breakover, shoe material, padding, breed and type of ground surface can all play into the concussion as the horse impacts the ground.