The anatomy of the hoof and its mechanisms of action impart unique abilities to the horse
Tough and resistant yet malleable and elastic; hard like rock yet full of life and cellular activity; streamlined and compact yet able to withstand and cushion massive forces, the equine hoof is an incredibly complex structure. It supports up to three times the horse’s weight yet is light enough to allow swift strides at a full gallop. Packed with several soft tissue types and an impressive network of blood vessels, it’s the fruit of millions of years of unique evolutionary design.
No other animal walks on a single digit, or toe, making the equine hoof a truly exceptional piece of anatomy. In this article we’ll explore the inner workings of this special body part.
Function 1: Shock Absorption
If you didn’t have shock absorbers on your car, you’d be in for a jarring ride. In fact, you’d probably step out of that car feeling pretty sore, with your body having taken on many of the forces of contact with the road. Your engine probably wouldn’t last very long, either, rattling from all the uncontrolled strain and eventually cracking and falling apart. In the event of an accident, your car’s design allows it to absorb much of the shock of impact—so your body doesn’t have to. It takes a lot of energy to crush metal and, as the laws of physics tell us, the energy (force) from any kind of impact must go somewhere and do something.
In horses the concept is similar, says Stephen O’Grady, DVM, of Virginia Therapeutic Farriery, in Keswick. The weight of the horse (and of you and your equipment) and the forces produced by his muscular effort create a substantial impact when the foot strikes the ground. When a horse pushes off the ground at a gallop, for instance, he bears as much as 2.5 times his body weight on the toe. Without shock absorption, that much force could easily snap bones, dislodge ligaments, rattle vertebrae, and shake up internal organs. (It would be a pretty painful experience for the rider, as well.)
While the musculoskeletal system is designed to absorb some of that shock—mainly through the stretchiness (elasticity or flexibility) of soft tissues and the movement of bones in the joints—a great deal of shock absorption happens in the hoof itself. Although it’s hard to the touch, the hoof capsule is made up of viscoelastic materials that allow it to deform under remarkably high pressure. Inside, soft tissue allows the hoof to change shape when it strikes the ground, spreading out, then rebounding, in a process that expends a great deal of energy and leaves little to jar the rest of the horse’s body.
Function 2: Energy-Efficient Speed
The most energy-efficient platform for moving a lot of mass at high speeds is, arguably, the wheel. Animals, of course, haven’t evolved to have wheels, but some—like horses—have developed incredibly energy-efficient legs for transporting relatively large masses over great distances at high speeds. Millions of years ago, when modern horses’ ancestors were the size of small dogs, they had as many as five toes on each foot.
Short legs require more movement and, therefore, more energy to cover similar distances as long ones. So as prey animals, prehistoric equids evolved to have longer, thinner, more lightweight legs that could take longer strides with less energy. As horses increased in size, the additional toes became, literally, a drag, explains Brianna K. McHorse, PhD, of Harvard University, in Cambridge, Massachusetts. Her work with fossil bones confirmed that as horses’ bodies got heavier, their middle toe got thicker and better able to bear forces. As a result, horses likely used their side toes less and less, to the point they were just slowing the animals down.
Evolution probably selected for stability of that middle toe, as well, she adds. Over time, horses didn’t need the side toes to keep their legs from tilting right or left during landing. The result, she says, is the current, single long toe that starts at the hock or knee and ends in the “nail”—the hoof. That smooth, resistant, lightweight, streamlined hoof gives horses their unique ability to be heavy creatures with remarkable speed.
Function 3: Communication
Every step reveals critical information the horse must know, says Catrin Rutland, PhD, PGCHE, MMedSci, SFHEA, FAS, associate professor of anatomy and developmental genetics at the University of Nottingham, in the U.K.
Rutland says she believes the hoof is constantly sending signals back to the central nervous system. “The central nervous system has to know where every part of an animal is at any one time, even when it’s standing still,” she says. “When a horse is walking, running, or jumping, the hooves quickly relay vital information about their interaction with the environment to the central nervous system so it can react and adjust the rest of the body accordingly.”
Rutland says she’s convinced that the hoof is well-equipped for that. “It looks inert but it’s actually full of nerves,” she says, and so complex that its neurons, or nerve cells, sometimes communicate directly with each other about hoof-centric issues, without going through the central nervous system (Al-Agele R. et al, 2019).
This sense of positional awareness, known as proprioception—or recognizing where your body is in space—hasn’t been proven to exist in regard to horses’ feet, however, says O’Grady. His work blocking the foot with local anesthesia has led him to believe that horses might have “very little sensation except for pain and pressure,” he says. “There’s no proof that horses experience proprioception with their feet. The structures are there, but we don’t yet know what they do.”
Alive and Active
Beneath the deceivingly simple hard outer shell of the hoof are tissues and cells with multiple functions, and they’re all very much alive, active, growing, and responsive, says Rutland.
“There’s this misconception that the hoof is made up of solid matter, with that hard shell on the outside and the hard bone on the inside,” she explains. “But within the hoof there’s a whole bunch of soft tissue, which plays a huge role in its proper function. That includes tendons and ligaments, of course, but also papillae, laminae, cartilage, blood vessels, nerves, the digital cushion, and more.”
In a healthy hoof the elastic tissues of the frog, digital cushion, and heels take on and diffuse shock to protect the upper musculoskeletal system, O’Grady says. The location of the heel makes it particularly subject to the force of impact, before the foot rotates over the toe in breakover.
In the heel area, the digital cushion helps dampen that blunt force. With its robust, flexible “padding,” it also covers and protects the navicular bone, bursa, and deep digital flexor tendon (DDFT), which extends into the foot from the leg.
The hoof also comes equipped with an efficient method to diffuse energy based on the circulatory system, O’Grady says. This “hemodynamic” system involves groups of blood vessels in the coronary band and sole and on either side of the lateral cartridges, which slope upward and backward from the wings of the coffin bone and reach above the coronary band margins. When the foot is off the ground, these “plexuses” fill with blood, he says. When the hoof strikes the ground, the structures flatten out, cushioning the blow and dissipating the energy of impact. “It’s like landing in bubble wrap,” O’Grady says.
The digital cushion is an inward extension of the V-shaped frog on the bottom of the hoof (Al-Agele R. et al, 2019), says Rutland. The frog is essential for shock absorption and also serves as a grip to prevent slipping. At the ground surface of the hoof wall, a network of soft, elastic tissue commonly called the “white line” binds the sole to the wall.
The more rigid tissue of the hoof wall also has a complex shock-absorbing design, says Christopher Pollitt, BVSc, PhD, of the University of Queensland, in Australia. If it didn’t, it would crack at every hoof strike. Thousands of long, thin, hollow cylinders called tubules make up the wall, he explains. The tubules and the “cement” that binds them to build a tough, composite wall are made of hard keratin, which has disulfide bonds known for their great physical strength, he says. The structure of the wall itself also makes it resistant. At the outer surface, the tubules are densely packed with little moisture, but they gradually become less dense with more water content toward the inner side of the wall. This intricate design is so resistant that for everyday movement, it only ever endures 10% of the force required to crack it, Pollitt says.
The hoof contains three bones. The coffin bone is suspended inside the hoof capsule by the lamellae, Rutland explains. The epidermal laminar tissues line the inner hoof wall in a layer that looks like leaves of a book, with each page branching out with hundreds of other tiny pages, all grasping onto dermal laminae reaching out from the coffin bone. Above this, the short pastern connects the coffin bone to the long pastern. Its “nearly cuboidal” shape makes it resistant to stress (Davies HMS et al, 2007). Behind these bones lies the small, smooth fibrocartilage-coated navicular bone that “has a pulleylike role,” helping the DDFT glide beneath the short pastern. The joint’s synovial fluid keeps it well-greased.
Meanwhile, each hoof step plays a vital role in healthy circulation, O’Grady says. “The veins in the equine foot have no valves, allowing the heart to push blood through,” he says. “Arterial blood comes into the foot when the horse lifts up his foot, the vessels fill with blood, and it gets utilized. So, when he puts weight on the foot, the blood in the veins coming from the thousands of capillaries across the entire solar surface of the foot get compressed, pushing blood back toward the heart.”
A remarkable thing about this structure is that it’s alive and capable of healing and regenerating—within limits, of course. Bones and ligaments can heal (often with scarring and weakening), but the rest of the hoof grows constantly, from the top down, says Pollitt. Areas in contact with the ground wear down or slough off and get replaced, and injury-related flaws in the wall grow out over time.
Keep in mind, however, that this description portrays the healthy hoof. When things go wrong—such as in times of sickness or injury, when nutrition is poor, when the horse is shod improperly or the footing isn’t managed well, or even when selective breeding favors genetically weaker hooves—the hoof fails to perform as well as designed. Lameness, in particular laminitis, is the risk associated with such an otherwise efficient design, says Pollitt.
“The tough hoof capsule protects the softer, more sensitive structures within and allows the natural horse to gallop over dry, rocky terrain with apparent impunity, but at a price,” he says. “Immobility and crippling result if the connection between hoof and bone fails.”
Resilient, aerodynamic, communicative, partially regenerating, and capable of dampening massive forces, the equine hoof makes for a highly functional structure. Its intricate single-toe design is unique in the animal kingdom. Understanding its anatomy and mechanisms of action allows us to better promote its proper function through care, management, and breeding.