Function
The primary function of the foot can be summed up in the word locomotion. A highly adapted aid to efficient locomotion, the foot absorbs concussion, stores energy in its elastic tissues, and provides leverage for muscles that insert on the bones within it.
A secondary function of the equine foot is standing support. The famous ability of horses to sleep while standing owes itself primarily to the ligamentous structures of the foot and other more proximal parts of the limb.Concussion and Storage of Energy
A large part of the mechanism for absorbing concussion depends on angulation of the joints of the limbs at the time of impact and immediately following contact of the foot with the ground. The muscles, tendons, and ligaments act as springs that absorb the shock of impact by permitting some flexion of the shoulder and elbow and hyperextension of fetlock, pastern, and coffin joints. Some of the energy of the foot striking the ground is stored as ligamentous and tendinous structures stretch; this energy is released as the foot leaves the ground. The rebound of ligaments and tendons straightens the joints and aids in lifting the foot, so that very little energy is expended on these parts of gait.
The hoof and its contents absorb concussion because of the elasticity of the hoof wall, ungual cartilages, digital cushion, and frog. As the frog strikes the ground, both the digital cushion and the frog are compressed, widening and thinning them. Pressure on the bars, the ungual cartilages, and the wall spreads the heels and forces blood out of the vascular bed of the foot. The direct cushioning effect of the frog and digital cushion is enhanced by the resiliency of the wall and the hydraulic shock-absorbing effect of the blood in the hoof. At the same time that the hoof is spread by frog pressure, blood is forced out of the vascular structures of the foot, which not only absorbs concussion but also pumps blood out of the foot and into the veins of the leg against gravity.
This pumping action of the foot is an important means of returning venous blood from the foot to the general circulation.Horses confined to a stall or otherwise immobilized for extended periods are often deprived of the pumping benefits of the active foot, with the result that tissue fluid accumulates in the distal limb. This gives the appearance of swelling, which is colloquially referred to as stocking up, in the pasterns, fetlocks, and sometimes even more proximal. The swelling, however, is not due to inflammation, is not painful, and very quickly resolves when the horse is encouraged to exercise for a time.
Flexibility of the hoof is also an important consideration in correct shoeing. For most purposes, the shoe should be nailed only as far back as the quarters so that the heels are free to expand over the top of the shoe. For the same reason, most farriers apply shoes that are slightly wider in the heel than the foot to which they are affixed. By this means, the expanding heels of the hoof are able to spread and still maintain contact with the shoe.