Physiology of the Thoroughbred

Physiology of the Thoroughbred

The evolution of the horse’s physiology has taken a different course from that of its primate brethren: instead of “articulating” arms, legs, hands, feet and digits, the horse’s development has in some cases radically compressed these body parts – and in more cases, drastically elongated them.

The horse’s hooves, for example, are the equivalent of all our finger-nails and toe-nails compressed into four curved shells. The startling inference – that the horse, therefore, must be running on the very tips of its fingers and toes – is correct! What looks to us like the “ankle” of a horse is actually the equivalent of the large band of knuckles on our hands; what we would assume to be the horse’s knee is actually equivalent to our wrist, and the human elbow is, on the horse, all the way up in the part of the leg nearest its girth. With this degree of attenuation of simple bones (on animals which weigh several time what we do), it becomes easier to appreciate the delicacy of the racehorse’s legs (especially in areas such as the cannon bone)…and their susceptibility to soreness, injury, strain and fracture.

Some of the most common ailments affecting the horse’s lower limbs and extremities are:

Bowed Tendon

An inflammation and tearing of one of the large tendons running behind the cannon bone. Usually occurring in a foreleg, this can be the result of conformation problems, over-strenuous training, or just an unfortunate step. Though a bowed tendon can mean the end of a horse’s racing career, some horses can return after a long lay-up.

Bucked Shins

Inflammation of the tissue covering the front of the cannon bone (most often afflicting the front legs, and most often occurring in young horses). Immature bones under too much stress (i.e., racing two year-olds) will often result in bucked shins. Fortunately, younger horses heal more readily, and this injury can usually be cured with rest.


Pieces of bone broken off of the knee or ankle (usually from racing stress). If chips remain attached, they may not interfere with the action of the horse’s leg, but can be extremely painful and usually require removal by arthroscopic surgery. A racehorse usually recovers from surgery for chips if identified and operated on early (although in some sales catalogues, the discovery of chips constitutes grounds for returning the horse).


When used in medical (vs. racing) terminology, this indicates a painful thickening in the ligament below the point of the hock, causing inflammation and lameness.

Quarter Crack

Under stress, or if improperly shod, the hard substance of the hoof (similar to the human fingernail) can crack and become a source of pain – sometimes including the development of an infection in the exposed soft tissue underneath. This ailment can be corrected with a fiberglass or epoxy patch, and shoeing.

Sesamoid Fracture

The sesamoids are two small, delicate bones located at the back of the fetlock, held in place only by ligaments.

Slab Fracture

A break in the knee whereby the “slab” of a carpal bone splits and the front part becomes detached. This can often be repaired surgically. While a slab fracture does not necessarily mean the end of the racehorse’s career, it is a serious injury.

Splint” (a.k.a. “Popped Splint”)

Injury to one or both of the two “splint” bones on either side of the cannon bone (and/or injury to their connecting ligaments). This condition is most commonly caused by concussion with a hard surface, and is indicated by a hard swelling below the knee or hock. The condition can be cured with rest.


Large ligaments running from the top end of the back side of the cannon bone (and knee or hock) down to the sesamoids and the pastern bone. These are among the most stressed of all tissues in the racehorse’s body, and are therefore one of the most common sites of injury.

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