Researchers are always working to determine which deviations from the norm are performance-limiting, and which ones are just unsightly.

Why are horses designed so poorly?”

Many a horse owner who has been saddled with weeks of stall rest, repeat veterinary visits, or injury rehab has likely uttered this point, complaining about horses’ tendency to strain tendons, chip knees, and manifest various musculoskeletal afflictions. Sometimes we wish horses came with a foolproof plan for promoting optimal performance without injury.

Conformation standards provide the closest thing to such a blueprint; conventional wisdom holds that conformation faults—deviations from the ideal proportions of the horse’s limbs and body and relationship of these parts to one another—can increase risk of injury and decrease performance ability.

Yet, conformation ideals vary among breeds and disciplines, and there are countless anecdotes about horses that appeared to be designed by committee but excelled at the highest levels. These anomalies provoke the question: What do we actually know about equine conformation?

In this article, we will start from the ground up, looking at limb anatomy and conformation’s relationship to biomechanics, injury risk, and performance.

Andrew Parks, MA, VetMB, MRCVS, Dipl. ACVS, professor of equine lameness and head of the Large Animal Medicine department at the University of Georgia’s School of Veterinary Medicine, describes conformation as “the sum of all the pieces and parts, the size and shape, and the way they relate to form the whole,” a collection of attributes that is set once a horse is fully developed.

The horse’s limbs are uniquely adapted for speed and weight-bearing. Instead of sporting five distinct digits on each limb, like humans do fingers for grasping, horses have a single set of sturdy phalangeal bones to help carry them forward over hard ground: P1 (the long pastern), P2 (the short pastern), and P3 (the coffin bone). The metacarpal bones, or cannon and splint bones, (called metatarsals in the rear) are similarly adapted. While human hand bones (metacarpals) are much shorter than the forearm bones (radius and ulna), the horse’s forelimb is almost equally proportionate from elbow to knee and knee to fetlock. And in contrast to a person’s long upper arm, the horse has a relatively short humerus, reducing the distance between shoulder and elbow.

There is an evolutionary explanation for horses’ soft tissue structures, as well. “For a running species, it is inefficient to have a heavy distal limb,” Parks says. “That’s why the muscles (of the horse) are high in the leg, and why horses have such long cannon bones and tendons.”

According to Adams and Stashak’s Lameness in Horses, 6th Edition, “The correct alignment of the skeletal components provides the framework for muscular attachments … . There should be a straight alignment of bones when viewed from the front and rear, large clean joints, high-quality hoof horn, adequate height and width of heel, concave sole, and adequate hoof size.”

Horse people frequently consider deviations from accepted conformation standards to be faults. However, those standards might have some wiggle-room, depending on breed and discipline.

In general, Lameness in Horses lists the following as forelimb conformation faults:

Base narrow Looking at horizontal pairs of feet, the distance between the center of each is smaller than the distance between the center of each corresponding limb at the chest. In other words, if you drop a line down the middle of each front leg, the feet would land toward the inside of the lines. Because this conformation causes the horse to bear more weight on the outside edge of the foot than the inside, affected horses (commonly, Quarter Horses) might develop osteoarthritis in the pastern or coffin joints, known as ringbone.

Base wide The opposite of base-narrow, in which the distance between the center of each foot is greater than the distance between the center of each corresponding limb at the chest. These horses bear more weight on the inner edges of their feet, also predisposing them to ringbone.

Toe out The toes point away from one another, increasing limb interference during movement.

Toe in The toes point toward one another. These horses might “paddle” during movement—in other words, legs travel in an outward arc—but this fault typically does not affect performance negatively.

Palmar (backward) deviation of the carpus (calf-kneed) When viewed from the side, it appears that the knee deviates toward the back of the leg. This potentially places more strain on soft tissue structures and increases a horse’s risk of knee injury.

Dorsal (forward) deviation of the carpus (buck-kneed) The knee deviates forward when viewed from the side. Mild forms do not tend to affect quality of performance.

Medial deviation of the carpus (carpus valgus, knock knees) The lower limb appears to deviate outward from the knee, increasing strain on the ligaments and bones on the inner (medial) part of the leg.

Lateral deviation of the carpus (carpus varus, bow legs) The lower limb appears to deviate inward from the knee, increasing strain on the outer (lateral) part of the knee. This fault can be far more performance-limiting than carpus valgus.

Bench (offset) knees The cannon bone is offset toward the outside of the leg, causing the horse to bear more weight on the splint bone and possibly increasing his risk of developing splints.

Standing under in front The entire forelimb (from the elbow down) is placed too far under the body. While this flaw can predispose a horse to stumbling due to forelimb overloading, it doesn’t usually cause problems otherwise.

Camped out in front The forelimbs are placed too far in front of the body, making a horse susceptible to concussion-related lameness issues such as laminitis and navicular disease.

Short, upright pastern The pastern is shorter and steeper (greater than 54° with the ground) than normal, putting an affected horse at increased risk for sustaining fetlock and navicular bone injuries.

Long sloping pastern The pastern is too long for the limb and slopes backward from the foot rather than upward, putting an affected horse at increased risk for experiencing sesamoid, navicular, flexor tendon, and suspensory ligament injuries.

The Adams and Stashak text lists the following as hind-limb conformation faults:

Excessive angulation of the hock (sickle hock) The hock angle is less than 150-153° (normal ranges from 155-165°). This conformation places greater stress on the hock joint, predisposing horses to developing osteoarthritis—particularly of the lower hock joints, also known as bone spavin—and other performance-limiting issues.

Standing under behind The entire hind limb is placed too far forward under the horse’s body, often seen in combination with sickle hocks.

Excessively straight limbs (straight behind) These horses have a hock angle greater than 165-170°, predisposing them to problems such as bog spavin (swelling on the inside of the hock joint), upward fixation of the patella, and suspensory ligament strains and tears.

Camped out behind The entire limb is placed too far behind the horse, predisposing him to back problems, an inefficient stride, and arthritis.

Base-wide The distance between the center of each foot is greater than the distance between the center of each of the limbs at the thigh, placing strain on the inner edges of the foot.

Base-narrow The distance between the center of each foot is less than the distance between the center of each of the limbs at the thigh. This places strain on the outer part of the limbs and hooves and can cause interference during movement.

Medial deviation of the hock (cow hocks) The hocks are too close together and point toward each other, while the feet stand too far apart. Severe deviations can lead to hind-limb lameness.

Horse’s feet are prone to conformation defects as well. The Adams and Stashak text lists the following important foot flaws:

Flat feet A lack of natural sole concavity (most common in draft breeds) can lead to sole bruising.

Contracted foot or heels The foot is narrower than normal, especially the back half. This might or might not lead to lameness, but it is generally undesirable in athletic horses.

Buttress foot A swelling above the front of the hoof wall at the coronary band, often as a result of ringbone or of coffin bone fracture, is a sign of advanced arthritis.

Club foot The hoof angle is greater than 60° and might or might not cause lameness issues.

Coon-footed The pastern has a shallower slope than the hoof wall, putting strain on the flexor support structures and extensor tendon.

Thin wall and sole A hoof sole that lacks adequate depth might be prone to bruising, while thin walls might be more likely to chip.

Although researchers have confirmed particular conformation faults’ relationship with injury risk in a number of studies, Parks and Sue Stover, DVM, PhD, Dipl. ACVS, of the University of California, Davis, J.D. Wheat Veterinary Orthopedic Research Laboratory, agree there’s actually surprisingly little research confirming what horsemen and women have historically observed about equine conformation. For instance, research and opinion on what impact the degree of a pastern’s slope (generally, 45° is desirable) has on injury and performance is surprisingly varied.

“In absence of documented knowledge, the intuitive expert commands a lot of respect,” says Parks. “But every once in a while, someone will come along with new research that may refute conventional wisdom.”

So let’s review what researchers do know about limb conformation:

“Essentially, the limb is a system of levers connected by joints,” Stover says. Deviations in bone or hoof length in turn change the lever arm length and the forces exerted on the joints. The greater the load, she says, the higher the level of strain on associated structures, and the greater the likelihood for injury. For example, a long toe and underrun heel conformation creates a larger lever arm that increases loads in the limb.

In a 2004 study Anderson et al. examined conformation’s impact in racing Thoroughbreds, finding that certain traits correlated closely with injury risk. Specifically, “offset knees contributed to fetlock problems, and long pasterns increased the odds of a fracture in the front limb.”

Yet, in these same racehorses, at least one “fault” appeared to actually be a positive trait. The researchers discovered that an increased carpal angle as viewed from the front (carpal valgus/knock knees) might serve as a protective mechanism, reducing the odds of carpal fracture.

In another study, in 2006, van Heel et al. determined that conformation altered foals’ grazing behavior and, subsequently, foot wear. They observed that foals with relatively long legs and proportionately smaller heads tended to habitually extend one leg while grazing in order to reach the grass. Over time, this behavior caused uneven hoof wear and growth that, if not managed, “might lead to a predisposition to overload injuries at mature age.”

Parks, who has a long-standing interest in the equine hoof, is not convinced that we know yet what limb angles are ideal. For instance, “we think the foot-pastern axis should be straight, and there is some information to suggest it should be straight,” he says. “But some horses are more upright and some sloped. We don’t actually know what its effects are in evidence-based medicine.

“Anecdotally, a sloping foot-pastern axis equates to increased strain on the soft tissues supporting the palmar (back) aspects of the fetlock and coffin joint, and upright conformation equates to excessive concussion in the osseous (bony) structures of the distal limb,” he continues. “However, the evidence to support these assertions is very thin or absent.”

Ultimately, other factors such as riding discipline, conditioning approach, and work surface might determine or impact conformation’s importance in individual horses.

Stover believes conformation plays a role in soundness, but she also emphasizes the importance of the soft tissues that stabilize joints. “Muscles and ligaments cross more than one joint,” she says. “We tend to look at (the impact of conformation) in an oversimplified way.”

In their 2006 study, van Heel and co-authors stated, “Breeding policies aim at producing an equine athlete that performs well and ‘looks nice.’ Some of the conformational characteristics are based on aesthetic considerations only, or their influence on performance has not been well-established.”

They go on to emphasize that the goals of performing well and looking nice might be mutually exclusive in the horse. Breeders might select for the aesthetic value of a long-limbed foal with a greater height at the withers, yet the authors found that taller foals were more likely to develop uneven feet.

The researchers’ conclusion in that study brings home a critical point when considering equine conformation: Breeding for beauty might sometimes negatively affect athletic performance. Perhaps the key to evaluating and selecting for conformation is the intended purpose of the horse. Horses aren’t necessarily poorly designed; some are simply better designed for specific uses that others.