The Science of Symmetry


Are horses innately “sided,” or do asymmetries originate in some other way?

By Hilary M. Clayton, BVMS, PhD, Dipl. ACVSMR, FRCVS
Images courtesy of Dr. Hilary Clayton

Reprinted from the May/June 2023 issue of USDF Connection.

One of the goals of dressage training is to develop the horse’s straightness. In dressage, straightness implies not only correct body alignment on straight lines and through turns, but also equality of impulsion, bending, and contact on the left and right sides.

Yet neither animals nor people are completely symmetrical, either structurally or functionally. Some scientists even question whether it’s desirable to try to create symmetry when our natural state is asymmetry. With regard to the locomotor system, however, it seems sensible to equalize weight-bearing and forces on the left and right sides of the body. If a horse habitually carries more weight on one limb than the other, for instance, the overweighted limb might be predisposed to develop a repetitive-strain injury.

There are several reasons that a sound horse may move asymmetrically. For example, an old injury that resulted in the formation of scar tissue during healing may restrict the range of motion. In this article, though, I want to discuss a specific type of asymmetry that arises in the cerebral cortex in the brain, referred to as laterality.

In humans, laterality gives rise to the preferential use of one leg, one hand, one eye, and one ear. Many other species—including mammals, birds, amphibians, and fish—have been shown to exhibit laterality. Sometimes, the majority of individuals within a species conforms to the same side preference. In others, each individual has a distinct left or right preference, but the numbers preferring left or right are fairly equal.

Laterality in horses is a subject on which a lot of people have opinions, but there is not yet much science to support or refute those opinions.

Let’s start by considering laterality in humans and how it might apply to horses. You use your preferred or dominant hand for tasks requiring manual dexterity, such as writing, hammering, or brushing your teeth. But this criterion is difficult to apply to horses because their single digit (the hoof) is not designed for dexterity.

For the legs, the situation is a little different. In humans, the dominant foot is the one that would be chosen to perform a task requiring dexterity, such as kicking a ball. More relevant, however, is the fact that the non-dominant leg provides stable support while the dominant leg is being used for another task.

In horses, both the forelimbs and the hind limbs are used for locomotion, which makes them more similar to human legs. Taking this idea a step further, the horse’s leg on the side that’s used for balance may be stronger. For example, if your horse always paws with the same foot, this might be his dominant limb, while the other limb provides support. This rationale would fit with the common belief that every horse has a strong side and a weak side, and that the strong and weak sides are associated with the horse’s crookedness pattern.

FIGURE 1. A foal showing markedly offset forelimbs during grazing. Notice how the withers are lowered in this position to allow the foal to reach the ground to graze.

Selective breeding has produced refined sport horses with long limbs and small heads, but as a result, some foals can’t easily reach the ground to graze. To solve the problem, a foal may bend its knees, straddle its forelimbs to the side, or stand with one front limb forward and one back. A study of 23 warmblood foals in the Netherlands found that by the time foals were six months old, almost half showed a preference for always placing the same forelimb forward when grazing (Figure 1). Five foals habitually grazed with the right limb forward, and six grazed with the left limb forward.

This habitual stance laid the foundation for future asymmetries. The researchers found that the forward limb positioning put more weight on the heels, so as a result that foot grew into a flat shape with a low heel and long toe. The hoof that was further back had more weight on the toe and grew into a more boxy shape with long heels (Figure 2).

By the time the foals reached six months of age, in spite of regular trimming every four to eight weeks, there was an average of 4 degrees difference in the left and right hoof angles, with a lower angle in the limb that was habitually forward during grazing. We could call this an acquired conformational asymmetry. Interestingly, after the foals were weaned, they were housed in large barns and fed from raised hay racks. Within a few months, the asymmetries in hoof growth resolved, illustrating the dynamic nature of hoof growth.

FIGURE 2. Uneven hooves as viewed from behind. The left hoof is wider, with low heels. The right hoof is narrower, with higher heels.

But a follow-up evaluation conducted when the horses were three years old revealed that, in some individuals, asymmetries persist. One-quarter of the horses still preferred to graze with the same limb forward as they had done as foals—and all of those showing a stance preference had been asymmetrical as foals. These horses still had longer limbs and smaller heads, and the asymmetry between their left and right hoof angles had increased to 6 degrees.

These findings lead to the question: Could a leg-position grazing preference be an early indication of laterality? What we learned from this study is that some horses have an inherent preference in the way they use their left and right limbs during grazing, and that this preference may be associated with asymmetrical hoof angles. What is not yet certain is whether a habitual grazing stance is a sign of innate laterality. Regardless, many horses have slightly different-shaped fore hooves, with one growing a little wider and flatter while the other grows narrower and more upright. No matter the cause, regular farriery should prevent such differences from increasing over time.

Study #1: Circling.

From the dressage trainer’s standpoint, an important aspect of asymmetry relates to the horse’s tendency to perform differently when tracking right versus tracking left. A two-part study evaluated a group of young horses at nine months of age and again at two years of age. In the first phase of the study, 29 unhandled nine-month-old foals were observed in a round pen as their dams were lunged in 20 circles to the left and 20 circles to the right. On the left rein, all but three foals followed the outside rail; the other three each deviated to the inside on a single circle. Tracking right, 20 foals stayed on the rail for all 20 circles, while nine cut across the circle on all 20 circles.

After the foals were weaned, they were handled equally from the left and right sides to remove the possible influence of asymmetrical handling. For the second phase of the study, 17 of the original group were reevaluated in the round pen at age two. On the left circles, 16 horses stayed on the rail throughout, and one horse deviated across the circle on all 20 revolutions. Circling to the right, only six horses
remained on the rail throughout, with 10 horses deviating across the circle every time.

Taken as a group, the majority of these young horses tended to drift toward the outside (the right) on the left rein and to “fall” to the inside (the right) on the right rein. The tendency to leave the rail and fall to the right increased with age, becoming close to 60% by age two. We don’t as yet have data that directly connect the behavior of young horses with that of ridden horses, but horses’ tendency to fall to one side is well recognized under saddle.

Study #2: Withers drop.

A group of Swedish researchers made an interesting observation as they watched seven highly trained dressage horses walk on a treadmill. In the walk stride, the two forelimbs are on the ground together for a short period at the start and end of each step, as the weight is transferred from one forelimb to the other. During the weight transfer, one forelimb is retracted under the body as it prepares to leave the ground, and the other forelimb is forward as it accepts weight (Figure 3).

FIGURE 3. Vertical oscillations of the withers during a full stride of walk. The illustrations show the stage of the stride corresponding with the high and low points on the graph. In this horse, the withers drop lower during the weight transfer from right fore to left fore.

At the walk, the horse’s withers are normally at their lowest point during the weight transfer from one forelimb to the other. The researchers
determined that, in six out of the seven horses, the withers dropped lower during the weight transfer on one side than on the other. The difference was quite small—about an inch—but was consistent under a variety of circumstances. Of the seven horses analyzed, five dropped the withers lower when the right forelimb was retracted; one dropped lower when the left forelimb was retracted; and one was symmetrical (Figure 4).

The amount by which the withers dropped was greater when the horses were ridden, which reflects the effect of the rider’s weight on the horse’s vertical motion. In addition, a high or restricted head and neck position was associated with greater asymmetry of withers motion, whereas a long-and-low or free position minimized the asymmetry.

FIGURE 4. Body alignment of horses and corresponding withers position during weight transfer between forelimbs. Of seven horses studied, one had lower withers during weight transfer from left fore to right fore, with the haunches deviating to the left (left). One horse showed no difference in withers height during weight transfer (center). The remaining five had lower withers during weight transfer from right fore to left fore, with their haunches deviating to the right (right).

The forelimb that was retracted during the withers drop stayed on the ground longer and was retracted further than the other forelimb. The
horse’s haunches were displaced toward the side of the more retracted forelimb, with the hind limb on that side taking a shorter step. In 71% of the horses in the study, the right forelimb was retracted further and the haunches were displaced to the right. These findings are in accordance with the German Equestrian Federation’s training manual, The Principles of Riding, which states that in most horses the right hind hoof is not aligned with the track of the right forehoof; the right hind “escapes” to the side and is set down further to the right and further forward than the left hind.

Study #3: Foot and leg twisting.
Another piece of the asymmetry puzzle is provided by a study that looked at how a horse’s hooves twist against the ground when the legs are bearing weight. In this study of nine horses, both hind hooves tended to twist the toes inward and the hocks outward. This is why the hocks may be seen to swivel outward when the horse is on loose footing that allows the hooves to twist. In the forelimbs, two of the nine horses showed the same pattern, with both forelimbs experiencing an inward twist. In the other seven horses, the left front twisted inward and the right front twisted outward. In 78% of the horses studied, the right forelimb was “different.”

The number of horses examined in research studies to date is small, and it will require many more studies with larger numbers of horses before we can hang our hats on the results. Interest in laterality is growing rapidly within the scientific community, as evidenced by the recent formation of the International Task Force on Laterality in Horses. This group is dedicated to investigating laterality in horses in general and especially with regard to locomotion. We hope that this will lead to information that can help riders and trainers to understand the issues around straightness.

Meet the Expert

Dr. Hilary Clayton is the professor and Mary Anne McPhail Dressage Chair emerita. She was the original holder of the Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University’s College of Veterinary Medicine, East Lansing, from 1997 to 2014.

A world-renowned expert on equine biomechanics and conditioning, Dr. Clayton is president of Sport Horse Science, LC, which is dedicated to translating research data into practical advice for riders, trainers, and veterinarians through lectures, articles, and private consultations. A USDF gold, silver, and bronze medalist, she is a longtime USDF Connection contributing editor and a past member of US Equestrian’s Dressage Committee. In 2020 she was inducted into the Roemer Foundation/USDF Hall of Fame.

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