horse moving symmetrically

When we want to see if a horse is moving symmetrically, we often focus on their gaits or the slip of the saddle. But Swedish researchers recently tested a new way to evaluate unevenness: the withers. Not side-to-side movements; rather, what up-and-down wither movement might reveal about asymmetry.

They found that, at the walk, some horses tend to drop their withers lower when bringing one foreleg forward than the other. In these initial investigations, they said, it appears that drop might have more to do with natural laterality than pathological (disease- or injury-related) asymmetry. Such information could be useful for both scientists and equestrians, the team said.

“To be able to better diagnose laterality is likely of benefit both for the trainer that tries to increase symmetry while training the horse, as well as for the clinician trying to determine if a slight asymmetry is lameness or ‘only laterality,’” said Agneta Egenvall, DVM, PhD, a professor in veterinary epidemiology at the Swedish University of Agricultural Sciences Faculty of Veterinary Medicine and Animal Science, in Uppsala.

Egenvall and colleagues measured wither drop—the difference in wither weight between left and right forelimb steps—in seven sound upper-level dressage horses walking freely on a treadmill.

They found that five of the horses consistently dropped their withers more when the left forelimb was forward, Egenvall said. One horse dropped his withers more when the right forelimb was forward, and one was essentially symmetrical.

Interestingly, she added, there appeared to be a connection between hind-limb engagement and wither drop. “We found wither drop in the early stance of walk to be related to hind-limb biomechanics, interpreted as the hind limb—the one on the same side as the more pronounced wither drop—not being as engaged. This is an interesting area for further research.”

Why horses drop their withers is a complicated question to answer, however. Forelimb stride length as well as the width between the forelimbs can affect withers height, which could explain a significant part of the differences observed, Egenvall said. But, “the asymmetry may well stem from more central parts of the body, perhaps stiffness in the shoulder area or the croup, or from difference in stiffness or movement ranges of the lower limbs, or even the hoof shape or mechanics,” she added.

Even in sound horses, it’s important to consider laterality when training horses to be straighter, at least to a certain extent, Egenvall said. “In general, we believe that training for increased straightness will also be beneficial for the locomotor health of the horse as well as for performance, in line with the opinions of riders and trainers.

“But the level at which asymmetry becomes a performance problem (if not pain-related) has really not been investigated,” she added. “Perfect symmetry may even be somewhat unnatural but could still be an important goal to strive for in order to avoid that natural asymmetries increase over time during training.”

She said they haven’t yet studied wither drop in horses with less prominent withers (such as stock breeds). Although wither shape “probably wouldn’t make a difference,” she said these breeds’ shorter strides might make wither drop more subtle and, therefore, more difficult to detect.

The study, “Biomechanical findings in horses showing asymmetrical vertical excursions of the withers at walk,” was published in PLOS One.