When it comes to evaluating horses, their movement has always been a primary focus. Now research suggests that more consideration should be given to what a horse’s posture says about his health.
“Standing is the horse’s most prevalent activity,” says Karen Gellman, DVM, PhD. “Horses stand nearly all day, but we usually only assess their locomotion.” Gellman, a veterinarian with a doctorate in animal locomotion biomechanics, became interested in equine posture while doing an acupuncture internship with Judith M. Shoemaker, DVM, more than 20 years ago.
“Dr. Shoemaker had observed that certain common problems were closely associated with horses using an abnormal compensatory posture,” explains Gellman. Specifically, horses who stand with their legs canted-in under their body are more likely to have chronic or recurrent lameness issues, including navicular syndrome, suspensory strains, hock arthritis and back pain. “This becomes a chicken-and-egg question,” Gellman adds. “Do these things make a horse stand this way or does standing this way cause the pathology?”
With a grant from the American Holistic Veterinary Medical Foundation, Gellman was able to begin investigating this question in a research setting. As part of this work, she teamed up with Andy Ruina, PhD, a professor of mechanical engineering at Cornell University.
Using principles of physics and mathematics, Ruina created a simplified model of a standing horse to explore postural stability. “Human postural stability is modeled in a similar way,” Gellman says. The equine model (simplified to 2D) consists of three elements: a “trunk” representing the back, head and neck as a horizontal line; and a single front “leg” and single back “leg,” rendered as vertical lines connecting to each end of the trunk and to the ground.
“The model—just two legs hinged at the trunk—is inherently unstable, in that the hinges can rock the legs to any angle,” says Gellman. “In the living horse, the leg-trunk connection is stabilized by muscles, tendons and ligaments. Our question was: How much neuromuscular effort would the model ‘horse’ need to stabilize its stance at different leg angles (postures)?” Ruina added theoretical springs to the model’s “shoulders” and “hips”: The stiffer the springs needed to stabilize the model, the more effort required to maintain the corresponding posture.
Using these calculations, Ruina tracked how much spring energy would be necessary to stabilize a horse’s stance across a wide range of leg positions, from canted-in (with fore hooves and rear hooves closer together) to standing square (with the hind and forelegs vertical under the body); to splayed-out (with the fore and rear hooves relatively far apart).
The researchers found that a horse must expend twice the amount of neuromuscular effort to stabilize his body in a canted-in stance versus a square stance. “We found it takes twice the amount of neuromuscular effort to stabilize a horse who is standing canted-in than one who is standing square,” says Gellman. Even a small difference in “cost of standing” can add up, since most horses are standing for long periods every day.
“A canted-in stance (abnormal compensatory posture) doesn’t just consume more energy,” continues Gellman. “It’s maladaptive to the way the equine musculoskeletal system is ‘designed.’ Standing canted-in puts abnormal stresses on the limb joints, muscles, the structures of the hooves, as well as the back.”
The model also showed that horses who stand splayed out, or “parked,” use even less stabilization effort than horses standing square. “We see horses assume a splayed-out posture naturally when challenged with a heavier load,” says Gellman, “like mares in late pregnancy or horses pulling a laden cart. And you have probably experienced something similar yourself—you instinctively take a wider stance when lifting something heavy. However, most feral and healthy domestic horses usually stand with vertical—not splayed-out—limbs. While splayed-out posture is slightly less costly, vertical posture can be more versatile for daily activities— allowing rapid mobilization for escape from predators, and strengthening leg structures in the vertical orientation for peak loading during fast locomotion.”
This study didn’t address why a horse might adopt a stance that requires more energy to maintain, but ongoing research by Gellman, Shoemaker and colleague Elizabeth Reese is designed to tease out the causes of abnormal compensatory posture.
“Our clinical experience has shown that canted-in posture in horses is closely associated with dental malocclusion and distorted morphology of hoof capsules,” says Gellman. “We believe these abnormalities, along with poll-muscle dysfunctions, are sending distorted proprioceptive signals to the postural control centers of the brain. Clinically, when we fix the structural and functional problems in the upper cervical region, hooves and teeth, the horse’s posture reverts to vertical (“neutral”). This theoretical study was our first pass at showing the physical basis for this phenomenon.”
While many questions remain, Gellman says the study does support the desirability of the square stance.
“In a dressage test, they require horses to stand square for the salute. Show dogs are expected to ‘stack,’ which is a vertical stance. Clearly, a square, vertical limb stance is a sign of a healthy quadruped. If you have to force or hold a horse into a square stance, I think it’s worth exploring why.”
Reference: “Standing horse posture: A longer stance is more stable,” Biology Open, April 2022