Lessons from analyzing sheep gait sheep in the Texan summer

“One thing that we’ve learned but never actually implemented is to not do it in the middle of summer in Texas,” jokes Aaron Henry, a PhD researcher at Texas A&M University’s Department of Multidisciplinary Engineering. He’s describing a 2021 motion capture project he co-led, under the supervision of Dr. Andrew Robbins and Dr. Michael Moreno, that saw a team of researchers and veterinarians analyze the gait of 20 often uncooperative sheep in a barn in 100-degree Texas heat.

While the project will sound like a worst-case scenario for many movement analysis practitioners, the research team hopes that it will advance the study of hypophosphatasia (HPP), an extremely rare metabolic bone disease that causes abnormal development of the skeleton and teeth. They also hope to develop a model similar to Plug-in Gait, which can be used for future quadruped studies.

“Dr. Gaddy and Dr. Suva, who are the principal investigators studying HPP, have a granddaughter who has hypophosphatasia, so they’re invested in this project from a personal standpoint,” explains Henry. “The disease is super rare and presents itself with extreme variability, so it’s difficult to study or even to put a cohort together. Even when you do have a cohort, the subjects have varying presentations of the disease.”

“In people with the same gene mutation, the symptoms can range from females who will lose their teeth post-menopause to, if you have the most severe form of it, something incredibly tragic like kids who can’t breathe without a respirator and can barely walk,” explains Jordan Ankersen, a Clinical Research Engineer and recent PhD graduate in Biomedical Engineering at Texas A&M, who co-led the project with Henry.

While the rarity and variability of hypophosphatasia make studying it in humans difficult, the condition can also be modeled in animals, offering alternative research routes.

Dr. Gaddy and Dr. Suva previously looked into using small animals to study the disease but found that they didn’t offer sufficient variety in the condition’s presentation. Mice don’t lose baby teeth, their bones form and remodel differently from humans and the mouse models did not show the altered gait of humans with HPP. As a result, large animal models, in particular sheep, who do have two sets of teeth, were the next step.

Despite the fact they’re quadrupeds, sheep have musculoskeletal similarities to humans that make them ideal candidates for investigating gait biomechanics. “They allow you to do much more gross locomotion biomechanics, which is particularly helpful for a disease that affects the musculoskeletal system and impacts gait,” says Henry.

Sheep with hypophosphatasia move, for example, “like Victoria’s Secret models, they cross their legs when they walk,” says Ankersen — something that corresponds with some of Dr. Gaddy and Dr. Suva’s observations of human subjects with the condition.

“Another thing was that they wiggle like snakes when they walk — the bottoms of the sheep with the mutations sway back and forth,” expands Ankersen.