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Skeletal Morphology Among Racing Thoroughbreds and Quarter Horses


By Kentucky Equine Research Staff, April 7, 2021

Do training and racing goals change the structural properties of bone among different types of racehorses? Researchers set out to answer this question using Thoroughbreds and Quarter Horse racehorses, with feral Assateague Island ponies serving as untrained controls.*

The two racing breeds chosen for this study train and race in distinct ways. While Thoroughbreds compete over long distances and around turns, Quarter Horses contest short distances in a straight line. While the elongated, tapered limbs of all horses are adapted for high speeds, artificial selection has created a morphological extreme that includes long limb bones more susceptible to fracture.+

Researchers sourced postmortem specimens of third metacarpal, or foreleg cannon, bones from a veterinary hospital, university, and the National Park Service. Bones from nine Thoroughbreds (two to seven years old), 11 Quarter Horses (two to four years old), and six Assateague Island ponies (16 to 32 years old) were used in this study. Horses were euthanized or died for reasons unrelated to the foreleg cannon bones. Researchers noted body mass and linear measurements of the bones. The structural properties of bone examined included bending strength, distribution of bone, cortical bone density, and epiphyseal bone density.

While the researchers found the basic morphology of the cannon bones similar between racing and non-racing horses, subtle differences were noted.
  • Thoroughbreds showed greater bone mineral density on the lateral condyle of the left limb, which corresponded to an area of high loading when negotiating counterclockwise turns at speed, a characteristic of Thoroughbred training and racing. A condyle is a rounded protuberance at the ends of some bones that helps shape a joint.
  • Assateague Island ponies had lower trabecular bone mineral density, but researchers felt this could be related to age because the ponies were much older than the racehorses. [Trabecular bone, also called cancellous bone, is porous bone composed of trabeculated bone tissue. It can be found at the ends of long bones like the femur, where the bone is actually not solid but is full of holes connected by thin rods and plates of bone tissue.]
  • Thoroughbreds were more rounded through the midshaft of their cannon bones than Quarter Horses, reflecting increased bending strength. Because this was noted in both Thoroughbreds and Assateague Island ponies, both of which were older than the Quarter Horses in this study, researchers speculated it might be another age-related adaptation.
In sum, the researchers concluded that “age and mechanical loading appear to affect bone structure in horses. The overall similarity in morphology between the breeds, however, may indicate evolutionary constraints—due to both natural and artificial selection—on remodeling of the third metacarpal in this highly specialized species. Such evolutionary constraints may increase the likelihood of fracture in racehorses, which are pushed to the mechanical limit when racing.”

High-performance athletes, like racehorses, require optimal nutrition to train and race successfully. Bone-remodeling processes, brought about by biomechanical stress, require nutrients to replace damaged bone with healthier, stronger bone. In addition to a high-calorie, fortified diet, performance horses may benefit from a nutritional supplement that provides key bone-building substrates. Triacton is a scientifically formulated supplement designed to improve bone density that contains a novel source of calcium and an array of other nutrients. Further, Triacton provides buffering for the stomach and the hindgut, important for racehorses, as they are often fed large concentrate meals to fuel athletic effort.

*Goldstein, D.M., J.B. Engiles, G.B. Rezabek, and C.B. Ruff. 2021. Locomotion on the edge: Structural properties of the third metacarpal in Thoroughbred and Quarter Horse racehorses and feral Assateague Island ponies. Anatomical Record 304:771-786.

+Currey, J.D. 2002. Bones: Structure and mechanics. Princeton: Princeton University Press.

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