When you buy a new horse trailer, chances are you'll also get lots technical information about the “fatigue life” of mechanical parts like the shocks or the clamp to close the hitch. That fatigue life refers to how long these parts can be used—opening and closing, absorbing shock, clamping, or whatever they do—before they break.
Recently, researchers began studying a far more important fatigue life for horse owners: that of racehorse bones. Specifically, a team of Australian researchers looked at the subchondral bone—the bone lying under the cartilage at the joints—of racehorse legs to determine how well that bone stood up to repeated pressure.
“Every step a horse takes causes a tiny bit of damage to bones,” explained Sandra Martig, DrMedVet, PhD candidate and researcher at the Faculty of Veterinary Science at The University of Melbourne. “But nature knows about this, and the bones can repair the damage. That’s why in normal life people and horses don’t break their bones just from doing everyday things.”
However, intense training programs can get ahead of nature, Martig said. “Because horses in the wild don’t gallop as much as racehorses do, bones are not made for racing, and racehorses’ bones sometimes are unable to deal with the damage that accumulates during galloping,” she said. She estimates that as many as 80% of racehorses worldwide experience subchondral bone injury during their careers.
Fortunately, progressive training can usually help nature along in the repair process, making bones stronger and more resistant to subchondral injury, Martig said.
To study racehorses' subchrondral bone fatigue life, the team took subchondral bone tissue specimens from 23 horses that had either died or been euthanized on racetracks in Victoria, Australia. They placed those specimens in a laboratory machine (termed a “material testing machine”). The team applied pressure to the bone repeatedly until the bone finally gave way and crumbled. Using the data from the material testing machine, Martig and colleagues investigated the relationship between the horses’ time in training and what they call “bone stiffness”—meaning how well the bone specimen resists compaction. Under pressure, the bone specimen will shorten slightly (a fraction of a millimeter). This “stiffness” test is easy for scientists to use, she said, and it can be used with fewer horses to get reliable results.
While it’s possible that the different training histories of the 23 horses in her study could have caused different levels of resistance to the laboratory crushing machine, Martig and colleagues found—to their surprise—the training history of the horses made no difference in the stiffness value.
She cautioned that the findings could be in part due to the wide range of meanings of the phrase “in training.”
“Some horses spend time on the walker; others do a lot of slow and a little fast galloping work, or the other way around,” Martig said. “This variability in training intensity could explain why it is difficult to demonstrate the effect of ‘time in training’ on bone stiffness.”
Studying the subchondral bone’s fatigue life is a first step in a much larger project, Martig explains. Working with a biomechanical and engineering group, she plans to create a computer model for understanding the relationship between training and bone health in racehorses, which can lead to concrete training recommendations for trainers, she said. For the moment, the fatigue life found in Martig’s study is expressed in “crushing machine cycles,” which doesn’t mean much to trainers and breeders. But as the project continues, those cycles will mean a lot to the researchers preparing useful recommendations for the field.
The study, "Compressive fatigue life of subchondral bone of the metacarpal condyle in thoroughbred racehorses," will appear in an upcoming issue of Bone.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.