High-Speed Exercise and Bone Response

Researchers are striving to better understand the balance between tolerable and excessive bone damage.
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Bone was once considered an inert material with its structure defined by genetics. But it turns out there’s a lot more at work, explained Larry Bramlage, DVM, MS, Dipl. ACVS: “Selective breeding dictates the initial skeleton, but adaptive training in response to exercise modifies it further.” He and other racehorse surgeons are striving to better understand the balance between tolerable and excessive damage—the adaptive kind that occurs naturally and the type that sidelines animals or ends their careers.

During his presentation at the 2013 American Association of Equine Practitioners’ Convention, held Dec. 7-11 in Nashville, Tenn., Bramlage, of Rood & Riddle Equine Hospital, in Lexington, Ky., explained that bone is the only tissue capable of entirely reconstituting itself. With this capacity to change, he noted, there are several ways long bones strengthen themselves in response to training, including modeling and remodeling. Modeling is the process in which bone adds to itself, both inside and out, while remodeling is how existing bone tissue alters itself.

Bramlage started by describing the dynamic nature of bone activity on a cellular level. Two types of bone cells are involved in bone modeling and remodeling: osteoblasts and osteoclasts. Osteoblasts become trapped in the bone and become osteocytes, which are key to sensing biomechanical loads on the skeleton during exercise and directing bone tissue response accordingly. As they detect mechanical loads, they prompt additions to (formation) or reductions in (dissolution) bone mass, to achieve correct bone density for current athletic demands. Osteoclasts then tunnel through and cut canals into the bone, with osteoblasts following to make new bone.

It’s important to note that bone becomes stiffer as it ages. “This can become a problem when bone formation stops for a time and then is reinitiated due to changing stimuli,” said Bramlage. As bone loses its homogeneous structure, a distinct interface forms between new and old bone that can promote failure, similar to the interface of two types of pavement in a patched pothole. “Bone trains in a stair-step fashion with adaptations being work-specific," Bramlage stressed. "Overloading results in microfractures that stimulate bone to strengthen (by way of osteoblasts creating new tissue to fill and repair the microfractures), but if there is inadequate repair in the face of exercise, damage accumulates

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Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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