Bulking Up, Not Adding On

Although training might make your yearling look like a bodybuilder, that physique doesn't guarantee athletic prowess. Evolutionary factors--not early speed or exercise programs--determine the amount of fast-twitch muscle horses have as adults. Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the Equine Center at the University of Minnesota, presented data from nine different studies on how young horses' muscles adapt to training at the 15th Annual Kentucky Equine Research Conference, which was held Oct. 16-17 in Lexington.

Researchers performing these studies examined young horses to see if implementing strenuous training regimes early in life would change the type and concentration of muscle fibers they had. The studies, which span the past 23 years, used different breeds of horses that were trained a variety of different ways. The depths of their muscle biopsies were inconsistent, but regardless, strong trends did emerge. Newborns across the board had a high number of fast-twitch muscles, but as horses aged, the amount of Type 1 (slow-twitch, fatigue-resistant fibers used for long-term non-strenuous work and postural support) increased. Type 2A fibers (fast-twitch, fatigue-resistant fibers used for sustained speed) also increased. Type 2B, the fast-twitch fatigable fibers that are used for bursts of speed, decreased with age.

This means that as horses age from foals to young adults, they have a decrease in the type of muscle used for speed, and an increase in the type used for endurance.

"There's a lot of influence in evolution as to what happens in foals," Valberg said, "At one year (of age), endurance becomes more important than speed."

This transformation within fiber types was consistent in both the exercised and non-exercised groups. So, whether an athlete was in an intense exercise regime or a he was a pasture potato, training didn't change the type or amount of young horses' muscle fiber. The only difference was that the exercised horses had more bulk in their muscle fibers and a greater oxidative capacity, although the fiber types were the same.

"In large part, the number of muscle cells you have a birth is the same as the number you have at the end of life," Valberg said.

Overall, Valberg says that these findings suggest that intense training regimens for young horses might not be worth the risk of injury that they pose.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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