Submaximal exercise training improves mitochondrial efficiency in the gluteus medius but not in the triceps brachii of young equine athletes.

We tested the hypothesis that, similar to humans and rodents, exercise training would enhance mitochondrial (Mt) biogenesis and function in skeletal muscle of young horses. Twenty-four Quarter Horse yearlings were randomly assigned to either submaximal exercise training or no forced exercise (untrained). Biopsies were collected from the gluteus medius and triceps brachii before and after 9 wk of treatment. Citrate synthase activity was lower (P < 0.0001) and cytochrome c oxidase activity per Mt unit was higher (P < 0.0001) in gluteus compared to triceps, but neither changed over the trial period. From wk 0 to 9, intrinsic Mt respiration (P CI , P CI+II ; P = 0.008) and electron transport capacity (E CI+II ; P = 0.01) increased, and LEAK-related flux control factor (FCFL; P = 0.02) decreased in both muscles. After 9 wk of training, gluteus muscle exhibited higher (P < 0.05) intrinsic P CI , P CI+II , E CI+II , and FCFCI and FCF CI+II , and lower FCFL (P = 0.0002). Mitochondrial content did not change from wk 0 to 9, and also not in response to submaximal exercise training. Improvements in Mt function were most directly related to ongoing growth of horses independent of muscle group, and training further enhanced Mt function in the gluteus medius.