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Exercise training improves functional capacity in aged individuals. Whether such training reduces the severity of exercise-induced muscle damage is unknown. The purpose of the present study was to determine the effect of 10 wks of treadmill exercise training on skeletal muscle oxidative capacity and exercise-induced ultrastructural damage in 6 aged female Quarter horses (>23 yrs of age). The magnitude of ultrastructural muscle damage induced by an incremental exercise test before and after training was determined by electron microscopic (EM) examination of samples of triceps, semimembranosus, and masseter (control) muscles. Maximal aerobic capacity ( V&O2max) increased 22 % after 10 wks of exercise training. The percent of type-IIA MHC increased in semimembranosus while the percent of type-IIX MHC decreased in triceps. Following training, triceps muscle showed significant increases in activities of both citrate synthase and 3-OH acyl CoA dehydrogenase. Attenuation of exercise-induced ultrastructural muscle damage occurred in the semimembranosus muscle at both the same absolute and the same relative workloads following the 10 wk conditioning period. We conclude that aged horses adap