Many older athletes are capable of endurance performances equal to those of young runners who have higher maximal O2 uptakes (VO2max). To determine whether this is a result of differences in skeletal muscle characteristics, gastrocnemius muscle biopsy samples were obtained from eight master athletes [aged 63 +/- 6 (SD) yr] and eight young (aged 26 +/- 3 yr) runners. The young runners were matched with the master athletes for 10-km running performance and for their volume, pace, and type of training. Despite similar 10-km run times, VO2max was 11% lower (P less than 0.05) in the master athletes. Fiber type distribution did not differ between groups, with both groups having 60% type I and very few type IIb fibers. Succinate dehydrogenase and beta-hydroxyacyl-CoA dehydrogenase activities, however, were 31 and 24% higher in the master athletes compared with the matched young runners, whereas lactate dehydrogenase activity was 46% lower (all P less than 0.05). The capillary-to-fiber ratio was also greater in the master athletes; however, capillary density was similar in the two groups, because of the master athletes' 34% larger (P less than 0.05) type I fibers. These differences in skeletal muscle characteristics may explain the master athletes' ability to perform as well as some young runners despite having a lower VO2max.