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In this study we elucidate the interaction of physical activity with aging as regards skeletal muscle fiber distribution and size. Thirty-three male athletes and 42 normally active counterparts served as subjects. They were assigned to younger (<25.5 years) and older (>25.5 years) subgroups. Serial cross-sections from muscle biopsy samples (musculus vastus lateralis) were stained to distinguish fiber type: fast glycolytic (type IIb), fast oxidative-glycolytic (type Ha), or slow oxidative (type I). We also measured fiber diameters. A greater mean diameter of type I fibers was seen in older as opposed to younger athletes. Older controls had a smaller mean diameter of type IIb fibers than did younger controls. Athletes had a smaller mean percentage of type Ha fibers and a greater mean percentage of type I fibers than did controls. There was a greater mean percentage of type I fibers in older as opposed to younger controls, but this was not the case in athletes. Athletes may have larger fibers and a greater percentage of type I fibers at the expense of type Ha fibers. Atrophy of fibers with aging might be retarded by training, which might also reduce the age-associated rate of type IIb percentage loss and type I percentage gain.