A 12-year longitudinal study examined age-related changes in skeletal muscle size and function in healthy sedentary men. Twelve men were studied in 1985–86 (T1) and nine were reevaluated in 1997–98 (T2). Isokinetic muscle strength of knee and elbow extensors and flexors decreased by 20–30% at slow and fast angular velocities. Computerized tomography showed reductions in cross-sectional area (CSA) of thigh muscles, quadriceps, and flexors. Analysis of covariance showed that strength at T1 and changes in CSA were independent predictors of strength at T2. Muscle biopsies revealed a reduction in type I fibers and a lower capillary-to-fiber ratio at T2. These findings suggest that quantitative loss in muscle CSA is a major contributor to age-related muscle weakness. The study highlights the importance of muscle strength at T1 and changes in CSA in predicting future strength. The results indicate that cross-sectional studies may underestimate age-related muscle function changes. The study also found that muscle fiber type distribution and capillary density changed with age, with a reduction in type I fibers and a decrease in capillary density. These findings emphasize the need for exercise recommendations to prevent sarcopenia and disability in older adults. The study underscores the complexity of age-related muscle changes and the importance of longitudinal research in understanding these changes.A 12-year longitudinal study examined age-related changes in skeletal muscle size and function in healthy sedentary men. Twelve men were studied in 1985–86 (T1) and nine were reevaluated in 1997–98 (T2). Isokinetic muscle strength of knee and elbow extensors and flexors decreased by 20–30% at slow and fast angular velocities. Computerized tomography showed reductions in cross-sectional area (CSA) of thigh muscles, quadriceps, and flexors. Analysis of covariance showed that strength at T1 and changes in CSA were independent predictors of strength at T2. Muscle biopsies revealed a reduction in type I fibers and a lower capillary-to-fiber ratio at T2. These findings suggest that quantitative loss in muscle CSA is a major contributor to age-related muscle weakness. The study highlights the importance of muscle strength at T1 and changes in CSA in predicting future strength. The results indicate that cross-sectional studies may underestimate age-related muscle function changes. The study also found that muscle fiber type distribution and capillary density changed with age, with a reduction in type I fibers and a decrease in capillary density. These findings emphasize the need for exercise recommendations to prevent sarcopenia and disability in older adults. The study underscores the complexity of age-related muscle changes and the importance of longitudinal research in understanding these changes.