The adaptability of human skeletal muscle to increased (training) and decreased (detraining) usage was studied in 11 athletes over a 42-month-long observation period. Biopsies were taken from the deltoid and the quadriceps muscle, together with measurements of maximum torque output during voluntary knee extensions at high relative to slow speeds of movement. A 16% and 14% decrease in the proportion of type I fibers was seen in the proximal arm and leg muscles, respectively, in the detraining subjects. This conformed with the changes in muscle function. On the other hand, in the training subjects, who increased their activity level through systematic daily physical training over an almost 4-year-long period, there were no significant changes seen in fiber type proportions of either arm or leg muscles. This was presumably due to the smaller net change in physical activity level caused by training as compared to detraining in the subjects of this study. Thus, the results show that fiber type proportions in intact human skeletal muscle are not exclusively determined by heredity, but may also be influenced by environmental factors, such as physical activity level.