The aim of this thesis was to expand the body of knowledge on the diminished aerobic capacity of individuals with post-polio syndrome (PPS). The studies described in this thesis were based on the assumption that, besides a reduced muscle mass, deconditioning contributes to the severely diminished aerobic capacity found in many of these individuals. Deconditioning may result from two factors, the disease process itself and a sedentary lifestyle. While the disease process itself is irreversible, deconditioning as the consequence of a sedentary lifestyle may be reversed by increasing physical activity in daily life or by following exercise programs. The efficacy of a high intensity home-based aerobic exercise program was evaluated in a randomized controlled trial. The results showed that the training program was not effective in improving the aerobic capacity or reducing fatigue in severely fatigued individuals with PPS. A process evaluation exploring the reasons for the lack of efficacy revealed that, despite high attendance rates, individuals with PPS were unable to adhere to the high intensity training program on a cycle ergometer. Although participants trained around the anaerobic threshold most of the training period, the program did not result in an improved aerobic capacity as muscle function nor cardiorespiratory fitness increased. It was also shown that fatigue resistance of the knee extensor muscles in individuals with PPS did not differ from healthy subjects. Together with the lack of efficacy of the training program on muscle function our findings do not support the assumption of deconditioning of the muscles of the lower extremities in PPS.