The primary aim of this pilot study is to evaluate the feasibility and safety of four different types of intradialytic exercise (exercise performed during dialysis) programs: resistance, aerobic, combined resistance and aerobic, and flexibility (attention control). The secondary aim of the study is to estimate the effect of aerobic and resistance exercise, each compared with the attention control on: health-related quality of life, tests of physical performance, strength, and physical activity outside of the dialysis unit. The results of this study will be used to inform the planning of a full-scale efficacy study.