Physical activity is considered essential for optimal health, development, socialization and well-being of children. However patients with end-stage renal disease (ESRD) are often restricted from participation in exercise activities. This is especially true for children on hemodialysis (HD). As a consequence their exercise capacity is reduced, both before, but most impressively after HD. In a nationwide randomized, multi-center design this study aims to proof the influence of an individualised endurance training program by bicycle ergometer performed during dialysis on the efficacy of HD, measured as single pool Kt/V. Secondary goals are to enhance physical performance, physical and mental well-being, and to improve measurable blood and treatment parameters (e.g. haemoglobin level, amount of medication). A positive impact of physical activity was observed in adults on HD, although most studies did not address this issue in a randomised protocol. Despite this beneficial evidence in adults, sport is still not integrated as part of standard care in patients on maintenance HD. The study protocol, developed in close collaboration with the German Sport University Cologne, differs substantially from previously published reports as it uses bicycle ergometer training in an upright position outside the dialysis couch and adapts the intensity of intervention to the patient's capabilities. Based on the expected results the investigators will develop an individualised training program to be integrated in the standard care of (pediatric) patients on maintenance HD.