Inclusion Criteria:  newly diagnosed pediatric cancer or relapse aged 6-18 years between the first and third cycle of chemotherapy diagnosis and treatment at the Kinderklinik München Schwabing, Department of Paediatrics and Children's Cancer Research Centre, Technical University of Munich German language informed consent as documented by signature  Exclusion Criteria:  Any medical contraindication with respect to a high-intensity interval exercise intervention (e.g., cardiovascular complications, comorbidities, bleeding risk, pain, infection, nausea, orthopedic impairment of the lower extremities) inability to follow the procedures and understand the intervention and assessment of the study, e.g., due to cognitive impairment, language problems, psychological disorders etc.