The investigators will study the effects of an supevised exercise intervention along the duration of intensive treatment (neoadjuvant \[solid tumours\]/intense chemotherapy \[leukemias\], expected median duration 5-6 months) on several health-related variables. Participants will be recruited from 4 hospitals in Madrid (Spain). Inclusion criteria: male/female aged 12-19 years, newly diagnosed--or relapse of-- a malignant extracranial tumour; not having received any type of therapy--except surgery--at the time of diagnosis; adequate health status (Karnofsky/Eastern Cooperative Oncology Group scale score ≥50/≤2); to understand Spanish language and to provide written informed consent. The investigators will recruit ≥136 participants and conduct a randomised controlled trial. The intervention group will follow a supervised concurrent exercise program from the start to the end of intensive treatment. The exercise intervention will be performed in the hospital gymnasium or in the patients' ward (during neutropenic phases), and will also include supervised online sessions. Additionally, the intervention group will perform include inspiratory muscle training. The following outcomes will be assessed at baseline (diagnosis), end of treatment, and at 3-month follow-up in all participants: echocardiography-determined left ventricular function (primary outcome); and other echocardiography-determined variables, cardiac biomarkers, blood pressure, blood lipids, body composition, physical activity levels, energy intake, cardiorespiratory fitness, muscle strength, functional mobility, health-related quality of life, cancer-related fatigue, clinical variables, and potential biological underpinnings of exercise multisystemic benefits (metabolic and inflammatory markers, plasma proteome, gut microbiome, and immune function).