This project is made possible through the Pediatrics Department of the University Hospital of Salamanca, in collaboration with researchers specialized in therapeutic exercise from other clinical trials and research projects, alongside physical therapists experienced in designing, programming, and supervising therapeutic exercise programs.

Furthermore, the project addresses current healthcare needs, as a significant portion of the patient population exhibits symptoms that impair functional capacity and overall quality of life-driven both by age and the oncological disease and its treatment. Consequently, this initiative aims to implement a strength training program to foster beneficial outcomes across various aspects of these patients' health, while sharing the findings with the scientific community.

Background and Current State of the Art Childhood cancer, although relatively rare compared to adult cancer, is one of the leading causes of morbidity and mortality due to disease in the pediatric population worldwide. It is estimated that more than 400,000 cases of cancer are diagnosed each year in children and adolescents under the age of 19, with leukemias, central nervous system tumors, and lymphomas being the most common types. Despite significant advances in diagnosis and treatment that have substantially increased survival rates in recent decades, many pediatric patients experience physical and psychological side effects derived from both the disease and oncological treatments, including chemotherapy, radiotherapy, or surgery.

Among the most frequent complications are cancer-related fatigue, muscle mass loss, decreased cardiorespiratory capacity, functional impairment, and psychological alterations such as anxiety, depression, or social adaptation difficulties. These effects can significantly impact the physical, cognitive, and emotional development of children, as well as their quality of life during and after treatment.

In this context, physical exercise has emerged as a promising non-pharmacological intervention to improve the health and well-being of pediatric patients with chronic diseases, including cancer. Various studies in the adult population have shown that exercise can help reduce fatigue, preserve muscle mass, improve cardiorespiratory function, and promote psychological well-being during oncological treatment. In recent years, research in the pediatric population has begun to explore these benefits, showing positive preliminary results.

Physical exercise, when prescribed safely and adapted to the patient's clinical and functional characteristics, can contribute to enhancing functional capacity, promoting physical recovery, and fostering participation in daily and school activities. Furthermore, it has been observed that physical activity can play an important role in regulating relevant physiological processes in cancer patients, such as systemic inflammation, energy metabolism, and immune function.

Despite these promising findings, scientific evidence regarding the effects of physical exercise in the pediatric cancer population remains limited compared to that available for adults. In particular, there are significant gaps regarding optimal exercise prescription, including the most appropriate type of activity, intensity, frequency, and duration for different ages, tumor types, and treatment phases. Therefore, further research into the impact of physical exercise on children with cancer is necessary to develop evidence-based intervention programs that contribute to improving their recovery and quality of life.

Background and Current State of the Problem In recent years, interest in the role of physical exercise in pediatric cancer care has increased considerably. Several studies have shown that supervised physical activity is safe and feasible during different phases of cancer treatment in children and adolescents, and that it can contribute to improving cardiorespiratory capacity, muscle strength, and quality of life.

One of the main problems associated with cancer and its treatments in the pediatric population is the decline in physical fitness. Prolonged chemotherapy, frequent hospitalization, and the reduction of habitual physical activity promote the development of muscle weakness, loss of muscle mass, and decreased functional capacity. These alterations can have long-term consequences, including an increased risk of cardiovascular disease, obesity, and metabolic syndrome in survivors of childhood cancer.

In this regard, physical exercise-and particularly combined training that includes aerobic and strength components-has proven effective in improving muscle strength, cardiorespiratory endurance, and functionality in children with chronic diseases. In pediatric cancer patients, some studies have reported improvements in mobility, exercise tolerance, and the ability to perform daily activities following the implementation of supervised exercise programs.

In addition to physical benefits, exercise can have a positive impact on the psychological well-being of children with cancer. Participation in adapted physical activities can boost self-esteem, reduce anxiety levels, and improve mood, as well as promote social interaction and the maintenance of routines similar to those of other children their age.

Another relevant aspect is the potential effect of exercise on physiological processes involved in disease progression and patient recovery, such as systemic inflammation and immune function. Some studies have suggested that physical activity can modulate the release of inflammatory cytokines and improve the immune response, which could contribute to a better adaptation of the organism to oncological treatment [17].

However, despite advances in this field, research on physical exercise in pediatric cancer patients remains limited and heterogeneous. There are major methodological differences between studies regarding the design of exercise programs, the duration of interventions, and the variables evaluated, which makes it difficult to establish clear recommendations for clinical practice.

Therefore, it is fundamental to continue developing research that allows for a better understanding of the effects of physical exercise in this population and to establish safe, effective intervention protocols tailored to the specific needs of pediatric patients. Generating evidence in this area will contribute to integrating physical exercise as part of a multidisciplinary approach to childhood cancer care, aimed not only at survival but also at improving the quality of life and holistic development of affected children.

Inclusion Criteria:

• People under the age of 18 who have been diagnosed with cancer and are currently undergoing treatment or living with the after-effects of the disease.
• People who have not engaged in regular physical activity in the last 8 weeks.
• Performance Status (ECOG) 0-1.
• Signing of the informed consent form.

Exclusion Criteria:

• Have any contraindications to physical exercise: severe musculoskeletal disorders, severe cardiovascular diseases, uncontrolled bone metastases, and other conditions as determined by a healthcare professional.
• Patients who are unable to undergo the initial assessment tests or who have difficulty performing basic exercises.
• Patients who must discontinue treatment due to intolerance to it.
• Other circumstances, at the discretion of the investigators, that may interfere with the purpose or conduct of the study.

In this research project, the evaluators will be blinded, as none of them will know which group the participants belong to. Due to the nature of the study, the participants cannot be blinded. Furthermore, the statistical analysis will be carried out by an independent statistician who will not know which intervention group the participants are in; therefore, they will also be blinded.