The aim of this randomized controlled trial on Physical Activity and Pediatric Cancer (PAPEC) is to assess the impact of an exercise program intervention in pediatric cancer patients undergoing chemotherapy for solid tumors. The investigators hypothesized the intervention will have a beneficial effect on the study outcomes.

Sixty patients, of both sexes, children will be recruited for this trial with an age range between 4 to 16 years old, undergoing treatment for primary tumors (excluding central nervous system tumors). Each participant will be assigned randomly (with blocking on sex) to either intervention or control (usual care) group.

The intervention group will participate in combined inpatient physical training (aerobic + strength) over neoadjuvant chemotherapy. The intervention will include three weekly exercise sessions of 60-90 minutes, and will be held in child's room or in a pediatric gym specifically enabled on the aforementioned hospital, depending on the children's health status.

Primary outcomes [cardio-respiratory capacity (peak oxygen uptake), muscle strength (6RM leg and bench press, and lateral row), functional capacity ('Timed Up and Down Stairs test', and '3 m and 10m Time Up and Go tests'), physical activity levels (determined by accelerometry) and quality of life "Child Report Form of the Child's Health and Illness Profile-Child Edition", Adolescent Edition and Parents Report Form )] and secondary outcomes [immune function (immune cell subpopulations, natural killer cells' cytotoxicity), inflammatory profile (blood levels of 47 cytokines) and leukocyte telomere length] will be measured in both groups in the following time points:

(i) before the exercise intervention (immediately after diagnosis and before the start of treatment); (ii) after the exercise intervention (upon termination of neoadjuvant chemotherapy); and iii) after a detraining period (2 months after the intervention).

Inclusion Criteria:

• age range between 4 to 16 years old, undergoing treatment for primary tumors (excluding central nervous system tumors) treated in the Hospital Infantul Niño jesús and residents in Madrid community. -

Exclusion Criteria:

• age<4 years or >16 years, other tumours