The primary objective of this research is to gain novel insights into the potential of physical activity in reducing fatigue, improving QoL and GI manifestations in children with IBD. The study design will be composed of two parallel groups to investigate the role of physical activity: on the one hand patients with higher exercise habits, on the other hand children with lower exercise habits. To this end, the two groups of pediatric IBD patients will undergo a 24 weeks exercise programme, adjunctive to their current treatment, quantified by a Health Smartwatch (Garmin Inc.). The primary outcomes will then be characterized by the PedsQoL-MFS, IMPACT-III, PCDAI and PUCAI questionnaires, as well as VO2-max quantification. The proposed research will confirm or refute current hypotheses about physical training suggesting an improvement in quality of life (QoL), fatigue and bowel symptoms in children with IBD. Furthermore, investigating the effectiveness on secondary outcomes including muscle strength and aerobic capacity will be a new contribution to current knowledge.

A 24 week exercise programme is designed by "Physical Activity on Prescription" (Bewegen Op Verwijzing). This multidisciplinary team develops personalized coaching programs designed to support physically inactive individuals in adopting a more active lifestyle. The intervention is characterized by professional guidance from a qualified and motivational coach, the development of a tailored physical activity plan, and continuous follow-up. The team will develop a holistic family-centered coaching trajectory for children aged six to 18 years. The intake session in which individualized physical activity goals are defined in collaboration with the child, encourages autonomy and active participation. Based on this initial assessment, a personalized coaching plan will be co-created with each child and their family, aiming to increase physical activity levels in a structured and supportive manner. The intervention will span a minimum duration of six months, in order to reduce the risk of drop-out and to promote long-term sustainability of behavioral change. All participants will participate in one supervised intake session with "Physical Activity on Prescription" (Bewegen Op Verwijzing) at baseline assessment. Furthermore, the online application UZA@Home provides digital support and guidance to patients throughout their trajectory at the University Hospital of Antwerp (UZA). Its primary aim is to empower patients by equipping them with the necessary tools to actively engage in their treatment, while ensuring high-quality care as close to home as possible. Through the patient portal, individuals receive feedback on their intervention program and are able to consult appointments or access their medical records. The primary outcomes of this trial include differences in cardiorespiratory fitness (CRF), as assessed by maximal oxygen uptake (VO₂max), in addition to patient-reported outcomes measured by the IMPACT-III, PedsQoL-MFS, and the disease activity indices PCDAI and PUCAI. Secondary outcomes encompass a range of physical health parameters, including heart rate variability (HRV), muscular strength, resting blood pressure, basal metabolic rate, aerobic capacity, and body mass index (BMI). Intramural inflammation will be quantified through fecal calprotectin levels. Furthermore, disease activity will be evaluated using intestinal ultrasonography, standard endoscopic procedures (no study specific procedures, only standard of care biopsies) and the analysis of gastrointestinal mucins. All measurements will be conducted during each physical follow-up visit at the hospital, which will occur at the beginning, mid-intervention (except for endoscopy) and after the 24 week intervention period.

Inclusion Criteria:

• Diagnose with IBD (Crohn's Disease or Ulcerative Colitis) according to the European Society for Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) Guidelines.

Exclusion Criteria:

1. Diabetes Mellitus (all types, according to the American Diabetes Association (ADA))
2. Malnutrition or Failure to Thrive, suspected or confirmed
3. Children with malignancy
4. Children with an acute phase of IBD disease activity
5. Children who are too fatigued to apply
6. Children < 120 cm, as VO2 max cannot be measured
7. Physical inability to perform a cardiopulmonary exercise test (CPET)
8. Participation in organized exercise training programs in a research setting
9. Medical contra-indications for exercise

No individual participant data (IPD) will be shared. All anonymous data will be collected prospectively in REDCap (Research Electronic Data Capture) during scheduled hospital visits at baseline, throughout the intervention period (one follow-up point), and at study completion. Patient-reported outcomes will be obtained through validated questionnaires (IMPACT-III, PedsQoL-MFS, PCDAI, and PUCAI), administered electronically in the UZA@Home application. All data will be entered into a secure, password-protected electronic database compliant with GDPR and institutional data protection policies. Any discrepancies or missing values will be addressed according to a predefined data management plan. The four researchers adhere to the 'Guide on Good Data Protection Practice in Research' of the European University Institute (EUI). Technical appendix, statistical code, and dataset available from the Dryad repository.