Applying and refining the 11 for Health fidelity checklist

Statement of Contribution What is already known on this subject?• Fidelity is rarely assessed systematically in youth physical activity interventions.• BCT-based fidelity checklists exist for 11 for Health but have only been used by their developers.• Programmes adapted for new cultural contexts are seldom checked for preserved delivery. What does this study add?• A manual can sustain high delivery fidelity when facilitators lack teaching expertise.• Checklists could not show whether health messages were taught through football activity.• First use outside the developer team produced twelve refinements; checklists updated.