Adapting a resistance training program for Danish physical education: teachers' preconceptions and participatory co-adaptation.

Introduction: Transferring and scaling school-based physical activity (PA) programs across educational contexts requires systematic adaptation to ensure implementation feasibility and contextual fit. This study examined Danish physical education (PE) teachers' preconceptions for resistance training and a participatory co-adaptation process used to adapt the evidence-based Resistance Training for Teens (RT4T) program for implementation in Danish lower secondary schools. Methods and materials: MEGAFiT, a Danish adaptation of RT4T, was developed using a sequential qualitative design. We conducted six online group interviews with 19 PE teachers and analysed data using the COM-B model to identify key implementation determinants related to capability, opportunity, and motivation. These findings informed four participatory co-adaptation workshops with 14 PE teachers, combining practical testing, reflection, and iterative program refinement. The resulting MEGAFiT program was structured as a six-week PE course consisting of modular, equipment-light resistance training sessions designed to integrate with PE content. Adaptations were documented using the Framework for Reporting Adaptations and Modifications-Expanded (FRAME). Results: Teachers' capability and confidence to deliver resistance training varied substantially and were closely linked to prior experience. Key implementation barriers included limited lesson time, large class size, and restricted access to facilities and equipment. During the co-adaptation process, teachers and researchers collaboratively refined the program by shortening its duration, introducing modular lesson structures, developing equipment-light exercise formats, and redesigning instructional materials to improve usability in PE settings. Conclusion: Explicitly addressing implementer preconceptions through structured co-adaptation strengthened the alignment between the RT4T program and the Danish school context. The findings illustrate how participatory adaptation can help tailor evidence-based PA programs to everyday PE practice by aligning program structure, organization, and materials with teachers' capabilities and contextual constraints.