Enabling sustainability through co-adaptation: a process evaluation of a school-based physical activity program

Schools represent strategic settings for adolescent health promotion across multiple domains, including physical activity (PA). Despite the development of numerous effective programs, sustaining their impact within the school context remains a key challenge. This process evaluation explores how mechanisms, participant experiences, and contextual factors shaped the design, implementation and potential sustainability of the replicated Sigue la Huella (SLH) program (originally designed to increase PA and reduce sedentary time) in a new secondary school in Spain. Grounded in the Replicating Effective Programs framework and co-creation principles, the intervention was co-adapted and implemented over a three-year period through an iterative, collaborative process. The approach emphasized contextual relevance, gradual transfer of responsibility, and long-term sustainability. Participants (ntotal = 43) included teachers (i.e. school implementers), the school principal, students, parents, and policymakers. Data were collected through field notes, focus groups, and interviews. Using the Framework Analysis, findings were organized into thematic matrices capturing stakeholder perspectives across co-adaptation and three implementation phases. While the core structure of SLH was retained, delivery strategies were adapted to reflect school priorities and constraints. Key enablers of sustainability included strategic planning, flexible implementation, stakeholder empowerment, and institutionalization. However, challenges such as training needs, coordination gaps, and competing initiatives persisted. In essence, the processes of co-adaptation, accompanied by a staged withdrawal of research implementation support, have been found to foster school ownership, high-fidelity delivery, and institutionalization creating optimal conditions for autonomous sustainability while recognizing context-dependent challenges.