Youth with physical disabilities experience restrictions to participation in community-based activities, leading to poor health outcomes. Personalized participation-based treatment approaches, implemented in the individual's natural environment, are considered recommended practice. However, it is unclear whether enhancing participation (e.g., engaging in sledge hockey) can simultaneously improve both body functions (e.g., grip, trunk control) and activity performance - two key outcomes in pediatric rehabilitation. Specifically, the impact of participation in a self-chosen real-life activity, that is meaningful to the youth, on a range of functions (motor, cognitive and affective) has not been established. Supported by Canadian Institutes of Health Research (CIHR), we first proved the effectiveness of the Pathways and Resources for Engagement and Participation (PREP) intervention in promoting youth's community participation; then, through Strategy for Patient-Oriented Research (SPOR), we successfully pilot- tested an innovative pragmatic clinical trial to improve body functions through participation, informing the design of this proposed larger study. Using the PREP, we aim to examine the impact of youth engagement in an 8-week community-based activity program (e.g., drawing, swimming) on underlying body functions or impairments (e.g., movement, attention, mood), as well as on performance of the selected activity. Specifically, change in 3 relevant body functions will be examined: motor (using the Functional Reach Test, Trunk Impairment Scale, and dynamometer), cognitive and affective (using the Behavior Assessment System for Children), and activity performance (via the Canadian Occupational Performance Measure). A 22-week interrupted time series design with multiple baselines systematically replicated across 30 youth with physical disabilities (e.g., cerebral palsy, spina bifida) aged 15 to 24 will be employed. Each activity, individually selected by the youth, will be analyzed using the task analysis approach to identify underlying body functions. These functions will be measured multiple times throughout the entire study, resulting in 90 trajectories of change in body functions (30 youth X 3 functions) and 30 trajectories representing change in activity performance. A linear model will be fit for each of the 120 trajectories. Motor-related scores will be standardized to allow comparison across different outcomes and mixed-effects models will be used to estimate an intervention effect across participants. Our interdisciplinary team includes researchers in the field of childhood disability, including occupational therapy, physical therapy and pediatric medicine. Partnered with the Centre of Integrated University Health and Social Services (CIUSSS) West-Central Montreal and CanChild Centre for Childhood Disability, as well as key local and national stakeholders, the team will collaborate throughout the research process. This novel patient-oriented study will support a paradigm shift in clinical practice and build knowledge that can guide clinicians, families and policy-makers in appraising the benefits of participation- based therapies on improving both functional capacities and performance in meaningful life activities. Describing the multiple benefits generated by one single intervention can facilitate efficient youth-engaging therapies, contributing to the provision of pediatric rehabilitation services. Findings can also enhance knowledge of pragmatic clinical trials for testing complex individual-based interventions highly applicable to practice.