Bridging the gap: Implementing BRIDGES (Brussels Resilience and Inclusion for Depression - Guidance, Exercise and Support) to shape adolescent outpatient mental health care. A pilot randomized controlled study protocol

Physical activity interventions for adolescents with depression often show limited effectiveness due to insufficient developmental tailoring, limited autonomy support, and poor contextual feasibility. BRIDGES (Brussels Resilience and Inclusion for Depression - Guidance, Exercise, and Support) is a theory-informed and co-designed physical activity intervention integrating the COM-B model, self-determination theory and motivational interviewing to support autonomous and competence-enhancing engagement. This pilot randomized controlled trial uses a hybrid type 2 implementation-effectiveness design to examine feasibility, acceptability, implementation processes and obtain preliminary estimates of clinical effectiveness to inform a future larger-scale trial.

Sixty physically inactive adolescents aged 12-19 years with depression will be recruited from an outpatient mental health service in Brussels, Belgium. Participants will be randomized in waves using covariate-adaptive minimization with a biased-coin component (p=0.85), balancing baseline depression severity, physical activity level, gender and age. The intervention consists of 18 individualized physiotherapist-led counselling sessions delivered over 12 weeks, including structured support to facilitate transition to community-based physical activity. Primary feasibility, acceptability, implementation outcomes will be assessed using quantitative measures and semi-structured interviews with adolescents, caregivers and community partners, guided by the Practical, Robust Implementation and Sustainability Model (PRISM) and the Reach, Effectiveness, Adoption, Implementation and Maintenance (RE-AIM) framework. Preliminary effectiveness outcomes are changes in physical activity measured using the Physical Activity Vital Sign (PAVS) and depressive symptoms measured using the Patient Health Questionnaire-9 modified for Adolescents (PHQ-A). Preliminary effectiveness will be analysed using linear mixed-effects models following an intention-to-treat approach to estimate cluster-level parameters for a future fully powered Hybrid Type 2 trial. Secondary outcomes include social support, exercise self-efficacy, cardiorespiratory fitness and waist-to-height ratio. Qualitative data will be analysed using the Qualitative Analysis Guide of Leuven.