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

INTRODUCTION: 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. METHODS AND ANALYSIS: 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. TRIAL REGISTRATION: ClinicalTrials.gov NCT07422909.