Mental health issues in young people are a priority for health and social care. Surf therapy is an innovative intervention that may help address this health burden globally. While increasing evidence demonstrates the effectiveness of surf therapy, there has been limited exploration as to how it achieves its outcomes. Such theoretical exploration is important for service optimisation, monitoring and proliferation. This research aimed to adopt, for the first time, a rigorous grounded theory approach to explore underlying programme theory within the Wave Project surf therapy intervention. Participants (n = 22, 14 males and 8 females; mean age = 14 years, SD = 3.5, range 8-23) were interviewed about their intervention experiences. Data were analysed through constant comparative analysis and memo writing. Two core categories reflected mediators by which surf therapy may achieve its outcomes: "Self-Selected Pacing and Progression While Surfing" and "Creation of Emotional and Physical Safe Space at Beach". Three antecedent (linking known inputs to core categories) and three consequent categories (linking core categories to associated outputs) were also identified. These demonstrate theorised pathways from known inputs to associated outcomes within the intervention. These important findings provide plausible evidence on how to optimise the Wave Project's delivery in tackling mental health burden.
Keywords: grounded theory; mental health; physical activity; programme theory; qualitative research; surf therapy.