| Summary: | Abstract Background This process evaluation explored the implementation and mechanisms of impact of a Park Prescription Intervention trial (PPI), including the effects of hypothesised mediators (motivation, social support, recreational physical activity [PA], park use and park PA) on trial outcomes. Methods Participants from the community were randomly allocated to intervention (n = 80) or control (n = 80) group. The intervention included baseline counselling, a prescription of exercise in parks, materials, three-month follow-up counselling and 26 weekly group exercise sessions in parks. Process evaluation indicators were assessed at three- and six-months. Implementation indicators included participation rates in intervention components and survey questions plus focus group discussions (FGDs) to understand which components participants valued. FGDs further assessed barriers and facilitators to intervention participation. To explore mechanisms of impact, linear regression was used to compare objectively measured PA between quantiles of group exercise participation. Structural equation modelling (SEM) explored hypothesised mediation of the significant intervention effects. Framework analysis was conducted for FGDs. Results Participants were middle-aged (mean 51, SD ± 6.3 years), predominantly female (79%) and of Chinese ethnicity (81%). All intervention participants received baseline counselling, the park prescription and materials, whilst 94% received the follow-up counselling. Mean minutes of moderate-to-vigorous PA/week (95% CI) differed by group exercise participation (p = 0.018): 0% participation (n = 18) 128.3 (69.3, 187.2) minutes, > 0–35.9% participation (n = 18) 100.3 (36.9, 163.6) minutes, > 35.9–67.9% participation (n = 17) 50.5 (− 4.9, 105.9) minutes and > 67.9% participation (n = 18) 177.4 (122.0, 232.8) minutes. Park PA at three-months had significant mediating effects (95% CI) on recreational PA 26.50 (6.65, 49.37) minutes/week, park use 185.38 (45.40, 353.74) minutes/month, park PA/month 165.48 (33.14, 334.16) minutes and psychological quality of life score 1.25 (0.19, 2.69) at six-months. Prioritising time with family and preferences for unstructured activities were barriers to intervention participation. Human interaction via follow-up or group exercise were facilitators. Conclusion This process evaluation showed park PA consistently mediated effects of the PPI, suggesting activity in parks was a mechanism of its effects. To optimise effectiveness, participants’ preference for prioritising time with family through family involvement and tailoring the intervention to participants’ preferences for structured or unstructured PA could be considered in future studies. Trial registration ClinicalTrials.gov NCT02615392 , 26 November 2015.
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