BackgroundIntervention trials that have demonstrated significant effects may not always replicate those effects when scaled up. This study aimed to test whether scaling-up a successful cluster randomized trial (the COMMUNICATE study, 9 intervention communities), which promoted population-level physical activity (PA), could promote PA in a broader citywide setting (29 communities) after two years, as a mid-term evaluation of the six-year scaled-up trial.MethodsThis is a single-arm, pre-post comparison of a multi-strategic community-wide intervention covering the entire Unnan City, Japan. The intervention for middle-aged and older people consisted of three components: information delivery, education, and support delivery. The intervention method followed the COMMUNICATE study but adapted and introduced new initiatives tailored to local resources. A baseline survey (n = 3,718) among randomly selected residents aged 40–79 years in 2016 and a follow-up survey with the same respondents two years later were conducted. The primary outcome was the change in the percentage of people who practiced the recommended levels of PA, analyzed with a generalized linear mixed model to account for clusters at individual and community levels. Additionally, we examined the dose–response relation of the effect based on the intervention doses in each community. The RE-AIM framework assessed intervention dissemination and implementation.ResultsThe two-year intervention was implemented in all communities, reaching and involving various groups. The rate of people engaged in PA significantly increased in two years (adjusted change: + 8.0 percentage points [95% confidence interval: 6.1, 10.0]). Based on the type of PA, only muscle-strengthening activity showed a significant increase (+ 11.5% points [9.6, 13.5]), whereas walking (-1.8% points [-3.6, 0.1]) and flexibility activities (+ 0.3% points [-1.5, 2.0]) did not. The increase in PA in higher-dose areas was not significantly different but slightly larger than that in lower-dose areas (+ 8.4% points vs. + 7.6% points, adjusted difference in change: 0.8% points [-3.8, 5.5]).ConclusionThe scaled-up citywide intervention promoted PA, especially muscle-strengthening activity. Collaboration with diverse organizations in different settings is crucial for multi-faceted interventions and requires balancing uncertainty in its implementation quality and quantity owing to collaborative decision-making.Trial registrationUMIN-CTR, UMIN000024682. Registered 02 November 2016, https://center6.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000028377
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