BACKGROUND: Digital interventions have the potential to increase physical activity in adults with the use of few resources, but evidence of long-term effectiveness is limited. This study aimed to evaluate the effects of three digital interventions on physical activity. METHODS: 183 self-reported inactive adults (< 150 min per week of moderate to vigorous physical activity (MVPA)) aged 22 to 55 years were included in a hybrid type 1 effectiveness-implementation trial over 18 months and were randomised to three fully web-based interventions: (A) activity tracker with the personalised metric Personal Activity Intelligence on a mobile app, (B) group A+home-based online training and (C) group B+online peer support through social media. Physical activity was measured with hip-worn accelerometers (ActiGraph GT3X-BT) at baseline, 6, 12 and 18 months. Outcome measures included MVPA, light and total physical activity, steps, adherence to physical activity recommendations, waist circumference (WC), quality of life, perceived competence for exercise, self-efficacy for exercise, social support and reasons for performing physical activity. Longitudinal changes in outcomes were evaluated using linear mixed models adjusted for baseline values. RESULTS: Mean MVPA in all groups at baseline was over two times higher than the criteria for inactive and decreased from 69.7 min per day (95% CI 67.3 to 72.1) to 60.2 min (95% CI 56.8 to 63.7) through 18 months (p < 0.001). No time by group interaction was observed (p = 0.97). Similar patterns were observed for light and total physical activity (main effect of time: both p < 0.02, time by group interaction: both p > 0.59). WC increased from baseline through follow-up (all p < 0.001), but with no time by group interaction (all p > 0.15). CONCLUSION: Self-reported physically inactive adults receiving an activity tracker with a mobile app accumulated high physical activity levels at baseline but decreased their activity levels over 18 months. Adding home-based online training and peer support did not provide additional effects. TRIAL REGISTRATION NUMBER: Prospectively registered, 23 of April 2021, identifier: NCT04526, https://clinicaltrials.gov/ct2/show/NCT04526444.

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