BACKGROUND: Our main objective in the current study was to evaluate the long-term effectiveness (12 months from baseline) of I Move (a web-based computer tailored physical activity intervention, based on self-determination theory and motivational interviewing). To this end, we compared I Move to a web-based computer tailored physical activity intervention based on traditional health behavior theories (active plus), and to a no-intervention control group. As a secondary objective, the present study aimed to identify participant characteristics that moderate the long term effects of I Move and active plus. METHODS: A randomized controlled trial was conducted, comparing three research conditions: (1) the I Move condition, participants in this condition received I Move; (2) the active plus condition, participants in this condition received active plus; (3) the control condition; participants in this condition received no intervention and were placed on a waiting list. Main outcome measures were weekly minutes of moderate to vigorous physical activity and weekly days with minimal 30 min of physical activity. All measurements were taken by web-based questionnaires via the study website. Intervention effects were analyzed using multilevel linear regression analyses. RESULTS: At 12 months from baseline, I Move was found to be effective in increasing weekly minutes of moderate to vigorous physical activity (ES = 0.13), while active plus was not. In contrast, active plus was found to be effective in increasing weekly days with > 30 min PA at 12 months (ES = 0.11), while I Move was not. No moderators of the effects of I Move were found. CONCLUSIONS: The results suggest that web-based computer tailored physical activity interventions might best include elements based on both self-determination theory/motivational interviewing and traditional health behavioral theories. To be more precise, it is arguable that the focus of the theoretical foundations, used in new web-based PA interventions should depend on the intended program outcome. In order to draw firm conclusions, however, more research on the effects of self-determination theory and motivational interviewing in web-based physical activity promotion is needed. TRIAL REGISTRATION: Dutch Trial Register NTR4129.

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