Just-in-Time Adaptive Interventions for physical activity: a systematic review of their public health impact and methodological quality.

BACKGROUND: Physical inactivity is a major public health challenge. Just-in-Time Adaptive Interventions (JITAIs) offer a promising digital approach to promote physical activity. However, the public health potential of JITAIs-their effectiveness and the quality of evidence supporting them-remains unclear. This review focuses on their potential for public health impact and the methodological quality of the evidence. OBJECTIVE: To systematically review the effects of JITAIs on physical activity behaviors and to critically appraise the methodological quality of the evidence from a public health perspective. METHODS: A systematic search was conducted in PubMed, Web of Science, Embase, Scopus, CINAHL, PsycINFO, and Cochrane CENTRAL from database inception to June 1, 2026. Interventional studies that employed JITAIs as the intervention strategy, measured physical activity as an outcome, and enrolled adults (≥18 years) were included. Two investigators independently screened studies and extracted data. Risk of bias for randomized controlled trials (RCTs) was assessed using the Cochrane RoB 2 tool. A narrative synthesis approach was used to present the findings. RESULTS: A total of 37 studies were included, comprising 6,939 participants. Steps were the most frequently reported outcome, measured in 64.9% of the included studies. Supportive RCTs reported effect sizes ranging from +371 to +1,327 steps/day, although exploratory meta-analysis revealed substantial heterogeneity (I 2 = 85%). For moderate-to-vigorous physical activity (MVPA), all RCTs that reported complete between-group data showed positive effects, with effect sizes ranging from +13 to +200 min/week. For sedentary time, two RCTs reported objectively measured data showing consistent direction of effect, although neither reached statistical significance. CONCLUSION: JITAIs show preliminary promise for promoting physical activity, but the evidence base is methodologically immature, characterized by substantial heterogeneity and inconsistent reporting. To realize their public health potential, future research must adopt standardized outcome reporting and prioritize head-to-head comparisons against static digital interventions to establish their true incremental value. These findings provide a clear roadmap for strengthening the evidence base needed to inform public health policy and practice. SYSTEMATIC REVIEW REGISTRATION: https://www.crd.york.ac.uk/PROSPERO/view/CRD420261432266, CRD420261432266.