Turning Goals into Action: How Reinforced Implementation Intentions Can Boost Physical Activity - A Randomized Controlled Trial Enhanced with Ecological Momentary Assessment

Background: While many people intend to engage in regular physical activity (PA), translating these intentions into actual behavior remains a challenge. We tested whether a theory-driven, minimally guided mobile Health (mHealth) approach using personalized, reinforced Implementation Intentions (IIs) increases moderate-to-vigorous PA (MVPA) and explored potential underlying mechanisms, including day-level intention formation, day-level intention–behavior gap, and behavioral automaticity.Methods: Participants were randomized into an intervention group (IG; N = 59) and a control group (CG; N = 59). In an individual coaching session, IG participants formed PA goals and IIs, which they revisited and reaffirmed online (‘reinforced’) every third day for 4 weeks. MVPA was measured via (1) daily self-report through ecological momentary assessment (EMA), (2) wrist-worn activity trackers, and (3) the International Physical Activity Questionnaire (IPAQ) at baseline (t0), post-intervention (t1; after 4 weeks), and follow-up (t2; 3 months after t1). Underlying mechanisms were explored through daily measurement of PA intentions, corresponding behavior, and recognition of contextual PA triggers to infer the effect of behavioral automatization.Findings: Across the 4-week intervention, the IG reported on average 17.75 more daily MVPA minutes than the CG. Wearable data also favored the IG although this difference did not reach statistical significance. On the IPAQ, both groups improved to a similar degree from baseline to post-intervention and largely maintained gains to follow-up. Potential underlying mechanisms include increased frequency of day-level intention formation, narrowed day-level intention–behavior gap, and context-triggered PA.Conclusion: Reinforced Implementation Intentions deployed in a minimally guided mHealth approach can significantly increase self-reported MVPA. EMA-based self-monitoring may have contributed to overall MVPA increases in both groups, underscoring the potential of combining reinforced IIs with frequent self-monitoring (e.g., via wearables or EMA) to strengthen and maintain behavior change.