RESUMEN
OBJECTIVES: (1) Test whether a mental imagery-based self-regulation intervention increases physical activity behaviour over 90 days; (2) Examine cognitive and affective precursors of change in physical activity behaviour. DESIGN: A randomized control trial with participants (N = 500) randomized to one of six intervention conditions in a 3 (risk communication format: bulleted list, table, risk ladder) x 2 (mental imagery behaviour: physical activity, active control [sleep hygiene]) factorial design. METHODS: After receiving personalized risk estimates via a website on a smartphone, participants listened to an audiorecording that guided them through a mental imagery activity related to improving physical activity (intervention group) or sleep hygiene behaviour (active control). Participants received text message reminders to complete the imagery for 3 weeks post-intervention, 4 weekly text surveys to assess behaviour and its cognitive and affective precursors, and a mailed survey 90 days post-baseline. RESULTS: Physical activity increased over 90 days by 19.5 more minutes per week (95%CI: 2.0, 37.1) in the physical activity than the active control condition. This effect was driven by participants in the risk ladder condition, who exercised 54.8 more minutes (95%CI 15.6, 94.0) in the physical activity condition than participants in the active control sleep hygiene group. Goal planning positively predicted physical activity behaviour (b = 12.2 minutes per week, p = 0.002), but self-efficacy, image clarity, and affective attitudes towards behaviours did not (p > 0.05). CONCLUSIONS: Mental imagery-based self-regulation interventions can increase physical activity behaviour, particularly when supported by personalized disease risk information presented in an easy-to-understand format.