Evaluating the Let’s Move It intervention programme theory for adolescents’ physical activity: theorised psychosocial mechanisms of behavioural change

Objectives: Behaviour change theories have extensively been used in health behaviour change interventions and their programme theories. However, they are rarely evaluated in randomised field studies. The Let’s Move It intervention targeted various psychosocial constructs to increase vocational school students’ physical activity. A theory-based process evaluation aiming to illuminate the trial findings, as well as to test the programme theory used is conducted. Specifically, we investigate whether the intervention influenced the theorised determinants of change immediately post-intervention and after one year, and whether these determinants were associated with changes in physical activity.
Design: A cluster-randomised controlled trial (n=1166, mean age=18.18).
Methods: We measured theorised determinants with self-report, and physical activity (PA) with accelerometry and self-report. The effects are evaluated with repeated measures ANOVA and regression models.
Results: No changes were detected in most theorised determinants but intervention arm reported higher enactment of behaviour change techniques used during intervention immediately post-intervention and lower descriptive norms for PA throughout. Autonomous motivation was associated with PA immediately post-intervention.
Conclusions: The intervention influenced only few theorised determinants and no long-term associations to PA change were found. Reporting these null effects advances understanding of behaviour change processes. We introduce methodologic possibilities for future intervention programme theory evaluation efforts.