Development and exploratory cluster-randomised opportunistic trial of a theory-based intervention to enhance physical activity among adolescents.

This article reports the development and exploratory testing of a school-based intervention programme designed to enhance levels of physical activity in adolescents. The intervention is based on social cognitive theory (SCT), self-regulation theory (SRT) and planning as evidence-based mediators of physical activity changes. Two classes, paired on socio-economic variables, were selected from each of eight Portuguese schools and randomly assigned to an intervention or control group (N = 291). Primary outcome was 'moderate to vigorous physical activity' (International Physical Activity Questionnaire) measured pre and post intervention and at three and nine months follow-up. SCT, SRT and planning variables were secondary outcomes measured pre and post intervention. At post test, participants in the intervention group reported 18 min per week more physical activity (PA), adjusted for pre-intervention, age and sex, than those in the control group (95% confidence interval -10 to 46; p = 0.249). This difference increased to 33 min (95% CI-4 to 71; p = 0.082) at three months and to 57 min (95% CI 13 to 101, p = 0.008) at nine month follow-up. Moreover, the intervention resulted in changes of some of the theoretical target variables, including outcome expectancies and coping planning. However, no evidence was found for the changes in theoretical moderators to mediate the intervention effects on behaviour. Implications for theory and for future research are discussed.