Abstract
OBJECTIVES: Physical activity declines during adolescence. The Theory of Planned Behaviour (TPB) is a useful framework for investigating activity but leaves variance unexplained. We explored the utility of a dual-process approach using the TPB and the Prototype Willingness Model (PWM) to investigate correlates of physical activity, and 1-year change in physical activity, among a large sample of adolescents. DESIGN: A cross-sectional and longitudinal analysis of baseline and follow-up data from the Fit to Study cluster-randomized trial. METHODS: A total of 9,699 secondary school pupils at baseline and 4,632 at follow-up (mean age = 12.5 years) completed measures of past week physical activity and constructs from both behaviour-change models, at time-points 1 year apart. Cross-sectional analyses used multilevel, stepwise regression models to measure the strength of associations between model constructs and physical activity, and variance in behaviour explained by PWM over and above TPB. In longitudinal analyses, change scores were calculated by subtracting follow-up from baseline scores. Models controlling for trial treatment status measured the strength of associations between change scores, and variance explained. RESULTS: At baseline, after controlling for past behaviour, physically active prototype similarity had the strongest relationship with activity after the intention to be active. Change in prototype similarity had the strongest relationship with change in activity after the change in intention and attitudes. Prototype perceptions and willingness explained additional variance in behaviour. CONCLUSION: A dual-process model incorporating prototype perceptions could more usefully predict physical activity than models based on rational expectations alone. Behaviour-change interventions promoting an active self-image could be tested for effects on physical activity.