Explaining the effect of a 1-year intervention promoting physical activity in middle schools: a mediation analysis.

OBJECTIVE: The aim of the present study was to examine the mediation effects of changes in psychosocial determinants of physical activity (attitude, social support, self-efficacy, perceived benefits and barriers) on changes in physical activity.
DESIGN: One-year intervention study with baseline and 1-year post measures of physical activity habits and psychosocial correlates.
SETTING: Fifteen middle schools.
SUBJECTS: Boys and girls (n = 2840) aged 11-15 years completed the validated questionnaires during class hours.
RESULTS: The product-of-coefficients test was used to asses the mediating effects. Self-efficacy for physical activity at school was found to be the only significant mediator of physical activity change. Specifically, self-efficacy for physical activity at school partly mediated the effect of the intervention on total and school-related physical activity change in the intervention group with parental support (P < 0.05). None of the other potential mediators, attitudes, social support, perceived benefits and perceived barriers, seemed to have had a positive effect. Even a suppressor effect was found for attitudes. Given that the effects of self-efficacy and attitudes were of opposite direction, the total mediated/suppressed effects of the intervention were not statistically significant.
CONCLUSIONS: Positive changes in total and school-related physical activity in adolescents could be partly explained by increases in self-efficacy for physical activity at school through a physical activity intervention in middle schools with parental support. However, the suppressor effect of attitudes decreased this effect. As this is one of the first true mediation analyses in this age group, further research is needed to replicate the importance of these mediators.