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.

One-year intervention study with baseline and 1-year post measures of physical activity habits and psychosocial correlates.

Fifteen middle schools.

Boys and girls (n = 2840) aged 11–15 years completed the validated questionnaires during class hours.

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.

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.

To evaluate the effects of a comprehensive physical activity (PA) promotion programme in elementary schools on children's total PA levels, leisure-time PA, physical fitness and psychosocial correlates of PA.

A pre-test–post-test design over two school years.

Sixteen elementary schools (764 children, mean age: 11.2 ± 0.7 years) were randomly assigned to the intervention condition (n = 8) and the control condition (n = 8). The intervention included a health-related physical education programme, an extracurricular PA promotion programme and classroom-based PA education lessons. In the total sample, leisure-time PA, psychosocial correlates of PA and physical fitness were measured using a PA questionnaire and the Eurofit test battery. In a sub-sample, total PA levels were measured using an accelerometer.

According to accelerometer data, children's moderate PA and moderate-to-vigorous PA (MVPA) levels decreased less in the intervention schools than in the control schools (P < 0.01). The average time spent on MVPA decreased by 9 min per day in the intervention schools compared with 33 min per day in the control schools. Children in the intervention schools reported significantly more moderate PA in leisure time than the controls (P < 0.05). No overall improvement of physical fitness and no effects on the psychosocial correlates of PA were found.

The comprehensive PA promotion programme was successful in preventing a decline in children's total activity levels. Furthermore, the intervention increased children's PA engagement in leisure time. Therefore, implementation needs to be encouraged.