Intervention effects and mediators of well-being in a school-based physical activity program for adolescents: the 'Resistance Training for Teens' cluster RCT

Objective: To examine the impact of a school-based physical activity intervention on adolescents' self-esteem and subjective well-being, and to explore moderators and mediators of intervention effects. Methods: Resistance Training for Teens was evaluated using a cluster RCT in 16 schools located in New South Wales, Australia. Adolescents (N = 508; 14.1 ± 0.5 years; 49.6% female) completed measures of global self-esteem, subjective well-being, and hypothesized mediators (i.e., perceived fitness, resistance training self-efficacy, and autonomous motivation) at baseline (April-June, 2015) and post-intervention (October-December). The school-based physical activity program was delivered by teachers over 10-weeks via Physical Education, co-curricular school sport, or an elective subject known as Physical Activity and Sport Studies, and involved once-weekly fitness sessions and additional lunch-time sessions. Intervention effects and moderator analyses were tested using multi-level linear regression analyses with interaction terms. Multi-level mediation analyses were used to explore potential mediators of changes in well-being outcomes. Results: Intervention effects for self-esteem (ß = 0.05, p =.194) and wellbeing (ß = 0.03, p =.509) were not statistically significant. Moderator analyses showed effects for self-esteem were greater for the overweight/obese subgroup (p =.069 for interaction), and resistance training self-efficacy was a significant mediator of changes in self-esteem (product-of-coefficients [AB] = 0.021, SE = 0.010, 95% CIs = 0.002 to 0.040). No other significant indirect effects were observed. Conclusion: Overall, Resistance Training for Teens did not improve adolescents' self-esteem or subjective well-being. However, our mediation findings lend support to resistance training self-efficacy as a mechanism explaining the positive effect of resistance training on self-esteem.