Supporting adolescents' participation in muscle-strengthening physical activity: protocol for the 'Resistance Training for Teens' (RT4T) hybrid type III implementation-effectiveness trial.

Introduction: In Australia, only 22% of male and 8% of female adolescents meet the muscle-strengthening physical activity guidelines, and few school-based interventions support participation in resistance training (RT). After promising findings from our effectiveness trial, we conducted a state-wide dissemination of the '' (RT4T) intervention from 2015 to 2020. Despite high estimated reach, we found considerable variability in programme delivery and teachers reported numerous barriers to implementation.