A school-based intervention based on self-determination theory to promote girls' physical activity: study protocol of the CReActivity cluster randomised controlled trial.

BACKGROUND: Physical inactivity is deemed to be the fourth leading cause for premature death. Nevertheless, only a minority of children and adolescents in Germany fulfil the guideline of the World Health Organization of at least 60 min of moderate-to-vigorous physical activity per day. Children and adolescents with a lower socioeconomic background and especially girls are regarded as a high-risk group in terms of physical inactivity. Aim of this study is to examine how a theory-based physical education intervention programme supporting students' autonomy, competence and relatedness affects physical activity both during these lessons and in leisure time. Based on the self-determination theory and the social cognitive theory, the extent to which autonomy, competence, relatedness, social support and self-efficacy in girls mediate the effect of the intervention programme on their physical activity will be examined. Moreover, the potential moderating role of socioeconomic status, environmental factors, teacher characteristics and BMI will be tested. METHODS: CReActivity is a two-arm cluster randomised controlled trial with a follow-up period of three months after the end of the intervention programme. A total of 600 sixth grade girls in lower secondary schools in Bavaria, Germany will be proactively recruited. The intervention is carried out by the physical education teachers of the participating classes for five months. Primary outcome is the girls' physical activity measured by accelerometers and systematic observations. DISCUSSION: We expect to provide an intervention programme that can contribute to the increase of physical activity levels in girls and offer insights into the mechanisms of physical activity behaviour change. TRIAL REGISTRATION: German Clinical Trials Register DRKS00015723 (date of registration: 2018/10/22 retrospectively registered).