Secondary schoolgirls have the most unhealthy profile regarding physical activity and sedentary behaviour of all age groups. Since both behaviours independently contribute to various health outcomes, an effective intervention to promote physical activity and to reduce sedentary behaviour needs to be developed.

Many interventions fail to induce changes in health behaviour, as they often use a top-down approach with limited input from the end-users (i.e. the secondary schoolgirls) of the intervention programmes. This project uses a participatory health research approach by actively involving the girls in the entire research process. A co-creation group was set up per intervention school (n=3), including girls and a researcher which co-created a school-specific intervention, tailored to the needs and interests of those girls.

The effect of participatory process were evaluated via a clustered randomized controlled trial using an intervention and control condition and a pre- and post-test design. Effects on physical activity and related determinants (e.g. self-efficacy, attitude, etcetera) were evaluated. The hypothesis was that girls from the intervention condition would improve their physical activity levels and related determinants compared to no changes among girls from the control schools. The process was evaluated as well using focus group research and the hypothesis was that girls who participate in the co-creation groups would be positive about the process and will feel empowered.

Inclusion Criteria:

• Healthy, female pupils of secondary schools (aim: 10th grade) who volunteer to participate in the study