Children's COOPeration Denmark (Child-COOP): A 3-year systems trial targeting physical activity behaviour in children – design and baseline characteristics

Background Many children do not meet the recommended levels of physical activity (PA), with particularly low levels among socioeconomically disadvantaged families and during the transition from childhood to adolescence. PA behaviours arise from dynamic, multilevel interactions across individual, social, organisational, and environmental levels, indicating that sustained improvements in childhood PA likely require approaches that address this complexity. This paper presents the Child-COOP trial design and reports the baseline characteristics of the participating communities and children (aged 6–12 years) in the intervention and wait-list communities. Methods A three-year quasi-experimental trial was conducted in ten Danish communities using a wait-list design. The intervention applies a participatory systems approach, incorporating group model building workshops to facilitate identification, development, and implementation of local actions. The baseline data collected in 2023 included community-level indicators (socioeconomic factors, readiness to change, and knowledge/engagement) and child-level measures, including accelerometer-assessed PA and secondary outcomes (physical performance, anthropometrics, sleep, screen time, diet, and well-being). Results All the participating communities were characterised by socioeconomic disadvantages relative to national averages. Baseline characteristics were broadly comparable between the intervention and wait-list communities, although the intervention communities had a greater proportion of children with an immigrant background and children with slightly lower cardiovascular fitness. Community readiness to change was predominantly at the ‘preplanning’ stage, with one community at the ‘preparation’ stage. Knowledge and engagement levels were moderate and broadly comparable. Conclusions Baseline characteristics were comparable between the intervention and wait-list communities and highlighted the shared socioeconomic disadvantage. These findings strengthen the validity of subsequent implementation and outcome evaluations within the Child-COOP trial. Trial registration: ClinicalTrials.gov NCT06185413, Registered December 29, 2023 - Retrospectively registered.