Teun Remmers

150 | Chapter 9 Abstract Introduction : Physical activity (PA) significantly declines from childhood to adolescence. Therefore, the transition period between primary and secondary school seems an important target period for PA promotion. Although several studies have assessed the tracking of PA over time objectively, objective information on the behavioral context in which potential changes occur is often lacking. Therefore, the aim of this study was to investigate the relationship between PA patterns and its context in the transition period between primary and secondary school to add in-depth insight in how, where, and the time-segments at which changes in PA patterns occur. Methods : Children participated in a 7-day accelerometer- and Global Positioning System (GPS) protocol at the last year of primary- and the first year of secondary school in 's- Hertogenbosch, the Netherlands. GPS data-points were overlaid with Geographical Information Systems (GIS) data using ArcGIS 10.1 software. Contextual data was identified based on the GPS locations of individual data-points at the participant's residence, school, sports grounds, shopping centres, and other places. Also, trips in active and passive transport were identified. Multi-level linear mixed models investigating changes in children's PA patterns were fitted adjusting for age, gender, meteorological circumstances, and the nested structure of measurement-days within children and children within schools. Outcome measures were minutes spent in sedentary time, light PA and moderate-to-vigorous PA, specified for the time-segments before school, during school, after school and during weekend days. Results : In total, 89 boys and 86 girls provided valid longitudinal data. Average age at baseline was 12.1 years. Total time spent in LPA and MVPA declined between primary and secondary school, especially after school and during weekends. Considerable declines in after school PA were found. Here, PA at other places (e.g. visiting friends or at places in the neighbourhood but outside the house) was replaced with increased sedentary time at children's residence. Transport-related activity significantly increased during weekdays over time, and stronger increases were found for children with higher increases in distance from the residence to their school between primary and secondary school. Conclusion : LPA and MVPA especially declined after school and during weekends. Although active transport increased due to increased distance from the residence to their school, the major drivers of decreasing PA levels from primary to secondary school are 1) increased sedentary time at the residence, 2) decreased LPA at sports grounds, and 3) decreased LPA and MVPA at other locations. Studies combining accelerometers, GPS, and GIS data provide valuable information in understanding the development of context- specific PA patterns.