Teun Remmers

Afterschool PA and the built environment using GPS, GIS and accelerometers | 125 Introduction The detrimental impact of prolonged sedentary behaviour bouts and insufficient physical activity (PA) on youth health profiles is well established. As physical inactivity tends to track from youth to adulthood (1), promoting physical activity (PA) of children is a crucial component of primary prevention strategies that combat overweight and obesity-related health consequences. The development of objective measurements (e.g. accelerometry) allows researchers to continuously monitor children's daily PA behaviour, and to investigate separate time periods that are promising for interventions by filtering data-outputs based on time- segments. Acknowledgement of these specific time-segments is essential, as children's PA patterns fluctuate during the day and can be highly context-specific (2, 3). The afterschool period is such a context-specific time-segment that is often referred to as 'critical hours' for PA promotion, because it contributes up to half of the daily amount of moderate to vigorous PA (MVPA) (4), and afterschool PA declines as children reach adolescence (5). During the afterschool period, children have more discretion over the activities in which they engage, leading to a competition of PA pursuits versus technology- or homework- related sedentary activities (5-7). Because of this, afterschool behaviours may reflect more autonomously regulated behaviour, which may be more likely to persist into habits and routines (8, 9). Hence, afterschool PA has been considered highly predictive of overall sustained PA patterns (10), which makes it a primary time-segment for PA interventions (11). From 2010 onwards, the number of studies that used objective measurements to investigate afterschool PA rapidly increased (12). However, interpretation of these studies’ results is limited by two main methodological challenges. First, there are inconsistent definitions of the afterschool time-segment (5). For example, while some studies used generic start time thresholds such as 3 PM (6, 7, 13, 14) or 3:30 PM (15-17), other studies used reported schools' schedules (4, 5, 18-21) or excluded activity during school hours (22). Second, although separately investigating afterschool PA is an important first step in examining context-specific determinants of PA (23), the afterschool period still consists of multiple distinct contexts of PA behaviour (e.g. active transport, organized sports participation, leisure time PA) (18). The ability to measure PA within these contexts is essential, as the influence of potential determinants may depend on these contexts. To date, these behavioural contexts have been unaccounted for. As a result, it is uncertain whether previous studies using objective measurements to investigate afterschool PA represent the actual self-sustained leisure-time afterschool behaviour. Besides its discretionary character, the afterschool period has the potential to expose children to diverse features (or outdoor PA opportunities) around the school- as well as in their residential environment. Therefore, the role of the social- and physical environment in children's afterschool PA is of special interest. To date, from the six studies that have investigated relationships between afterschool PA and the built environment (16, 19, 22-