Teun Remmers

General introduction | 5 In the current literature, results from studies investigating individual determinants of PA in children and adolescents are still mixed, inconclusive, and primarily based on self- reported outcomes. Studies with more complex conceptual models and objective measurements of PA are therefore warranted to unravel this relationship. Therefore, the second aim of this thesis was to investigate individual-level determinants of PA, including both socio-demographical and psychological determinants of objectively measured PA. Environmental Determinants of PA Several reviews have been conducted on potential direct relationships between environmental factors and PA in children (41-44). There is some marginal evidence suggesting that residential density, land-use mix and access to PA facilities are potential determinants of PA in the physical environment (41), while social support and parental influences are potential determinants of PA in the sociocultural environment (45-47). However, results from these studies are difficult to compare due to differences in assessing both PA and characteristics of the environment (42). In addition, results from previous studies are still largely based on cross-sectional designs, and focus on either environmental factors or individual-level factors. Future studies that investigate both individual and environmental determinants of PA in longitudinal designs are warranted. In addition, complex conceptual models that investigate moderation between environmental determinants and behavior may be the next step in understanding pathways between exposure to certain environments and daily PA behavior in children (48, 49). Moderation exists when the strength or direction of a relationship differs for a third moderating variable. For example, boys may perceive other environmental attributes as PA-supportive than girls. Consequently, our third aim was to investigate longitudinal associations between the perceived environment and children's outside play, and potential pathways by which the interplay between perceived environmental determinants influences this behavior. Methodological Advances in Measuring PA and Environmental Determinants Previously, evidence for environmental determinants of PA was largely based on self- reported or parent-reported data. Although subjective reports can provide valuable insights into clearly defined and understandable PA-contexts (e.g. organized sports participation or outside play), it lacks information about the intensity and the location in which PA is performed. In addition, several studies pointed out significant biases in self- reporting and parent-reporting children's PA (50-52).