Teun Remmers

6 | Chapter 1 Towards Objective Measurement of context-specific PA Recently the use of objective measurements has become the standard in PA research, which significantly improved researchers' abilities to accurately quantify the frequency, intensity, and duration of PA in children (53, 54). One of the most used devices are accelerometers, which have shown good reliability in assessing children's PA (55, 56). However, accelerometers alone do not provide information about the context in which PA occurs (e.g. school or transport). The context is essential in understanding differences in total daily PA between children, and provides future interventions with valuable information regarding the specific behavioral context to target (e.g. promoting active transport vs. organized sports participation). Based on the internal timestamps (as recorded, in, for example, accelerometers), studies have recently demonstrated time- segmented associations. For example, in children, their school-schedules provide important insights into relative contributions of school hours and after school periods to their total objectively measured PA (28, 57). In addition, based on these accelerometer timestamps, additional subjective data (e.g. dairies) or objective data (e.g. registries of meteorological information) can be aligned with the accelerometer data. Towards Measurement of Objective Environmental Determinants of PA Comparing results from studies that investigate environmental determinants of PA is hampered by differences in measurement mode of both PA and the environment (41). Previous studies generally conceptualized the physical environment as either the perceived environment or objective environment. The perceived and objective environment may be conceptually different, as the perceived environment may differ from one person to another, irrespective of their residential neighborhood (58, 59). An important consideration in studies investigating environmental determinants is the conceptual match between the environment and PA (60, 61). This means that environmental determinants are behavior-specific (e.g. cycling paths as determinant of active transport), and that increased specificity of both PA and environmental attributes is necessary to understand how the environment influences PA (60). Due to its accuracy and reliability, some researchers stated that objective measurements of the environment are preferred. For example, Geographical Information Systems (GIS), which hold extensive data about location and characteristics of the environment, may be used for these purposes. Combining Accelerometers with Global Positioning Systems Recently, frameworks have been developed for linking objective measures of PA with measurements of location data using Global Positioning Systems (GPS) (62-64). This allows for passively collecting continuous objective data of both PA and geographical location, which subsequently can be combined based on timestamps. Moreover, when these data are presented within GIS, information about the contextual environment in which PA occurs can be inferred from the contemporaneously measured location or travel speed (62, 65-67). In this way, specific objectively measured environmental attributes can be linked to conceptually matched PA activities.