Combining Accelerometers with GPS to Investigate Children’s Outdoor Physical Activity

Introduction With increasing interest in how the physical environment may influence physical activity, methods are needed that can be used to investigate the environmental context of activity. The aim of this study is to use combined accelerometer and GPS data to describe the level and location of children’s physical activity outdoors.

Methods Data were collected in the PEACH project, a longitudinal study of 1300 children measured in their final year of primary school and 1 year later in the first year of secondary school. Children wore an accelerometer (Actigraph GT1M) for 7 days and a GPS (Garmin Foretrex 201) for 3 days after school and on one weekend day. Both instruments recorded at 10 second intervals. Accelerometer and GPS data were matched based upon the accelerometer time stamp. Any GPS data matched to an accelerometer record were considered to be outdoors. A Geographical Information System (ARCGIS 9.3) was used to identify the proportion of combined GPS and accelerometer points falling inside/outside of greenspace.

Results Primary school children (n=1010) spent 41.7 ± 46.1 minutes (approx. 13% of total time) outdoors each day between 3.30 and 8.30pm. Time outdoors did not differ between the sexes and was lower in the winter than the summer. Physical activity levels outdoors were >2.5 times higher than indoors [1]. Time spent in greenspace after school accounted for only 2.4% of total time, but was more likely to be spent in activity of moderate to vigorous intensity than outdoor time elsewhere [2]. In secondary school, time outdoors after school was lower (35.1 ± 39.5 minutes; n=466) but showed very similar associations with gender, season and physical activity. Preliminary analyses indicate that more time is spent outdoors and in greenspace at weekends than during the week and that greater time in greenspace may be associated with active travel to school.

Discussion and Conclusion Combining accelerometer and GPS data provides a method for objectively measuring time outdoors and segmenting physical activity into that occurring indoors or outdoors, and with GIS allows the level and location of outdoor physical activity to be described. This study shows that physical activity is substantially higher outdoors and in greenspace, but only a small amount of time is actually spent in green-space. Describing these associations supports policy initiatives to promote outdoor activity and use of parks and other green areas, and improves our understanding of how the physical environment may influence physical activity.

References [1] Cooper AR, Page AS, Wheeler BW, Hillsdon M, Griew P, Jago R. Patterns of GPS measured time outdoors after school and objective physical activity in English children: the PEACH project. IJBNPA, 2010;7:31. [2] Wheeler BW, Cooper AR, Page AS, Jago R. Greenspace and children’s physical activity: quantifying the association with GPS/GIS in the PEACH project. Prev Med, 2010;51:148-152.