Minute-level smartphone GPS-derived exposure to greenness and consumer wearable-derived physical activity in the Nurses’ Health Study 3

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BackgroundGrowing research finds that exposure to natural vegetation, or greenness, is associated with physical activity. However, many studies have been limited to residence-based exposure to greenness and self-reported, time-aggregated measures of physical activity.MethodsThe Nurses’ Health Study 3 (NHS3) Mobile Health Substudy includes a subset of participants in the US-based, nationwide prospective NHS3 cohort who were asked to use a smartphone application and consumer wearable device (Fitbit) for seven-day periods, four times over a year. The smartphone application measured global positioning systems (GPS) data every ten minutes and the consumer wearable measured minute-level steps and heart rate. We estimated momentary exposure to greenness from Landsat satellite data using the Normalized Difference Vegetation Index (NDVI) in the 270m area around each smartphone GPS coordinate. Physical activity based on average steps and heart rate were measured based on wearable data in the ten-minute period prior to each GPS coordinate. We constructed generalized additive mixed models to examine nonlinear relationships and to account for correlations of observations within person. Analyses were adjusted for age, race, marital status, smoking status, and BMI.ResultsBased on data from 407 participants, comprising over 538,156 observations of steps, heart rate, and GPS data, we observed an average of 6.6 steps per minute and an average heart rate of 76.2 beats per minute. We observed adjusted nonlinear relationships between greenness and both steps and heart rate suggesting that between NDVI of 0.0 to 0.2, NDVI was positively associated with steps and heart rate, but above this point, NDVI was inversely related with both outcomes.ConclusionsUsing mobile health measures embedded into a prospective cohort, this study demonstrated complex nonlinear relationships between greenness and physical activity, suggesting that participants were less likely to be physically active above a certain threshold of greenness.