US Population Profile of Time-Stamped Accelerometer Outputs: Impact of Wear Time

Background: We examined the effects of wear time on a population profile of time-stamped accelerometer outputs using the 2005-2006 National Health and Nutrition Examination Survey (NHANES) data representing 3744 adults >= 20 years of age. Methods: Outputs included activity counts, steps, and time variables: nonwear (macro-determined), sedentary behavior (<100 activity counts/minute), and time in low (100-499 activity counts/minute), light (500-2019 activity counts/minute), and moderate-to-vigorous physical activity (MVPA; >= 2020 activity counts/minute) intensities. We describe mean values according to a 24-hour clock. Analysis was repeated in a reduced data set with only those who wore the accelerometer for 60 minutes within each considered hour of the day. Results: Between 12:00 and 17:00, U.S. adults spend approximately 31 minutes each hour in sedentary behaviors, and approximately 14 minutes, 10 minutes, and 2 minutes in low, light, and MVPA intensity activity, respectively. Removing the effect of nonwear time, sedentary behaviors are reduced in the morning hours and increase in the evening hours. Conclusion: At either end of the day, nonwear time appears to distort population estimates of all accelerometer time and physical activity volume indicators, but its effects are particularly clear on population estimates of time spent in sedentary behavior.