Impact of accelerometer wear time on physical activity data: a NHANES semisimulation data approach.

BACKGROUND: Current research practice employs wide-ranging accelerometer wear time criteria to identify a valid day of physical activity (PA) measurement.
OBJECTIVE: To evaluate the effects of varying amounts of daily accelerometer wear time on PA data.
METHODS: A total of 1000 days of accelerometer data from 1000 participants (age=38.7 ± 14.3 years; body mass index=28.2 ± 6.7 kg/m(2)) were selected from the 2005-2006 National Health and Nutrition Examination Study data set. A reference data set was created using 200 random days with 14 h/day of wear time. Four additional samples of 200 days were randomly selected with a wear time of 10, 11, 12 and 13 h/day(1). These data sets were used in day-to-day comparison to create four semisimulation data sets (10, 11, 12, 13 h/day) from the reference data set. Differences in step count and time spent in inactivity (<100 cts/min), light (100-1951 cts/min), moderate (1952-5724 cts/min) and vigorous (≥5725 cts/min) intensity PA were assessed using repeated-measures analysis of variance and absolute percent error (APE).
RESULTS: There were significant differences for moderate intensity PA between the reference data set and semisimulation data sets of 10 and 11 h/day. Differences were observed in 10-13 h/day(1) for inactivity and light intensity PA, and 10-12 h/day for steps (all p values <0.05). APE increased with shorter wear time (13 h/day=3.9-14.1%; 12 h/day=9.9-15.2%, 11 h/day=17.1-35.5%; 10 h/day=24.6-40.3%). DISCUSSION: These data suggest that using accelerometer wear time criteria of 12 h/day or less may underestimate step count and time spent in various PA levels.