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/m²) 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/day¹. 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/day¹ 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.