How many hours are enough? Accelerometer wear time may provide bias in daily activity estimates.

BACKGROUND There is little consensus on how many hours of accelerometer wear time is needed to reflect a usual day. This study identifies the bias in daily physical activity (PA) estimates caused by accelerometer wear time. METHODS 124 adults (age = 41 ± 11 years; BMI = 27 ± 7 kg·m⁻²) contributed approximately 1,200 days accelerometer wear time. Five 40 day samples were randomly selected with 10, 11, 12, 13, and 14 h·d⁻¹ of wear time. Four semisimulation data sets (10, 11, 12, 13 h·d⁻¹) were created from the reference 14 h·d⁻¹ data set to assess Absolute Percent Error (APE). Repeated-measures ANOVAs compared min·d⁻¹ between 10, 11, 12, 13 h·d⁻¹ and the reference 14 h·d⁻¹ for inactivity (<100 cts·min⁻¹), light (100-1951 cts·min⁻¹), moderate (1952-5724 cts·min⁻¹), and vigorous (≥5725 cts·min⁻¹) PA. RESULTS APE ranged from 5.6%-41.6% (10 h·d⁻¹ = 28.2%-41.6%; 11 h·d⁻¹ = 20.3%-36.0%; 12 h·d⁻¹ = 13.5%-14.3%; 13 h·d⁻¹ = 5.6%-7.8%). Min·d⁻¹ differences were observed for inactivity, light, and moderate PA between 10, 11, 12, and 13 h·d⁻¹ and the reference (P < .05). CONCLUSIONS This suggests a minimum accelerometer wear time of 13 h·d⁻¹ is needed to provide a valid measure of daily PA when 14 h·d⁻¹ is used as a reference.