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.