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 (-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.