Reliability of Objectively Measured Sedentary Time and Physical Activity in Adults.

Eivind Aadland,Einar Ylvisåker,Guillermo López Lluch

doi:10.1371/journal.pone.0133296

Abstract

BackgroundIn adults, a minimum of 3–5 days of accelerometer monitoring is usually considered appropriate to obtain reliable estimates of physical activity (PA). However, a longer period of measurement might be needed to obtain reliable estimates of sedentary behavior (SED). The aim of this study was to determine the reliability of objectively assessed SED and PA in adults.MethodsEighty-seven adult subjects (28 men; mean (standard deviation) age 31.3 (12.2) years; body mass index 23.7 (3.1) kg/m2) wore the GT3X+ accelerometer for 21 subsequent days, for which the reliability of different wear time criteria (8 to 12 h/day and 3 to 5 d/week) was explored. Variance partitioning along with the Spearman-Brown prophecy formula was used as the basis for determining intraclass-correlation coefficients (ICC) and the number of monitoring days needed (N) to achieve an ICC = 0.80. Week-by-week reliability was reported using ICC, Bland-Altman plots and absolute measures of agreement.ResultsSeven-10 days of monitoring was needed to reliably assess overall- (axis 1 and vector magnitude (VM) counts per minute (CPM)) and moderate-to-vigorous PA (MVPA), 3–4 days was needed for light PA (LPA), whereas the number of days needed for SED depended on whether adjustments were made for wear time (6–8 days) or not (13–15 days). The week-by-week ICC was ≥0.70 for all variables, with limits of agreement being ±267.8 cpm for CPM, ±352.3 cpm for VM CPM, ±76.8 min/day for SED, ±57.8 min/day for LPA and ±43.8 min/day for MVPA, equal to 1.0–1.6 standard deviations, when adjustment was made for wear time.ConclusionsFor most variables, more than one week of measurement was needed to achieve an ICC = 0.80. Correcting for wear time was crucial to reliably determine SED. Considerable week-by-week variability was found for all variables. Researchers need to be aware of substantial intra-individual variability in accelerometer-measurements.

Highlights

Accelerometry has gained widespread popularity as a valid tool for the assessment of free-living physical activity (PA) and sedentary behavior (SED) in diverse study settings
Seven-10 days of monitoring was needed to reliably assess overall- (axis 1 and vector magnitude (VM) counts per minute (CPM)) and moderate-to-vigorous PA (MVPA), 3–4 days was needed for light PA (LPA), whereas the number of days needed for SED depended on whether adjustments were made for wear time (6–8 days) or not (13–15 days)
The weekby-week intraclass-correlation coefficients (ICC) was !0.70 for all variables, with limits of agreement being ±267.8 cpm for CPM, ±352.3 cpm for VM CPM, ±76.8 min/day for SED, ±57.8 min/day for LPA and ±43.8 min/day for MVPA, equal to 1.0–1.6 standard deviations, when adjustment was made for wear time

Summary

Introduction

Accelerometry has gained widespread popularity as a valid tool for the assessment of free-living physical activity (PA) and sedentary behavior (SED) in diverse study settings. Given the inherent variation in behavior over time, an important aspect of accelerometer measurements is how many days of measurement to be considered to obtain reliable results. In adults, !3–5 days of monitoring are normally considered appropriate, which is in accordance with recommendations given [2]. Estimates of how many days of monitoring that should be included to obtain a reliable result vary considerably between studies [3,4,5,6,7, 2, 8], and might vary between outcome variables of interest [6, 8]. A minimum of 3–5 days of accelerometer monitoring is usually considered appropriate to obtain reliable estimates of physical activity (PA). Editor: Guillermo López Lluch, Universidad Pablo de Olavide, Centro Andaluz de Biología del DesarrolloCSIC, SPAIN

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