The consequences of using different epoch lengths on the classification of accelerometer based sedentary behaviour and physical activity.

Kathryn L. Weston,Mai J. M. Chinapaw,Teatske M. Altenburg,Niels Wedderkopp,Lars Bo Andersen,Xinhui Wang,Evi van Ekris,Niels Christian Møller

doi:10.1371/journal.pone.0254721

Abstract

We examined the influence of using different epoch lengths on the classification accuracy of laboratory-controlled sedentary behaviour (SB), and free-living total time and time spent in bouts of SB and moderate-to-vigorous physical activity (MVPA), in children and adolescents. We used two studies including accelerometer-derived data of: 1) controlled activities, i.e. seven sedentary, one standing and one dancing (n = 90); 2) free-living activities (n = 902). For the controlled-activity data, we calculated percentages of time classified as SB and MVPA. For the free-living data, we calculated medians (25th-75th percentiles) of total time and time spent in bouts of SB and MVPA. Applying 8counts/5seconds, 25counts/15seconds and 100counts/60seconds for SB on controlled-activity data revealed respectively (1) 92-96%, 89-99% and 98-100% of sedentary time accurately classified as SB (activity- and age-dependent); (2) 91-98%, 88-99% and 97-100% of standing time classified as SB (age-dependent); (3) 25-37%, 20-25% and 25-38% of dancing time classified as SB (age-dependent). Using longer epochs, children's total time in SB and MVPA decreased while time accumulated in bouts of SB and MVPA accumulated in bouts increased. We conclude that a 60-second epoch seems preferable when the aim is to classify sedentary behaviour, while a shorter epoch length is needed to capture children's short bursts of MPVA. Furthermore, we should be aware that a longer epoch results in averaging of intensities to the middle category.

Highlights

Accelerometers are widely used to assess time spent in physical activity and sedentary behaviour in children and adolescents
The aims of the present study were to examine the influence of using different epoch lengths (i.e. 5, 15 and 60 seconds) on (1) the classification accuracy of controlled sedentary activities in children and adolescents and (2) estimates of children’s free-living sedentary behaviour and physical activity, both for total time as well as time accumulated in bouts
This study examined the influence of different epoch lengths (i.e. 5, 15 and 60 seconds) on (1) the classification accuracy of controlled sedentary activities in children and adolescents; and (2) estimates of free-living sedentary time and time spent in physical activity at different intensities in children, both for total time and time spent in bouts of different durations

Summary

Introduction

Accelerometers are widely used to assess time spent in physical activity and sedentary behaviour in children and adolescents. When applying cut-points to classify accelerometer data into sedentary behaviour and physical activity of different intensities, data is aggregated in specified epoch lengths. Studies validating cut-points for sedentary behaviour, light, moderate and vigorous physical activity (LPA, MPA and VPA, respectively) generally applied epoch lengths ranging from 15 to 60 seconds [1,2,3,4,5,6,7]. Despite cut-points being established in a specific epoch length, they are adjusted to match to other epoch lengths. The Puyau cutpoint of 3200 counts per minute for classifying moderate-to-vigorous physical activity (MVPA) can be adjusted to match a 15-second epoch, resulting in a cut-point of 800 counts per 15-seconds. As cut-points are generally referred to as ‘counts per minute’ it could be assumed that a 60-second epoch is applied [10]

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