title: Filtering for productive activity changes outcomes in step-based monitoring among children.
abstract: Wearable activity monitors are increasingly prevalent in health research, but there is as yet no data-driven study of artefact removal in datasets collected from typically developing children across childhood. Here, stride count data were collected via a commercially available activity monitor (StepWatch), which employs an internal filter for sub-threshold accelerations, but does not post-process supra-threshold activity data. We observed 428 typically-developing children, ages 2-15, wearing the StepWatch for 5 consecutive days. We developed a minimum per-minute stride-count below which the data outputted from the StepWatch could be considered 'idle' and not 'productive'. We found that a threshold stride count of 10 steps per minute captured 90% of samples in a weighted average among isolated non-zero stride-count samples offset by inactivity. This threshold did not vary by age, gender, or by an age-gender interaction. Filtering the activity data according to this threshold reduced overall stride count by 8-10% by age group, from 8177  ±  2659 to 7432  ±  2641 strides per day. The impact on number of bouts per day decreased from an overall average of 79.3  ±  17.2 to 72.7  ±  12.1; this effect varied by age group. This study delivers the first data-driven estimate of a minimum activity threshold in step- or stride units that may extend to other studies. We conclude that the impact of production-idle filtering on activity data is substantial and suggests a possible impetus for re-contextualizing extant studies and guidelines reported without such filtering.
authors.affiliation: Department of Rehabilitation Sciences, University of Hartford, 200 Bloomfield Avenue, West Hartford, CT 06117, USA. Department of Biostatistics, Yale School of Public Health, New Haven, CT, USA. Cooperative Studies Program, Department of Veterans Affairs, West Haven, CT, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Wininger
authors.forename: Kristie
authors.initials: K
authors.lastname: Bjornson
mesh_terms: D061725:Accelerometry / Q000295:instrumentation; D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D016138:Walking / Q000502:physiology*; D000076251:Wearable Electronic Devices