title: Issues in accelerometer methodology: the role of epoch length on estimates of physical activity and relationships with health outcomes in overweight, post-menopausal women.
abstract: BACKGROUND
  Current accelerometer technology allows for data collection using brief time sampling intervals (i.e., epochs). The study aims were to examine the role of epoch length on physical activity estimates and subsequent relationships with clinically-meaningful health outcomes in post-menopausal women.
  
  
  METHODS
  Data was obtained from the Woman On the Move through Activity and Nutrition Study (n = 102). Differences in activity estimates presented as 60s and 10s epochs were evaluated using paired t-tests. Relationships with health outcomes were examined using correlational and regression analyses to evaluate differences by epoch length.
  
  
  RESULTS
  Inactivity, moderate- and vigorous-intensity activity (MVPA) were significantly higher and light-intensity activity was significantly lower (all P < 0.001) when presented as 10s epochs. The correlation between inactivity and self-reported physical activity was stronger with 10s estimates (P < 0.03); however, the regression slopes were not significantly different. Conversely, relationships between MVPA and body weight, BMI, whole body and trunk lean and fat mass, and femoral neck bone mineral density was stronger with 60s estimates (all P < 0.05); however, regression slopes were similar.
  
  
  CONCLUSIONS
  These findings suggest that although the use of a shorter time sampling interval may suggestively reduce misclassification error of physical activity estimates, associations with health outcomes did not yield strikingly different results. Additional studies are needed to further our understanding of the ways in which epoch length contributes to the ascertainment of physical activity in research studies.
  
  
  BACKGROUND
  Clinical Trials Identifier: NCT00023543.
authors.affiliation: Division of Epidemiology and Disease Control; University of Texas Health Science Center; Austin, TX 78701, USA. Kelley.P.Gabriel@uth.tmc.edu.
authors.forename: Kelley Pettee
authors.initials: KP
authors.lastname: Gabriel
authors.forename: James J
authors.initials: JJ
authors.lastname: McClain
authors.forename: Kendra K
authors.initials: KK
authors.lastname: Schmid
authors.forename: Kristi L
authors.initials: KL
authors.lastname: Storti
authors.forename: Robin R
authors.initials: RR
authors.lastname: High
authors.forename: Darcy A
authors.initials: DA
authors.lastname: Underwood
authors.forename: Lewis H
authors.initials: LH
authors.lastname: Kuller
authors.forename: Andrea M
authors.initials: AM
authors.lastname: Kriska