title: Epoch length and accelerometer outputs in children: comparison to direct observation.
abstract: OBJECTIVE
  To determine the effects of epoch length and activity count cutpoints on ActiGraph (AG; ActiGraph Health Services, Pensacola, FL) accelerometer estimates of time in moderate-to-vigorous physical activity (MVPA) in fifth-grade children monitored during physical education (PE) compared with a direct observation (DO) criterion standard.
  
  
  METHODS
  A sample of 32 fifth-grade males and females (mean age = 10.3 +/- 0.5 yr) wore an AG attached at the waist for a 30-min PE class. Participants were concurrently videotaped, and the Computerized System for Observing Fitness Instruction Time (C-SOFIT) was used to create a DO measure of MVPA time (DO_MVPA). AG data were collected in 5-s epochs then integrated into 10-, 15-, 20-, 30-, and 60-s epochs. AG activity counts were converted into time (in s) in MVPA using validated (and epoch-adjusted) children's activity count cutpoints established by Treuth et al. (AG_T), Mattocks et al. (AG_M), and Freedson et al. (AG_F).
  
  
  RESULTS
  All AG_T and AG_M epoch detected significantly lower time in MVPA than DO_MVPA. The percentage of DO_MVPA detected by AG_T and AG_M epochs ranged from 46% to 61% and from 26% to 47%, respectively. All AG_F epochs yielded similar (i.e., nonsignificant) mean estimates of MVPA versus DO_MVPA, with modest increases in root mean squared error (RMSE) with increasing epoch length. The percentage of DO_MVPA detected by AG_F epochs ranged from 93% to 100%.
  
  
  CONCLUSIONS
  All AG_F epoch lengths provide comparable mean estimates to DO-detected MVPA time in fifth-grade children during PE. To minimize error among individual estimates, shorter epoch lengths should be used, with 5-s epochs yielding the lowest RMSE in the current study. Considerations of both epoch length and activity count cutpoint are important to improved detection of intermittent bouts of MVPA among fifth-grade children.
authors.affiliation: Cancer Prevention Fellowship Program, Office of Preventive Oncology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA. james.mcclain@nih.gov
authors.forename: James J
authors.initials: JJ
authors.lastname: McClain
authors.forename: Teresa L
authors.initials: TL
authors.lastname: Abraham
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Brusseau
authors.forename: Catrine
authors.initials: C
authors.lastname: Tudor-Locke
mesh_terms: D002648:Child; D003625:Data Collection / Q000295:instrumentation / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity; D019370:Observation; D015203:Reproducibility of Results