title: Calibration and validation of the ActiGraph GT3X+ in 2-3 year olds.
abstract: OBJECTIVE
  To calibrate and validate the ActiGraph GT3X+ to measure sedentary behaviour and physical activity in 2-3 year olds, using 5-s epochs; and to compare the predictive validity of the resulting cut-points with that of NHANES', Trost's, and Pate's 15-s cut-points.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Eighteen children (2.86 ± 0.60 years) wore an ActiGraph GT3X+ during video-recorded semi-structured calibration activity sessions. Activity was coded following Children's Activity Rating Scale. Receiver Operating Characteristic analysis was used to derive Axis1 and vector magnitude cut-points for sedentary behaviour and moderate-to-vigorous physical activity at 5-s epochs. Agreement with Children's Activity Rating Scale was assessed with Cohen's kappa, Lin's concordance, and Bland-Altman plots. Predictive validity of all cut-points was assessed in an independent sample of 20 children (2.99 ± 0.48 years) video-recorded during free-play, using the same procedures as the calibration phase.
  
  
  RESULTS
  During calibration, vector magnitude cut-points (sedentary behaviour ≤ 96.12 counts; moderate-to-vigorous physical activity ≥ 361.94 counts) showed slightly better classification agreement with Children's Activity Rating Scale than Axis1 cut-points (sedentary behaviour ≤ 5 counts; moderate-to-vigorous physical activity ≥ 165 counts), but the latter showed the lowest bias in estimated sedentary behaviour and moderate-to-vigorous physical activity time. In the validation sample, 5-s Axis1 cut-points showed the best predictive validity and lowest mean differences of all cut-points between predicted and observed sedentary behaviour (-2.31%), light physical activity (-24.40%), and total physical activity time (-0.95%). Moderate-to-vigorous physical activity time was significantly overestimated by all cut-points (128.33-184.17%).
  
  
  CONCLUSIONS
  Because moderate-to-vigorous physical activity was highly overestimated, using only the 5-s Axis1 sedentary behaviour cut-point to distinguish sedentary behaviour from total physical activity is advised. The high accuracy indicates that these cut-points are useful for epidemiological studies involving the sedentary behaviour and physical activity of 2-3 year olds.
authors.affiliation: Loughborough University, United Kingdom; University College London, United Kingdom. Electronic address: silvia.costa@ucl.ac.uk.
authors.forename: Sílvia
authors.initials: S
authors.lastname: Costa
authors.affiliation: Bradford Institute for Health Research, United Kingdom.
authors.forename: Sally E
authors.initials: SE
authors.lastname: Barber
authors.affiliation: Loughborough University, United Kingdom.
authors.forename: Noël
authors.initials: N
authors.lastname: Cameron
authors.affiliation: Loughborough University, United Kingdom.
authors.forename: Stacy A
authors.initials: SA
authors.lastname: Clemes
mesh_terms: D056044:Actigraphy / Q000295:instrumentation*; D002138:Calibration; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D009749:Nutrition Surveys; D057185:Sedentary Behavior
keywords: Accelerometer; Children; Cut-points; Physical activity; Sedentary behaviour; Triaxial