title: Free-living activity counts-derived breaks in sedentary time: Are they real transitions from sitting to standing?
abstract: BACKGROUND
  Previous research has demonstrated a link between free-living accelerometer-measured breaks in sedentary time and health related variables. Breaks in sedentary time are typically inferred from time-stamped accelerometer data indicating a transition from lack of movement (recording of <100 activity counts/min) to relatively more movement (≥100 activity counts/min). However, it remains unknown whether these breaks actually represent sit-to-stand postural transitions in free-living. Thus, the purpose of this study was to compare free-living accelerometer-derived and posture-derived estimates of breaks in sedentary time using the ActiGraph GT3X+ (AG) and the activPAL™ (AP), respectively.
  
  
  METHODS
  A total of 15 participants concurrently wore an AG at their waist and an AP on their right thigh for 7 consecutive days (24h/day - removing them only when in contact with water). Data from both devices were matched on minute-by-minute timestamps while also applying a 3-min allowance window to account for clock drift. Dependent t-test was used to evaluate differences in mean breaks between AG and AP.
  
  
  RESULTS
  The AG detected 74±4.1 breaks/day (mean±SEM) while the AP detected 39±3.1 breaks/day (P<0.001). On average, the AG detected 67% of the AP breaks while 65% of the AG breaks did not correspond with AP breaks. Of the non-corresponding AG breaks, 52% occurred when participants were sitting, 42% when standing, and 6% when transitioning from standing to sitting.
  
  
  CONCLUSIONS
  The AG detected a significantly higher number of breaks in sedentary time, the majority of which do not correspond to sit-to-stand transitions as measured by the AP.
authors.affiliation: Syracuse University, Syracuse, NY, United States; Pennington Biomedical Research Center, Baton Rouge, LA, United States. Electronic address: tvbarrei@syr.edu.
authors.forename: Tiago V
authors.initials: TV
authors.lastname: Barreira
authors.affiliation: Pennington Biomedical Research Center, Baton Rouge, LA, United States.
authors.forename: Theodore W
authors.initials: TW
authors.lastname: Zderic
authors.affiliation: Pennington Biomedical Research Center, Baton Rouge, LA, United States; Oregon State University, Corvallis, OR, United States.
authors.forename: John M
authors.initials: JM
authors.lastname: Schuna
authors.affiliation: Pennington Biomedical Research Center, Baton Rouge, LA, United States.
authors.forename: Marc T
authors.initials: MT
authors.lastname: Hamilton
authors.affiliation: Pennington Biomedical Research Center, Baton Rouge, LA, United States.
authors.forename: Catrine
authors.initials: C
authors.lastname: Tudor-Locke
mesh_terms: D061725:Accelerometry / Q000295:instrumentation*; D056044:Actigraphy / Q000295:instrumentation*; D000328:Adult; D000465:Algorithms; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity; D011187:Posture; D057185:Sedentary Behavior; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*; D012984:Software
keywords: Accelerometry; Agreement; Posture; Sitting; Validity