title: Intraindividual variation of objectively measured physical activity in children.
abstract: OBJECTIVE
  This study examined the seasonal and intraindividual variation in objectively measured physical activity in 11- to 12-yr-olds.
  
  
  METHODS
  Children were asked to wear a uniaxial accelerometer for 7 d four times throughout the course of about a year. A random-intercepts model was used to separate the inter- and intraindividual components of physical activity. Gender, age, body mass index (BMI), height, and month of measurement were fitted to the model as potential confounders.
  
  
  RESULTS
  A total of 315 children had valid data for at least two measurement occasions, and 244 had data for all four measurement occasions. The unadjusted intraclass correlation coefficient (ICC) for total activity (counts per minute) was 0.54; 0.49 after adjusting for gender, age, and BMI; and 0.53 after adjusting for gender, age, BMI, and month. Further adjustment for pubertal status at baseline had no effect on the ICC. Restricting the analysis to only those with data for all four measurement occasions (N=244), or to measurements taken on school days only, had no effect on the ICC. The fully adjusted ICC was 0.51 for weekdays only and 0.39 for weekend days only. For minutes of moderate to vigorous physical activity, minutes of vigorous activity, minutes of sedentary behavior, and number of 30-min blocks of sedentary behavior, the fully adjusted ICC were 0.45, 0.37, 0.59, and 0.39, respectively. The analysis was repeated for boys and girls separately, but the differences in ICC were small.
  
  
  CONCLUSIONS
  There was substantial intraindividual variation in the objectively measured physical activity of these children. Studies using single a measurement occasion where physical activity is the exposure should take this into account to adjust for regression dilution.
authors.affiliation: Department of Social Medicine, University of Bristol, United Kingdom. c.mattocks@bristol.ac.uk
authors.forename: Calum
authors.initials: C
authors.lastname: Mattocks
authors.forename: Sam
authors.initials: S
authors.lastname: Leary
authors.forename: Andy
authors.initials: A
authors.lastname: Ness
authors.forename: Kevin
authors.initials: K
authors.lastname: Deere
authors.forename: Joanne
authors.initials: J
authors.lastname: Saunders
authors.forename: Joanne
authors.initials: J
authors.lastname: Kirkby
authors.forename: Steven N
authors.initials: SN
authors.lastname: Blair
authors.forename: Kate
authors.initials: K
authors.lastname: Tilling
authors.forename: Chris
authors.initials: C
authors.lastname: Riddoch
mesh_terms: D002648:Child; D003625:Data Collection / Q000295:instrumentation / Q000379:methods; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D018670:Monitoring, Ambulatory / Q000295:instrumentation*; D015203:Reproducibility of Results; D012621:Seasons; D006113:United Kingdom