Intraindividual variation of objectively measured physical activity in children.
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.
Mattocks
Calum
C
Department of Social Medicine, University of Bristol, United Kingdom. c.mattocks@bristol.ac.uk
Leary
Sam
S
Ness
Andy
A
Deere
Kevin
K
Saunders
Joanne
J
Kirkby
Joanne
J
Blair
Steven N
SN
Tilling
Kate
K
Riddoch
Chris
C
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