Week-to-week differences of children's habitual activity and postural allocation as measured by the ActivPAL monitor.

Background: Quantification of sitting and standing is possible with the ActivPAL accelerometer, using algorithms to classify activity into time spent sitting, standing and stepping. The purpose of this study was to determine children's week-to-week differences in time spent sitting/lying and standing along with other measures as provided by the ActivPAL accelerometer during continuous wearing of the ActivPAL monitor.

Methods: Fifty-six children (age 10.2±0.9 years, mean±SD) were recruited from 30 urban schools in Auckland, New Zealand. Children wore the monitor for 24h per day up to 14 days. Analyses were performed with mixed modeling. Reliability was expressed as change in the mean from week to week, intraclass correlation coefficient (ICC) and standard error of measurement (SEM).

Results: Mean week-to-week differences in percent time spent sitting/lying, standing and stepping on week days and weekend days for girls and boys were typically small. Step and sit-to-stand counts differences were small and unclear respectively. ICC values ranged from 0.40 to 0.79 during week days and 0.25-0.60 during weekends. SEM for time spent sitting/lying, standing and stepping were ∼3.5%, ∼2.5% and ∼1.5% respectively.

Conclusion: The ActivPAL showed moderate to low week-to-week reliability for habitual activity and postural allocation under free living conditions in boys and girls. Interventions aimed at a moderate reduction in sitting time (∼5% of the day) will require modest sample sizes for adequate precision.