Whole-day and segmented-day physical activity variability of northwest England school children

This study aimed to (1) assess the day-to-day variability of children's weekday physical activity for the whole-day, and when segmented into discrete periods of the day; and (2) compare boys' and girls' physical activity variability.

Fifty-eight children (aged 7-11 years; 31 boys) from a northwest England city wore accelerometers for 4 consecutive weekdays during November and December 2005. Intra-class correlations (ICCs) were calculated for 1 and 4 monitored days and segments of days to determine reliable estimates of moderate-to-vigorous physical activity (MVPA). The Spearman-Brown prophecy formula established the number of monitoring days required for 80% reliability.

Higher ICC coefficients were typically observed among boys. Eight days of boys' whole-day monitoring were needed to achieve a reliability of 0.8, while 10 days were required for girls. The pre-school segment was the most stable (boys' ICC=0.862; girls' ICC=0.770). Compared to girls, fewer days of boys' monitoring would be required during all segments other than the school day.

Day-to-day physical activity variability was sex-specific, with boys' MVPA generally more stable than girls'. The greatest reliability occurred between 7 am and 3 pm, suggesting that physical activity behaviors are more consistent in the school environment.