Time-specific effects of a multifaceted intervention on accelerometer-measured physical activity in primary school children: A cluster-randomized controlled trial

Background It is unclear whether intervention effects on school-aged children's physical activity differ across specific periods of the week or day. This study aimed to assess the time-specific intervention effects on accelerometer-measured physical activity in primary school children. Methods This was a nested study in a cluster-randomized controlled trial conducted from September 2018 to June 2019 in China. The intervention group included 4 schools (119 children) and the control group included 4 schools (99 children) in Beijing. The obesity prevention intervention engaged schools and families to improve children's physical activity. Outcome measures included accelerometer-assessed intensity and amounts of physical activity within specific periods of a week (weekday/weekend day) or a day (in-school/out-of-school periods). Linear mixed models were used to estimate intervention effects. Subgroup analyses were also conducted to examine potential moderators including sex, age, body mass index, physical activity, and accelerometer compliance. Results The intervention lead to an increase in time engaged in moderate-to-vigorous physical activity (MVPA) within in-school periods of a day (adjusted mean difference: 0.54 minutes/hour; 95% confidence interval: 0.13, 0.94, P = 0.012) but it did not improve physical activity within out-of-school periods (P > 0.05) compared with the control group. There was no difference in the effect size across most of the moderators except for age, as younger children appeared to benefit more from the intervention in the improvement of in-school MVPA (Pinteraction = 0.035). No between-group differences were observed in physical activity within the whole weekday or weekend day (P > 0.05). Conclusion The intervention effectively increased MVPA within in-school periods but did not improve out-of-school physical activity. Findings support the tailoring of intervention components to specific periods of a day to improve school-aged children's whole pattern of physical activity.