title: A systematic literature review with meta-analyses of within- and between-day differences in objectively measured physical activity in school-aged children.
abstract: BACKGROUND
  Targeting specific time periods of the day or week may enhance physical activity (PA) interventions in youth. The most prudent time segments to target are currently unclear.
  
  
  OBJECTIVE
  To systematically review the literature describing differences in young people's objectively measured PA on weekdays vs. weekends, in school vs. out of school, weekends vs. out of school and lesson time vs. break time.
  
  
  METHODS
  Electronic databases were searched for English-language, cross-sectional studies of school-aged children (4-18 years) reporting time-segment-specific accelerometer-measured PA from 01/1990 to 01/2013. We meta-analysed standardised mean differences (SMD) between time segments for mean accelerometer counts per minute (TPA) and minutes in moderate-to-vigorous PA (MVPA). SMD is reported in units of standard deviation; 0.2, 0.5 and 0.8 represent small, moderate and large effects. Heterogeneity was explored using meta-regression (potential effect modifiers: age, sex and study setting).
  
  
  RESULTS
  Of the 54 included studies, 37 were eligible for meta-analyses. Children were more active on weekdays than weekends [pooled SMD (95 % CI) TPA 0.14 (0.08; 0.20), MVPA 0.42 (0.35; 0.49)]. On school days, TPA was lower in school than out of school; however, marginally more MVPA was accumulated in school [TPA -0.24 (-0.40; -0.08), MVPA 0.17 (-0.03; 0.38)]. TPA was slightly lower on weekends than out of school on school days, but a greater absolute volume of MVPA was performed on weekends [TPA -0.10 (-0.19; -0.01), MVPA 1.02 (0.82; 1.23)]. Heterogeneity between studies was high (I (2) 73.3-96.3 %), with 20.3-53.1 % of variance between studies attributable to potential moderating factors.
  
  
  CONCLUSIONS
  School-aged children are more active on weekdays than weekend days. The outcome measure influences the conclusions for other comparisons. Findings support the tailoring of intervention strategies to specific time periods.
authors: UKCRC Centre for Diet and Activity Research (CEDAR) and MRC Epidemiology Unit, University of Cambridge School of Clinical Medicine, Box 285 Institute of Metabolic Science, Cambridge Biomedical Campus, Cambridge, CB2 0QQ, UK.
authors: Hannah L
authors: HL
authors: Brooke
authors: Kirsten
authors: K
authors: Corder
authors: Andrew J
authors: AJ
authors: Atkin
authors: Esther M F
authors: EM
authors: van Sluijs
mesh_terms: D061725:Accelerometry; D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D015444:Exercise / Q000502:physiology*; D006801:Humans; D013997:Time Factors