title: Longitudinal changes in objectively measured physical activity differ for weekdays and weekends among Chinese children in Hong Kong.
abstract: BACKGROUND
  Cross-sectional investigation showed that Chinese children in Hong Kong were more physically active on weekends than weekdays, which is contrary to previous findings. However, little is known as to whether these time-segment-specific differences persist with age. This study aimed to compare the 2-year changes in accelerometer-assessed physical activity (PA) and sedentary time (ST) between weekdays and weekends among Chinese children in Hong Kong.
  
  
  METHODS
  Children aged 6-8 years were recruited from primary schools in Hong Kong. Time spent in ST (<100 counts per minute [cpm]), moderate-to-vigorous PA (MVPA), and light-intensity PA (LPA) were measured by accelerometer at baseline and then at 1-year and 2-year follow-ups. Mean annual changes were determined using mixed-effects linear models for children who provided 3-day valid data (including 1 weekend day) for at least two time points (n = 412). Magnitude of changes between weekdays and weekends was compared using age × time-segment interactions.
  
  
  RESULTS
  At each assessment wave, the percentage of time spent in MVPA (% MVPA) and LPA (% LPA) was consistently high, whereas the percentage of time spent in ST (% ST) was lower on weekends than weekdays. A decrease in % MVPA was found for both weekdays (mean annual change: boys, -0.7, 95% CI = -0.9 to -0.1; girls, -0.8, 95% CI = -1.0 to -0.6) and weekends (boys, -1.2, 95% CI = -1.5 to -0.9; girls, -1.4, 95% CI = -1.6 to -1.1). An increase was found in % ST for both weekdays (boys, 1.3, 95% CI = 0.7 to 1.9; girls, 2.4, 95% CI = 1.9 to 3.3) and weekends (boys, 1.8, 95% CI = 1.1 to 2.5; girls, 2.6, 95% CI = 1.9 to 3.3). Mean annual change in MVPA time (min) was greater on weekends than weekdays (difference: boys, 3.0, 95% CI = 0.3 to 5.7; girls, 3.5, 95% CI = 1.1 to 5.8).
  
  
  CONCLUSIONS
  Age-related decline in MVPA was more marked on weekends than weekdays. Interventions to hinder age-related changes in PA and ST should target both time segments, but weekends warrant particular attention for interventions targeting PA maintenance due to the greater declines.
authors.affiliation: Department of Sports Science and Physical Education, The Chinese University of Hong Kong, Shatin, Hong Kong, China. hsswong@cuhk.edu.hk.
authors.forename: Stephen Heung-Sang
authors.initials: SH
authors.lastname: Wong
authors.affiliation: Department of Physical Education, AAB924, Academic and Administration Building, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China. wendyhuang@hkbu.edu.hk.
authors.forename: Wendy Yajun
authors.initials: WY
authors.lastname: Huang
authors.affiliation: Department of Sports Science and Physical Education, The Chinese University of Hong Kong, Shatin, Hong Kong, China. hegang@cuhk.edu.hk.
authors.forename: Gang
authors.initials: G
authors.lastname: He
mesh_terms: D061725:Accelerometry; D000367:Age Factors; D002648:Child; D003430:Cross-Sectional Studies; D015444:Exercise; D005260:Female; D006723:Hong Kong / Q000453:epidemiology; D006801:Humans; D016014:Linear Models; D008297:Male; D012737:Sex Factors