This was a cross-sectional study conducted between October and December 2012 in Fangshan District, Beijing, China. The method of stratified cluster sampling was applied. The students from grade 2 to 5 in primary schools, grade 7 to 8 in middle schools and their parents were recruited from 16 schools (four urban middle schools, four urban primary schools, four rural middle schools, and four rural primary schools). Students with serious organic diseases (e.g. heart, lung, liver, kidney, and other related diseases), or abnormal body development, or cause-based obesity (e.g. endocrine disease or drug side effect) were excluded. There were 3,441 questionnaires distributed to students and parents, respectively. There were 2,670 complete and valid questionnaires received from students, and the paired questionnaire from parents were excluded if parental support behaviors were not filled in the questionnaire. There were finally 1939 valid parental questionnaires, including 517 completed by fathers and 1,422 by mothers. Ethical approval was obtained from the Ethical Committee of Peking University. Written informed consent were obtained from all participating children and their parents before any assessment.
Children and adolescents’ physical activity was assessed using consecutive 7-day physical activity diary, including five weekdays and two weekend days. Students’ activities and time length were required to be recorded at the end of each day. Combined with children’s daily activities and adapted from previous study, the diary contents were designed by a group of experts. The items included (1) How to go to school? (2) How to leave school? The answers for these two items are no schooling, walking, bicycling, and taking a car. Those students with walking, bicycling, and taking a car were required to record time length. (3) Did you walk outside today? E.g. shopping, visiting relatives or friends, going to a park. (4) Did you do homework today? (5) Did you watch TV today? (6) Did you play computer or e-games today? (7) Did you have PE class today? (8) Besides PE class, did you do MVPA in school today? (9) Did you do MVPA out of school. MVPA in item (8) and (9) referred to those activities that make you sweat, gasp or feel a little tired or very tired, such as running, playing football, cycling, dancing and so on. The answers for item (3)~(9) were yes or no. If yes, the time length was required to be recorded.
In this study, bicycling to school or leaving school, having PE class, doing MVPA in school and doing MVPA out of school were regarded as MVPA. Time length of each activity were summed as every day total MVPA time. The dairy is validated if it was recorded more than or equal to three days on weekdays and more than or equal to one day on weekends. The mean time of MVPA on weekdays equals to total validated MVPA time from Monday to Friday dividing by the number of validated days. The mean time of MVPA in weekend days equals to total validated MVPA time from Saturday to Sunday dividing by number of validated days. The mean time of every day equals to total validated MVPA time from Monday to Sunday dividing by number of validated days. In analysis, the daily continuous MVPA time on weekdays, on weekends, on the whole week were transformed into categorical labels according to physical activity recommendation released by the WHO and by China. The cut-off value is 60 minutes. Namely, the labels of daily MVPA time on weekdays, weekends and the whole week were ≥60 minutes (i.e. meeting MVPA recommendation) and＜60 minutes (i.e. not meeting MVPA recommendation) , respectively.
Parental support for physical activity
Questionnaire data to evaluate parental support behaviors for children’s physical activity were collected. Parental support behaviors were evaluated by the following three questions on a 5-point Likert scale from never (1) to always (5). Q1, “As a parent, do you share your knowledge of physical activity/exercise with your child?” (marked as share PA knowledge with child below); Q2, “Do you tell your child and help your child cultivate good exercise habits after you requiring knowledge of physical activity/exercise?” (marked as cultivate child’s PA habits below); Q3, “Do you reserve time for your child to participate physical activity/exercise?” (marked as reserve PA time for child below). In the current sample, the Cronbach’s alpha of physical activity support was 0.762 and 0.778 for fathers and mothers, respectively. The sum of these three variables was used as a measure of the paternal or maternal total score of support behaviors.
Gender, grade, school name were collected from the physical activity diary; and information on children’s district and school stage was obtained. Children’s age was calculated according to their birthdate and examination date recorded. Children’s weight and height were measured objectively by trained postgraduates, and BMI (kg/m2) was further calculated. Children’s weight status (normal or wasting, overweight or obesity) was defined based on the age- and gender-specific BMI cutoffs according to the standards of screening for overweight and obesity among school-aged children and adolescents, as published in China in 2018.
Paternal or maternal age, education level, height and weight were collected from the questionnaire. Paternal or maternal BMI was calculated the same as children’s. Paternal or maternal weight status (normal or wasting, overweight or obesity) was classified according to Chinese criteria of weight for adults.
General description for normally distributed variables was presented as means and standard deviations. Categorical data were summarized by frequency and percentage. Differences in proportions were tested by Chi-square test for independent sample comparation or McNemar test for matched sample comparation. Differences in means for continuous variables were compared using student’s t-test. Multivariate logistic regression was used to examine the association between paternal or maternal support behavior score and children’s meeting MVPA recommendation on weekdays, on weekends and on the whole week. Owing to correlation between parental BMI and children’s BMI (r=0.171 between father and children, and r=0.220 between mother and children) and correlation between parental age and children’s school stage (t=-6.622, P<0.001 between father and children; t=-14.204, P<0.001 between mother and children), logistic regressions were conducted after controlling for children’s school stage, gender, district, weight status, and paternal or maternal educational level. All analyses were performed by SPSS (version 22.0), with P＜0.05 considered as statistically significant.