Study site and sampling
Tabriz, a city in the northwest of Iran with approximately 1,700,000 inhabitants, was the site of the present cross-sectional study. High school students (grades 9 to 11) included 62,714 students, of which 29,935 were female and 32,779 males during the 2013–14 academic years. A two-stage cluster sampling was used to select representative high schools and classes. At the first stage, high-schools were selected with probability proportional to the enrolment size, and at the second stage classes were randomly selected, and all students in the selected classes were then eligible to participate in the study. Overall, thirty high-schools, including sixteen girls’ and fourteen boys’ high-schools, and ninety classes (grades 9 to 11) [30] were chosen to participate in the study. The full details of the methodology have been reported earlier [31, 32].
Data collection
A reliable, valid and anonymous, self-administered, Persian Version of the GSHS PA module questionnaire was used for data collection [29, 33]. The questions in this module measure physical activity, travel to and from school, participation in physical education classes and participation in sedentary leisure behaviour [34].
Data collection took place over approximately three months from 26 December 2013. GSHS is a global, youth behavioural surveillance system designed by WHO, with technical support from the US Center for Disease Control and Prevention (CDC). Questionnaires were administered to students during an ordinary school day in selected schools and classes by Province Health Center staff, who, one week prior to the survey date, had attended a workshop on GSHS methodology. Answering time for questions was around 15 minutes.
Data Processing and Analysis
Measures
There are two dependent outcome variables: leisure time PA, which was defined as “any activity that increases your heart rate and makes you breathe hard, such as playing with friends, walking to school, running, walking fast, cycling, dancing and playing football”, and leisure time sedentary behaviour, which was assessed by asking about the time that students spend mostly sitting when they are not in school or doing homework. Leisure time PA was assessed by the question, “During the past 7 days, on how many days were you physically active for a total of at least 60 minutes per day?”, and leisure time sedentary behaviour by, “How much time do you spend during a typical or usual day sitting and watching television, playing computer games, talking with friends, or engaging in other sitting activities?”. Detailed response options are presented in Table 1, and the prevalence of physical activity and sedentary behaviour in Table 2.
Physical activity was defined as having at least 60 minutes’ moderate to vigorous physical activity per day on at least 5 days per week. Less than 5 days/week was defined as physical inactivity, with sedentary behaviour being defined as sitting 3 or more hours per day when not attending school or doing homework [9, 10].
Body Mass Index (BMI) was calculated (age and gender-specific) from self-reported body height and weight according to the formula (weight (kg)/height (m2)). Cut off points defining overweight and obesity were used. Students were considered overweight if their BMI was > +1SD from the median and obese if their BMI was > +2SD from the median for BMI for their age and sex [35]. According to the CDC and WHO, insufficient fruit consumption was defined as eating fruit less than twice a day and inadequate vegetable consumption defined as less than 3 times a day [36, 37, 38].
Parental support and encouragement was addressed by the question, “During the past 30 days, how often did your parents or guardians support and encourage you?” and the alternative answers were, “always,” “most of the time,” “sometimes,” “rarely,” and “never.” The answers were then classified into “Yes” or “No”. “Always,” and “Most of the time,” were classified as “Yes,” and “Sometimes,” “Rarely,” and “Never,” were classified as “No”.
Peer support in school was addressed by the question, “During the past 30 days, how often were most of the students in your school kind and helpful?” and parental or guardian connectedness by the question, “During the past 30 days, how often did your parents or guardians understand your problems and worries?”. The responses were classified in the same way as with the parental support question.
Data Analysis
Data analysis was carried out using SPSS software version 24.0. Descriptive statistics were used to report frequency, percentages, mean, and standard deviations. The two main outcome variables were self-reported physical activity and sedentary behaviour. A multivariate logistic regression was used to assess the association between relevant independent variables such as age, gender, grade, being bullied and the dependent outcome variables (physical activity and sedentary behaviour? Yes/No). Reporting of adjusted odds ratios (AORs) and unadjusted odds ratios (UAORs) was done after controlling for factors identified as significant in the bivariate analysis. AORs and UAORs with 95% confidence intervals (CI) were reported. A p-value less than 0.05 was considered as statistically significant for other statistical tests.