Participants and inclusion/exclusion criteria
This study is part of an extensive cross-sectional study focused on the description of family PA, sedentary behaviour, and obesity in parents and their offspring aged 4-16 from Czechia [32]. Participants were recruited by means of two-stage stratified random sampling. In the first stage, nine out of 14 administrative regions, three of each in the lowest, middle, and highest terciles for gross domestic product in Czechia, were randomly selected. In the second stage, seven public kindergartens located in urban areas and three in rural locations and 36 public primary schools located in urban areas and 15 in rural locations were randomly selected. Private schools/kindergartens were not addressed because public schools/kindergartens prevail strongly, and the number of private schools/kindergartens is still negligible in Czechia [32].
A total of 2,389 families were addressed in writing with an invitation to participate in the cross-sectional study, of whom 65.3% agreed to take part in the research (written informed consent received). The participating children and their parents were predominantly white Caucasian (>98%), which is representative of the ethnic demographics of the Czech Republic [32]. The family dyads consisted of a mother-child or father-child couple. The participation of at least one family dyad was a necessary precondition for being included in the research. In the initial stage of the study, information meetings were held to describe the process of the research. Figure 1 provides a detailed flowchart of the inclusion of the participants in the study. The analysis included 566 families (10.6% with a single child (n=60) and 89.4% with more children (n=506)) with complete data of family members on their weight status and ambulatory PA monitored with a Yamax pedometer during a regular school/work week during the spring and autumn between 2013 and 2019 (Figure 1).
Procedures and measurement
After recruitment and submission of a written consent, each family received a self-monitoring package including: 1) a letter describing the study design and the ethical approval, 2) a family logbook for recording the anthropometric and PA data of all family members, 3) Yamax Digiwalker SW-200 (Yamax Corporation, Tokyo, Japan) pedometers for each participating child and parent, 4) a detailed and illustrated guide describing how to operate the pedometer, 5) an explanatory letter to the teachers/coaches about the study explaining why a pedometer is worn by children during lessons/training, and 6) an illustrated instruction leaflet for home measurement of the body weight and height of the family participants.
To maintain the comparability of the family-related PA data with previous studies [29, 31, 32], the use of the Yamax Digiwalker SW-200 pedometer and the family logbook for recording the anthropometric and PA data of all family members was retained. The participants were asked to wear the pedometer attached to their right hip for eight consecutive days for at least eight hours per day and record their daily SC and possible active participation in organized leisure-time sport in the family logbook that was provided [33]. The SC data from the first day of PA monitoring was not included in the final analyses because of the novelty of wearing the pedometer, which might have affected the level of the participants' PA [34, 35]. The inclusion of participants in the analyses was subject to a record of valid PA data for at least six days, including both weekend days. The parents and children were instructed to wear the pedometer throughout the whole day (during their journey to school/work, during classes and breaks, and during participation in organized leisure-time sport or leisure organizations) except when dressing, performing personal hygiene, and showering/bathing. The elimination of seasonal differences was sought by choosing spring and autumn months in weeks without excessive examinations in schools and without multi-day school holidays and public holidays. The Yamax Digiwalker SW-200 pedometer is an unobtrusive, simple, valid, and reliable quantifier of all-day ambulatory PA across a wide population of children, adolescents [34, 36], and adults [37] designed for an analysis of the relationship between daily SC and health outcomes [38].
The parents were asked to fill in the anthropometric data (date of birth of child/children, age of parents, gender, body height/weight (with 0.5-cm/kg accuracy)) of all the participating family members in the family logbook before the start of the eight-day SC monitoring. The parents were thoroughly instructed how to measure their own body height and weight, as well as the height and weight of their offspring, according to the illustrated instruction leaflets for home measurement. Parental measurement of the body height and weight of their offspring at home [39–42] seems to be a sufficiently valid method to identify overweight/obesity according to the calculated BMI compared with objective or laboratory/researcher measurements [39, 40, 43]. Although self-reported body height and weight is a widely used method for determining underweight, normal body weight, overweight, and obesity, previous studies point to possible inaccuracies compared with laboratory/researcher measurements and the absence of a uniform measurement consensus [43–46]. The chronological age of all family members was calculated from the date of birth until the starting day of the PA monitoring.
Ethics
The study design, all procedures, and the measurement and method of feedback were approved by the Ethics Committee of the Faculty of Physical Culture, Palacký University Olomouc separately for families with preschool children (ref. No.: 57/2014 on 21 December 2014), families with 6-11-year-old children (ref. No.: 20/2012 on 12 December 2012), and families with 12-15-year-old adolescents (ref. No.: 14/2018 on 21 February 2018). The parents’ written consent was obtained prior to the start of the data collection. The parents of the children gave their consent to participation in this study. Participation in the project was voluntary and without financial incentives.
Data management and statistical analysis
All data management and statistical analyses were conducted using the Statistical Package for the Social Sciences (SPSS) for Windows, v.22 (IBM Corp. Released 2013. Armonk, NY, USA). To maintain the comparability of the prevalence of childhood BMI categories with previous studies [29, 31, 32], the BMI categories (underweight, normal weight, overweight, or obese) were derived using age- and gender-specific WHO growth charts [47, 48]. Overweight and obesity in the children was represented by a BMI from the 85th to 97th and >97th percentile of the WHO growth charts, respectively [47, 48]. Overweight and obesity in the parents was classified using a BMI from 25 kg/m2 to 29.9 kg/m2 and greater than or equal to 30 kg/m2, respectively [49]. The daily step count variable represented the mean difference between the morning (pedometer turned on) and evening (pedometer turned off) step counts on the days of the week that were monitored. Daily SC values below 1,000 and above 30,000 were truncated to these recommended values, respectively [35, 50], and included in the analyses. If step counts were recorded during four weekdays, data for the one missing weekday that was based on the participant’s personal mean scores was added. The participants whose step count data was missing for more than one day were excluded from the analysis. The average daily SC was calculated separately for school/workdays and for weekends as the sum of the individual daily SC divided by the number of days. The daily SC recommendation was set at a value of ≥13,000/≥11,000 steps/day for 5-12-year-old sons/daughters and ≥10,000 steps/day for 12-16-year-old adolescents [51] and adults [52]. The percentage of achievement of the recommended daily SC by individual family members was quantified separately for school/workdays, weekends, and the whole week. Pearson’s chi-square test (χ2) was used to compare the prevalence of obesity, overweight, normal body weight, and underweight, achievement of the daily SC recommendation, and frequency of participation in organized leisure-time sport between the single children and children with siblings. Neither the Shapiro-Wilk test nor the Kolmogorov-Smirnov test confirmed the normal distribution of SC variables. As a result of the non-normal distribution of SC variables, the Mann-Whitney U test was used to compare workday and weekend SC. The parent-child daily SC (BMI or BMI-for-age Z score) relationship was quantified using the Spearman's rS correlation coefficient. Binary logistic regression analyses (Enter method) were used to investigate whether the achievement of the daily SC recommendation by children/parents and parental obesity were associated with the obesity of offspring in single-child families and families in which the children had siblings. Because of previous differences in the relationships between the PA of mothers/fathers and their offspring [29, 31, 32], regression models were calculated separately for mother-child and father-child dyads. The results of the logistic regression analyses were expressed using the odds ratio (OR) and 95% confidence interval (95% CI). The alpha level of significance was set at the minimum value of 0.05.