Study participants

In this observational study, we approached to children between ages 11 and 14 years (50.8% girls) from randomly selected 10 primary schools located in the city of Zagreb. To be included, children had to be healthy and participated in physical education classes at the time of study and had to be not specifically trained for performance in the administered tests. According to the Croatian Bureau of Statistics for the year 2020 [24], there were 29 358 children aged between 11 and 14 years in total. Our sample size was estimated to be 3730 by using 95% confidence level, 1.5% margin error and the significance of p < 0.05. At the beginning, we recruited 4625 children. After the initial screening, 163 were absent and 110 did not attend physical education classes when the tests were being administrated, because of personal issues. Our final sample was comprised of 4352 children (mean age ± SD = 12 ± 1 years, mean height = 156 ± 10 cm, mean weight = 47 ± 11 kg, mean body-mass index = 19 ± 3 kg/m²; 66% of girls). Before the testing, physical education teachers responsible for undertaking the tests in each school were briefly instructed about the measurement procedures. The study was approved by the Faculty of Kinesiology, University of Zagreb, Croatia. Parent of each participant and all participants gave informed written consent before enrollment into the study. Analyses and procedures performed in the study were anonymous and conducted in accordance with the Declaration of Helsinki regulations.