**2. Materials and Methods**

### *2.1. Study Sample and Design*

The study design was cross sectional, including 13–16-year-old students from the capital area of Reykjavik, Iceland. A total of 439 participants (235 boys and 204 girls) were selected for the present research. Finally, 387 adolescents (209 boys (54%), Mage = 13.57, SD = 1.13 and 178 girls (46%), Mage = 13.38, SD = 1.14) took part in it, which yields a participation rate of 88.15%.

The subjects were recruited from two different schools in Reykjavik: Seljaskóli School (54.2%) and Ölduselsskóli School (45.8%). The inclusion criteria for this research were: male and female

participants between 13 and 16 years old who had submitted the signed informed consent by their parents/guardians. Related to their health status, participants in the present investigation were those subjects who, due to their state of health, could participate regularly in the subject of Physical Education. The participants did not have any type of cognitive or physical/motor limitations The National Committee of Bioethics of Iceland approved the present study (Ref.: VSNb2017030026/03.01), which ensured that all the procedures related to research involving human beings would be carried out in complete safety. Written informed consent was obtained from the parents of the participants. The adolescents were asked for verbal consent, while they were informed that participation was voluntary and that they could leave the study at any time.
