*2.3. Questionnaire*

Regarding the perceived health status of adolescents, self-rated health represents a meaningful subjective indicator for general health [19]. According to the Health Behavior in School-Aged Children (HBSC) study [20], self-rated health was measured by a single item with the question "Would you say your health in general is . . . ". For answering this question, we used a five-point Likert scale (very good, good, moderate, poor, very poor). Participants were subsequently divided into four health categories (very good, good, moderate, poor/very poor), which are comparable to the four HBSC categories of self-rated health (excellent, good, fair, poor) [20]. Participants also reported their average daily time (hours/day) of electronic media use (smartphone, tablet, computer, TV, etc.) outside of school as well as the presence of a TV in their bedroom [16]. Sports club participation was determined via self-report (yes/no). In addition, participants reported mean duration (hours per week) of practicing sports in a club setting as well as during leisure time. Weekly hours of sports club participation and weekly hours of physical activity in leisure time were then summed up to obtain a total amount of weekly leisure time sports activity. Finally,

smoking and alcohol consumption, as well as current back pain during the previous 7 days, were reported using a dichotomous variable (yes/no), respectively.
