## 2 Materials and Methods
### Variables and Testing  
Apart from age and sex, variables in this study were: community of residence (urban vs. rural), anthropometrics (body height, mass, and calculated body mass index), indices of fitness status, and PAL as measured by the Physical Activity Questionnaire for Adolescents (PAQA) [34; 35; 36]. The PAQA was used for measuring PALs at baseline and follow-up. Participants completed the questionnaire on the online platform Survey Monkey (SurveyMonkey Inc., San Mateo, CA, USA). Similar samples of respondents from Croatia and neighboring countries were assessed using the PAQA before, and it was shown to be a valid test for evaluating PAL [34; 35]. Participants had to recall the past seven days and report it through nine items in the questionnaire. The first eight sections included questions about types of physical activity (i.e., activities during free play, sports, physical education classes, and active transportation). The ninth item was not considered in the final score but was used for noting participants who had some injuries or illnesses that could cause reduced activity. The first eight items were scaled from 0 to 5, and the overall score ranged from 0 to 5, representing the minimum and maximum PAL, respectively. The scores were categorized as baseline-PAL, follow-up-PAL, and the \"delta score\", which represented the difference between baseline-PAL and follow-up-PAL and provided the range of change in PAL that occurred due to the regulations of social distancing during the COVID-19 pandemic. For statistical analyses (described later in detail), the baseline-PAL and follow-up-PAL were dichotomized. Specifically, scores below 2.73 were considered low-level-PAL, while scores above 2.73 were considered normal-level-PAL as suggested in previous studies [35; 36]. Measures of physical fitness were measured only during the baseline testing, and included six tests: (i) Standing broad-jump test (broad-jump), (ii) hanging on the bar with bent arms (bent-arm-hang), (iii) sit-and-reach (sit-and-reach), (iv) multilevel fitness test (multilevel-test), (v) 400-m run (run-400m), and (vi) sit-ups for 60 s (sit-ups). These tests are regularly used in the Croatian educational system and were measured by experienced physical education teachers [37; 38]. The standing broad-jump test was used to assess power (jumping) capacity. The test was performed using standardized equipment (Elan, Begunje, Slovenia) in the gym. Participants started from a standing position with feet placed shoulder-width apart and performed the test by bending their knees and swinging their arms to perform a maximal forward jump. Participants had three test trials with 20-30 s of rest in between, and the best (the longest) jump was used as the final score. The bent-arm-hang test was used to measure static upper-body strength. The test was conducted once in the gymnasium. Participants were assisted to reach the position with bent arms and with their chin over the horizontal bar. Participants were instructed to hold that position as for long as they could. The maximal recorded time in seconds was the test result. The sit-and-reach test was used to assess flexibility. It was performed in the gymnasium using a standardized box. Participants were sitting on the floor with both legs maximally extended at the knees and with the soles of their bare feet placed flat on the box. Participants were instructed to reach forward as far as possible on the measuring line positioned on the box and to hold that position for 1-3 s. Participants had three trials, and the best score, measured in centimeters, was recorded. The multilevel endurance test was used to test aerobic endurance [38]. Participants had to continuously run between two lines set 20 m apart according to the sound signals on the pre-recorded audio track. For the start of the test, participants were standing behind one line and facing the second line, and began to run on the sound signal (\"beep\").