Kilanowska, J., and R. Muszkieta. “Physical Activity As One of the Elements of Healthy Behavior. Comparative Analysis of 11-, 12- and 13-Year-Old Students in Selected Schools in Poland and Norway”. Pedagogy and Psychology of Sport, vol. 5, no. 2, Nov. 2019, pp. 191-08, doi:10.12775/PPS.2019.05.02.001.