Tzetzis Γ., Goudas Μ., and Kyratsou Ε. “A Comparison of Physical Activity Levels, Goal Orientations and Perceived Competence in 5th and 6th Grade Elementary School Students at Physical Education Classes”. Inquiries in Physical Education and Sport, vol. 3, no. 1, Feb. 2005, pp. 1-12, https://journals.lib.uth.gr/index.php/inquiries/article/view/1064.