Zafeiriou Παναγιώτης, Christoforidis Χρήστος, Kampas Αντώνης, Douroudos Ιωάννης, and Taxildaris Κυριάκος. 2007. “Comparison of Exercise Programs With and Without the Use of the Ball for the Improvement of the Maximum Speed, the Speed Endurance and the Countermovement Jump in Young Football Players 14-15 Years Old”. Inquiries in Physical Education and Sport 5 (3):424-30. https://doi.org/10.26253/heal.uth.ojs.ispe.2007.1221.