Jamurtas Α. Ζ., and Fatouros Ι. Γ. “The Effects of Exercise on the Levels of Beta-Endorphin in Blood”. Inquiries in Physical Education and Sport, vol. 2, no. 1, July 2004, pp. 93-102, doi:10.26253/heal.uth.ojs.ispe.2004.1037.