Indah Zunita, et al. “The Relationship Between Self-Confidence and Concentration With the Ability to Smash Volleyball Athletes 12-17 Years Old in Pervas Club ”. Jorpres (Jurnal Olahraga Prestasi), vol. 21, no. 2, Sept. 2025, pp. 57-66, https://jurnal.uny.ac.id/index.php/jorpres/article/view/95718.