Although previous research has investigated the training-specific adaptations to training in adults, there is a paucity of research aimed at investigating these adaptations in adolescent athletes. As such, adolescent athletes’ training-specific adaptations from three different training interventions were investigated in this study. Sixteen adolescent athletes participated in this study, whereby eight performed both training interventions and eight the non-training control. Pre- and post-testing was performed for each intervention with the testing battery: ultrasonography of the vastus lateralis and lateral gastrocnemius, countermovement jump, squat jump, and isometric mid-thigh pull. The resistance training group had large significant increases in isometric mid-thigh pull relative peak force (p < 0.01, g = 0.85 (−0.01, 1.71)) and vastus lateralis fascicle length (p = 0.04, g = 0.94 (0.07, 1.80)). The gymnastics and plyometric group demonstrated large significant changes in vastus lateralis pennation angle (p = 0.03, g = −0.94 (−1.81, −0.08)) and fascicle length (p = 0.03, g = 1.07 (0.19, 1.95)), and moderate significant increases in lateral gastrocnemius thickness (p = 0.01, g = 0.63 (−0.21, 1.47)) and eccentric leg stiffness (p = 0.03, g = 0.60 (−0.24, 1.44)). No significant changes were observed for any variables in the non-training group. The resistance training evoked increases in lower-body force producing capabilities, whereas the gymnastics and plyometric training evoked changes in muscle structure and inherent muscle qualities. © 2017, © The Author(s) 2017.