Effects of 5-Weeks In-Season Combined Resistance Training Using Isoinertial Training and a Vertical Cone-Shaped Flywheel Device on the Performance of U-16 Male Volleyball Players

(1) Background: The main aim of this study is to assess the effects of a strength-training program, including unilateral and bilateral actions and multidirectional, vertical and horizontal exercises, on the performance of young volleyball players. (2) Methods: Twenty young elite Spanish male volleyball players (U-16) participated in this study. Players from 2 different teams were divided into an experimental group (EXP, n = 12) and a control group (CON, n = 8). In addition to their regular volleyball training, the EXP group participated in an extra strength training program once or twice a week for five weeks. This program focused on exercises such as the back squat, vertical loaded coun-termovement jump (CMJ), hip thrust, and lateral crossover step (COD right (COD-R) and left (COD-L)), utilizing a vertical cone-shaped flywheel device. (3) Results: The results revealed significant Time by Group interactions in 5 m sprint time (η²p = 0.235), 10 m sprint time (η²p = 0.311), CMJ height (η²p = 0.243) and COD-R (η²p = 0.232), all favoring the EXP group. The simple main effects analysis on Time showed a significant difference for the EXP group in the CMJ (p= 0.017, ES= 0.79) and COD-R performances (p=0.021, ES=0.866). (4) Conclusions: According to our findings, implementing a combined strength-training program that incorporates isoinertial training, a vertical cone-shaped flywheel device, and triplane exercises enhances jumping ability and agility in young volleyball players.