Plyometric training with additional load improves jumping performance and isokinetic strength parameters of knee extensors and flexors in young male soccer players

This study investigated the effect of plyometric training with and without additional load on young male soccer players' jumping ability and isokinetic strength. Methods: In this randomized controlled trial, 39 U-17 male trained soccer players were randomly divided into plyometric training with additional load (PT+AL), plyometric training with just bodyweight (PTBW) and control (CON) groups. PT+AL and PTBW were performed for six weeks (2 days/week). Absolute peak torque (APT), relative peak torque (RPT), average peak torque (AvPT), time-to-peak torque (TPT), average rate of force development (AvRFD), vertical jump height (VJH), standing long jump (SLJ) and 15-second repeated jump tests (RJ15s) were assessed before and after the interventions. The findings showed that the performance of knee extensors in TPT-60 degrees/s and AvRFD-60 degrees/s, and knee flexors in APT-60 degrees/s, RPT-60 degrees/s, AvPT-60 degrees/s, AvPT-120 degrees/s, AvRFD-60 degrees/s and AvRFD-120 degrees/s significantly increased after PT+AL, compared to the CON (p < 0.05). Also, a significant improvement in jumping ability was observed in PT+AL compared to CON (p < 0.05). Additionally, PTBW also improved the performance of knee flexors in TPT-120 degrees/s and AvRFD-120 degrees/s, as well as RJ15s performance compared to the CON (p < 0.05). Furthermore, knee flexors AvRFD-60 degrees/s increased significantly after PT+AL, compared to PTBW (p < 0.05). SO, plyometric training, with or without additional load, improved young male soccer players' strength and jumping ability. However, strength parameters - especially the rate of force development - showed a greater increase following PT + AL compared to PTBW.