Purpose: To compare the effects of plyometric drop jump training against those induced by regular soccer training, and assess the transference effect coefficient (TEC) of drop jumps (“trained exercises”) performed from 20- [DJ20] and 40-cm [DJ40] height boxes with respect to different physical qualities (jumping; linear and change of direction speed; kicking; endurance; maximal strength) in youth male soccer players. Methods: Participants were randomly divided into a control group (n=20; age: 13.5±1.9 years) and a drop jump (DJ) training group (n=19; age: 13.2±1.8 years); and trained for 7 weeks. A two-way ANOVA for repeated measures with the within-subject factor time (pre- and post-intervention) and between-subject factor group (intervention vs. control) was performed. To calculate the TECs between DJ20-DJ40 and the physical tests, the ratios between the “result gains” (effect-size [ES]) in the analyzed physical qualities and the result gains in the trained exercises were calculated. The TECs were only calculated for variables presenting an ES ≥0.2. Results: Significant improvements (ES=0.21-0.46; P< 0.05) were observed in the DJ training group, except in linear sprint performance. The control group improved only maximal strength (ES=0.28; P < 0.05). Significant differences were observed in all variables (ES=0.20-0.55; P< 0.05) in favor of the DJ training group, except for maximal strength (group x time interaction). Conclusion: A plyometric training scheme based on dropjumps was able to significantly improve the physical performance of youth male soccer players. Overall, greater TECs were observed for DJ40 (0.58-1.28) than DJ20 (0.55-1.21)

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