Sanchez-Sanchez, J, Ramirez-Campillo, R, Petisco, C, Gonzalo-Skok, O, Rodriguez-Fernandez, A, Miñano, J, and Nakamura, FY. Effects of repeated sprints with changes of direction on youth soccer player's performance: impact of initial fitness level. J Strength Cond Res 33(10): 2753-2759, 2019-The aim of this study was to assess the effect of an 8-week repeated sprint with change of direction (RS-COD) training program on RS, intermittent endurance, and change of direction performance in youth soccer players with different aerobic fitness levels. Athletes were randomly assigned into a soccer-only (n = 9; age, 14.9 ± 0.4 years) and soccer plus RS-COD training programs for players with high (n = 10; age, 14.4 ± 0.5 years) and low (n = 10; age, 14.4 ± 0.5 years) aerobic fitness. Repeated sprint with change of direction was completed 2 days per week, involving 3 sets of ten 18-m sprints with 2 changes of direction of 90°. The soccer-only group achieved greater improvements in intermittent endurance (effect size [ES] = 0.61) and sprint decrement during RS ability test (ES = 0.77) compared with the RS-COD groups (ES = 0.19-0.24; ES = -0.27 to 0.02; respectively). Repeated sprint with change of direction training improved RS (ES = 0.16-0.38) and change of direction (ES = 0.48) performance in high but not in low-fitness players (ES = 0.03-0.13; ES = 0.16; respectively). Moreover, improvements in RS and change of direction were (possibly) greater in high compared with low fitness and soccer-only trained players (possibly to very likely). In conclusion, although RS-COD training may positively affect RS and change of direction performance, its effects may vary according to the initial aerobic fitness of youth soccer players, with trivial effect on intermittent endurance in high-aerobic fitness players and possible beneficial effect on low-aerobic fitness players.