Effects of repeated-sprints with changes of direction on youth soccer player's performance: Impact of initial fitness level.Nakamura, Fábio Y.; Sanchez-Sanchez, Javier; Ramirez-Campillo, Rodrigo; Petisco, Cristina; Gonzalo-Skok, Oliver; Rodriguez-Fernandez, Alejandro; Miñano, JavierJournal of Strength & Conditioning Research: Post Acceptance: September 06, 2017 doi: 10.1519/JSC.0000000000002232 Original Research: PDF Only Abstract The aim of this study was to assess the effect of an 8-week repeated sprint with change of direction training program on repeated sprint, 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 yr) and soccer plus repeated sprint with change of direction (RS-COD) training programs for players with high (n=10; age, 14.4+/-0.5 yr) and low (n=10; age, 14.4+/-0.5 yr) aerobic fitness. RS-COD was completed two days per week, involving three sets of ten 18-m sprints with two changes of direction of 90[degrees]. The soccer-only group achieved greater improvements in intermittent endurance (ES = 0.61) and sprint decrement during RS ability test (ES = 0.77) compared to the RS-COD groups (ES = 0.19 to 0.24; ES = -0.27 to 0.02; respectively). RS-COD training improved repeated sprint (ES = 0.16 to 0.38) and change of direction (ES = 0.48) performance in high, but not in low fitness players (ES = 0.03 to 0.13; ES = 0.16; respectively). Moreover, improvements in repeated sprint and change of direction were (possibly) greater in high compared to low fitness and soccer-only trained players (possibly to very likely). In conclusion, although RS-COD training may positively affect repeated sprint 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. Copyright (C) 2017 by the National Strength & Conditioning Association.