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Abstract Background The effects of caffeine have been widely explored using a repeated measures approach, but this does not represent real competition events. Scarce evidence with independent conditions and high participant numbers has been developed to understand the caffeine effects on the performance in athletes. The aim of this study was to evaluate the effects of acute caffeine supplementation on performance between two independent groups of soccer players. Methods 81 soccer players participated in this study, who were divided into caffeine (experimental group n = 44; administered 6 mg.kg −1 of caffeine) and control (placebo group n = 40; administered 6 mg.kg −1 of cellulose) groups. After 60 min of caffeine supplementation, the players performed a battery of tests to assess flexibility, strength, speed, power, and aerobic power. Student’s t test was used to compare exercise performance between the experimental and control groups. To test the consistency of the results, the variables that showed improvements were divided into quartiles and compared using Student’s t test. The limits and likelihoods of substantial change (harm or benefit) and conversion to the smallest substantial change were used to infer the true (population) effects of caffeine on performance. The significance level was set at p < 0.05. Results Right knee force (64.6 ± 13.8 vs. 58.1 ± 14.5 kgf, p < 0.05; ES = 0.46), peak power (877.9 ± 99.2 vs. 809.1 ± 163.1 W, p = 0.02; ES = 0.50), VO 2max (44.3 ± 4.7 vs. 42.1 ± 5.5 ml.kg −1 .min −1 , p = 0.04; ES = 0.48), and covered distance (1671.7 ± 297.7 vs. 1534.5 ± 279.1 m, p = 0.04; ES = 0.47) were higher in the caffeine group than in the control group. Conclusion In conclusion, caffeine improved the performance in some variables related to strength, power, and aerobic power profile in soccer players. These results suggest that caffeine may be a valuable supplement for improving match-relevant physical performance in sub-elite level soccer players.
Oliveira et al. (Mon,) studied this question.