BACKGROUND: Fatigue, sex, and asymmetries greater than 10% between limbs are factors that influence noncontact injury rates. The purpose of this study was to investigate the effects of a fatiguing bout of exercise on strength and power inter-limb functional asymmetries in elite soccer players. METHODS: 38 high-level soccer players (females n =20) completed the study (Mage= 21.2 ± 1.8y). The study consisted of two visits within a week. At the first visit, participants were familiarized with the jump tests and 5-RM single-leg press was determined. At the second visit, each participant executed performance tests pre and post a 90-minute simulated soccer game on a treadmill. This protocol was developed based on prior GPS data on elite soccer players. Performance tests included single-leg press at 90 % of their 5-RM till failure, countermovement jump (CMJ), unilateral countermovement jump (SLCMJ), drop jump (DJ), and unilateral drop jump (SLDJ) on force plates. Asymmetry index was measured for SLCMJ, SLDJ and single leg press. All metrics were analyzed using a paired t-test of pre vs. post and sex comparison analysis. Statistical significance was set at α=0.05. RESULTS: Significant decreases in jump heights pre vs post were observed in the overall sample for CMJ, DJ, SLMCMJ, and SLDJ (p<0.05), as well as a significant reduction in repetitions for single-leg press (p<0.05). Sex-specific analyses reveled differences between males and females for these reductions. CMJ (M: p=0.02; F: p=0.40), SLCMJ on R leg (M: p=0.05; F: p=0.41), SLCMJ on L leg (M: p=0.02; F: p=0.16), SLDJ on R leg (M: p<0.001; F: p=<0.001). and SLDJ on L leg (M: p<0.001; F: p=0.004). In addition, decreases in strength were seen in both legs in the single leg press test (M: p=0.002, F: p=0.04 on R leg; M: p<0.001, F: p=0.04 on L leg). Asymmetry indexes for SLCMJ, SLDJ, and single leg press were nonsignificant for both males and females (p>0.05). CONCLUSION: A 90-minute exercised-induced fatigue protocol that simulated soccer physical demands decreased performance metrics of power and strength in elite soccer players. Even though fatigue was detrimental on performance for both genders, there were no significant changes in asymmetry index pre vs post. Men presented more significant changes in inter-limb asymmetries than females. More research is needed to develop a standardized method to measure inter-limb asymmetries and their influence on sports performance.

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