The aim of this study was to compare reactive strength index (RSI), muscle stiffness, and aerobic capacity between male and female soccer players, and to examine their relationships with physical performance. A total of 32 soccer players participated in the study, including 16 female players (age: 18.06 ± 3.21 years; height: 1.66 ± 0.09 m; body mass: 56.87 ± 7.51 kg; BMI: 20.78 ± 2.03 kg/m²; training age: 7.09 ± 2.49 years) and 16 male players (age: 21.37 ± 3.86 years; height: 1.79 ± 0.06 m; body mass: 71.87 ± 8.93 kg; BMI: 22.51 ± 2.20 kg/m²; training age: 11.37 ± 4.22 years). To compare performance characteristics between sexes, several key physical parameters were assessed, including linear sprint velocity, aerobic capacity (via the 30-15 Intermittent Fitness Test), maximal ball velocity, leg stiffness, and Reactive Strength Index (RSI). Significant sex-based differences were found in sprint (p = 0.000, ES = 2.34), 30-15 IFT (p = 0.005, ES = 1.07), kicking velocity (p = 0.000, ES = 3.31), RSI (p = 0.043, ES = 0.75), and stiffness (p = 0.007, ES = 1.02). In females, stiffness was negatively correlated with kicking velocity (p = 0.017, ES = -8.34), while in males, RSI (p = 0.013, r = 0.604, ES = 2.47) and stiffness (p = 0.028, r = 0.548, ES = 1.82) were positively correlated with kicking velocity. Additionally, stiffness was positively related to 30-15 IFT performance in males (p = 0.049, r = 0.499, ES = 1.98). Male players showed higher RSI and stiffness values, which were more strongly associated with performance, while lower stiffness in females may support technical skills but increase injury risk. These findings highlight the importance of sex-specific training approaches that consider neuromechanical differences in performance and injury prevention.
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