Exploring the relationship between reactive strength index, leg stiffness, and physical performance in adolescent male soccer players: The mediating role of maturation

The purpose of this study was twofold: i) to examine the association among maturation, reactive strength index (RSI), leg stiffness, and physical performance tests, and ii) to investigate how maturation influences the relationship among RSI and leg stiffness and physical performance tests. Forty-six male soccer players (age: 11.8 ± 1.5 years; height: 152.9 ± 10.2 cm; weight: 45.9 ± 10.5 kg) participated in this cross-sectional observational study. Players’ biological maturity was assessed using peak height velocity (PHV). Physical tests were conducted in two separate sessions on different days: one included anthropometry, standing long jump, sprint, dribbling and agility; the other included RSI, leg stiffness, and kick velocity. Correlations and bootstrap-based mediation analyses were used to examine the mediating role of PHV between RSI, leg stiffness, and performance. Mean years to PHV was −1.61 ± 0.99 years, and all players were in the pre-PHV period. RSI was strongly correlated with sprint (r = −0.73; CI 95% = −0.840 to −0.533), agility (r = −0.67; CI 95% = −0.803 to −0.471), and SLJ (r = 0.68; CI 95% = 0.487 to 0.811), while leg stiffness was moderately correlated with sprint (r = −0.38; CI 95% = −0.602 to −0.102) and kick velocity (r = 0.43; CI 95% = 0.166 to 0.642). PHV was moderately to highly correlated with all performance indicators (r = 0.37–0.65). Mediation models showed that PHV partially explained the relationship between RSI and performance tests (17–70%),whereas the relationship between leg stiffness and performance tests was fully mediated by PHV (68–96%)(p < 0.05). In conclusion, the findings indicate that considering PHV's mediating effect in assessing neuromuscular performance in young soccer players is essential and should not be overlooked in research and practice.