Correlation of the Asymmetry Index from the Single-Leg Countermovement Jump with the Asymmetry Index from Isokinetic Strength in Elite Youth Football Players

Football is a sport in which athletes perform many movements using one of their legs, which is characterized as dominant. This differentiation in usage often creates asymmetries between the limbs, which has been noted to be related to the occurrence of injuries as well as negative effects on performance. To assess these asymmetries, vertical jump tests, as well as tests using an isokinetic dynamometer, were employed. The aim of this study was to examine the relationship between the asymmetries observed in the application of the single-leg countermovement jump (SLCMJ) and those observed through isokinetic strength assessment at 60°/s, 180°/s, and 300°/s in high-level youth football players. The study involved 63 elite youth football players. For statistical analysis, correlation analysis and linear regression analysis were used. The results showed a significant positive correlation between the CMJ Bilateral Asymmetry Index and the BAI Anterior Quadriceps (60°/s) (r = 0.262, p = 0.038, 95% CI [0.016, 0.479]), indicating a small-to-moderate effect size (Fisher’s z = 0.269, SE = 0.129). Additionally, a significant negative correlation was identified between the CMJ Bilateral Asymmetry Index and the BAI Posterior Hamstrings (60°/s) (r = −0.319, p = 0.011, 95% CI [−0.525, −0.077]), demonstrating a moderate inverse relationship (Fisher’s z = −0.331, SE = 0.129). The overall regression model was significant, F(6,56) = 2.42, p = 0.038, R2 = 0.206, indicating that the predictors collectively explained approximately 20.6% of the variance in the CMJ Bilateral Asymmetry Index. From the findings of this study, we conclude that the SLCMJ asymmetry index cannot replace the asymmetry index from isokinetic strength tests.