Lower Functional Bilateral Deficit Is Associated with Superior Multidirectional Performance in Soccer Players

Bilateral deficit (BLD) is traditionally defined as the reduced capacity to produce force during simultaneous bilateral contractions compared with the summed output of unilateral actions. However, in applied sport settings, BLD is frequently estimated from countermovement jump (CMJ) performance, representing a functional rather than a direct mechanical measure of force production. Therefore, the aim of this study was to examine the association between a CMJ-derived functional BLD index and multidirectional performance in soccer players. Forty male university soccer players (age: 23 ± 1 years) performed unilateral and bilateral CMJ. The BLD index was calculated from jump height values obtained during these assessments. Participants subsequently completed the 505 change-of-direction (CoD) test, which was analyzed using two-dimensional video-based motion analysis. Participants were classified according to BLD magnitude into low, moderate, and high BLD groups. Group differences were assessed using Kruskal–Wallis tests with Bonferroni-adjusted post hoc comparisons. Additionally, Spearman correlation analyses were performed using BLD as a continuous variable. Significant between-group differences were observed across all temporal phases of the 505 test (p < 0.001), with players exhibiting lower BLD values demonstrating superior acceleration, deceleration, reacceleration, and overall CoD performance. Significant negative correlations were also observed between BLD and reaction time, acceleration, deceleration, reacceleration, CoD time, and CoD deficit (rs = −0.42 to −0.69; p < 0.001). No significant associations were found for stride length, acceleration ability, or inter-limb asymmetry. These findings suggest that lower magnitudes of a CMJ-derived functional BLD index are associated with superior multidirectional performance in soccer players. However, given that BLD was estimated from jump performance, the results should be interpreted as associations with a functional neuromuscular performance index rather than as direct evidence of bilateral force production capacity.