Balancing the Game: The Relationship of Lower-Limb Asymmetry With Youth Soccer (Football) Performance - Laboratory vs. Field Insights.

Bartusevičius, D, Stanislovaitienė, J, and Conte, D. Balancing the game: the relationship of lower-limb asymmetry with youth soccer (football) performance - laboratory vs. field insights. J Strength Cond Res XX(X): 000-000, 2026-This study aimed to assess the relationships between: (a) laboratory- and field-based leg symmetry indexes (LSIs); (b) laboratory- and field-based LSIs and physical and ball-kicking velocity (BKV) performances; (c) physical and BKV performances. Twenty-nine male youth outfield soccer (football) players (age = 14.9 ± 0.9 years, stature = 177.1 ± 7.2 cm, body mass = 64.9 ± 9.8 kg, maturity offset = 1.6 ± 0.9 years) were recruited for the study. Laboratory-based testing included concentric peak torque of the knee extensors and flexors using an isokinetic dynamometer at 60 and 180°·s-1. Field-based testing encompassed the Y-balance test, single-leg countermovement jump, single-leg long jump, countermovement jump, 10- and 30-m sprints, and BKV. LSIs were calculated for both laboratory- and field-based tests using unilateral tasks. The relationships were assessed using Pearson's (r) and Spearman's rank correlation (ρ). Results showed trivial-to-small (p > 0.05) relationships between laboratory- and field-based LSIs. Furthermore, laboratory- and field-based LSIs showed trivial-to-moderate relationships with physical and BKV performances (p > 0.05). Moreover, BKV showed large positive relationships with countermovement jump (r = 0.531-0.572) and single-leg countermovement jump (r = 0.594-0.649), a moderate-to-large relationship with single-leg long jump (r = 0.488-0.646), and a large negative relationship with 30-m sprint (r = -0.503 to -0.536). Our findings suggest laboratory and field-based LSIs cannot be used interchangeably, and LSI should not be relied upon as an indicator of physical and technical performance. Coaches should focus on enhancing lower-limb explosiveness, given its strong association with BKV performance among youth soccer players.