Directional Agreement of Inter-Limb Asymmetry Between Isometric Knee Strength and Unilateral Jump Tests in Elite Female Football Players: A Cross-Sectional Study

Background Screening batteries in women's football commonly infer limb-specific weakness from a single test, which presumes that inter-limb asymmetry is a property of the athlete rather than of the task.Objective To determine the level of agreement in the direction of inter-limb asymmetry across isometric knee extension, isometric knee flexion, unilateral squat jump and unilateral countermovement jump in elite female football players.Methods Cross-sectional study of the complete senior squad of the reigning Bulgarian women’s national champion club (N = 19; 14 with complete strength data, 17 with complete jump data). Isometric knee strength was assessed with a Kineo dynamometer and jump height with an Optojump Next optical system. Directional agreement was quantified with Cohen’s kappa and associations with Spearman’s rho, both with bootstrap 95% confidence intervals.Results Complete data were available for 14 players in strength, 17 in jumping and 12 in both. Directional agreement did not exceed the fair level for any task pair (κ = −0.09 to 0.31): isometric knee extension against both jump tasks κ = −0.09, knee flexion against both κ = 0.31, and the two jump tasks κ = 0.06, with every confidence interval crossing zero. Only 3 of 12 players favoured the same limb across all four tasks. Mean asymmetry ranged from 5.2 ± 6.0% (knee extension) to 9.7 ± 5.8% (unilateral squat jump), and the proportion exceeding 10% varied from 21.4% to 52.9% by task. Six of eight correlations between asymmetry magnitude and bilateral jump height were trivial or small.Conclusion Both pre-specified hypotheses were supported. Asymmetry appears to be a property of the task rather than of the athlete, so a single screening test cannot support limb-specific intervention directed at a quality it did not measure — a constraint of particular consequence where screening is necessarily limited to one test.