Knee Extensor and Flexor Strength, Inter-Limb Asymmetry, and Force-Based H/Q Ratio in Professional Soccer Players: A Kinetic and Isometric FEMD Study

Abstract

Objective: This study aimed to characterize maximal quadriceps and hamstring force, inter-limb asymmetry, and the force-based hamstring-to-quadriceps (H/Q) ratio in Tier 3 professional male soccer players, and to compare these outcomes between kinetic and isometric conditions. Methods: Twenty-four male players underwent assessment of maximal force, asymmetry, and the H/Q ratio in the dominant (D) and non-dominant (ND) limbs. Descriptive statistics, paired tests, Benjamini–Hochberg FDR-adjusted correlations, and two 2 × 2 repeated-measures ANOVAs were applied. Results: Under the kinetic condition, force was greater in D than ND for quadriceps (589.46 ± 112.58 vs. 494.38 ± 100.64 N; p < 0.001) and hamstrings (504.63 ± 89.11 vs. 440.04 ± 96.82 N; p < 0.001). Isometric force was also greater in D for both muscle groups. Mean quadriceps/hamstring asymmetries were 20.48%/16.91% under the kinetic condition and 17.44%/13.47% under the isometric condition. Fourteen of 120 exploratory correlations remained significant after FDR correction. ANOVA showed a main effect of limb for quadriceps (η2p = 0.611) and hamstrings (η2p = 0.654), with no significant condition effect or interaction. Conclusions: The limb was the factor most consistently associated with force differences, whereas between-condition differences did not reach statistical significance. These findings characterize this sample but do not support causal, injury-risk, or test-modality superiority inferences. The fixed testing order limits the ability to rule out sequence-related effects; therefore, the observed inter-limb differences cannot be attributed exclusively to functional limb dominance.