Bilateral strength asymmetry in knee extension: a reliability and consistency analysis for scientific and practical applications.

Background: Muscular strength imbalances are associated with musculoskeletal injuries and performance deficits. This study aimed to assess the reliability and consistency of bilateral strength asymmetry (BSA) in quadriceps muscles as determined by dynamometry. Methods: Twenty-nine physically active participants underwent a familiarization and two test sessions, assessing isometric (100°, 140° knee angles) and isokinetic (30° s-1, 400° s-1) knee extension. BSA was calculated from Peak torque (PT), and reliability was evaluated using paired t-test, intraclass correlation coefficients (ICC), standard error of measurement (SEM), minimum detectable change (MDC), and Kappa coefficients for limb dominance consistency. Bland-Altman plots assessed agreement between sessions. Results: No significant differences in mean PT or BSA were found between sessions. However, considerable individual variability was observed, with 21%-38% of participants showing between session limb dominance reversals. ICC values ranged from 0.33 to 0.70 (poor to moderate reliability), SEM from 5.0% to 8.3%, and MDC from 13.9% to 22.9%. Kappa coefficients indicated less than chance to moderate agreement for limb dominance between sessions (0.19-0.59) and within sessions (-0.05-0.50). Bland-Altman analyses revealed small bias but wide limits of agreement. Conclusions: BSA measurements demonstrated low reliability and inconsistency, with significant individual variability and frequent limb dominance reversals. These findings raise concerns about the use of fixed asymmetry thresholds for injury risk assessment. Task-specificity and individual motor control variations may contribute to these inconsistencies. Future studies should consider habituation and standardized training protocols before evaluating BSA reliability. The results emphasize the need for cautious interpretation of BSA data and highlight limitations in its direct clinical application.