Comparison of Limb Symmetry Index Values Across Different Knee Flexor Strength Testing Conditions in Healthy Male Recreational Athletes

Background/Objectives: Restoring lower-limb strength and symmetry is crucial after ACL injury and reconstruction. The limb symmetry index (LSI) is often used to assess strength symmetry for return-to-sport decisions, but various assessment methods can influence outcomes. This study aimed to compare LSI across common knee flexor testing methods in healthy male athletes and to examine associations between absolute strength outcomes, thereby establishing baseline reference values for LSI in a healthy population. Methods: Twenty-two healthy recreationally active males participated in this prospective cross-sectional study. Knee flexor strength was assessed bilaterally using three force plate isometric tests, a static dynamometer-based test (isometric), and isokinetic dynamometer-based tests. Absolute strength values were normalized to body mass. LSI values were calculated for each testing condition. Differences in LSI across modalities were analyzed with repeated-measures ANOVA, and associations between normalized strength outcomes were assessed using Pearson correlation coefficients. Results: LSI values ranged from 96.69 to 101.83 across the testing conditions, with no significant differences observed between measures. Normalized absolute strength outcomes demonstrated very strong correlations within the same measurement category (r = 0.86–0.94 for force plate tests and r = 0.88–0.96 for isokinetic tests). In contrast, correlations between isometric and isokinetic strength outcomes were moderate (r = 0.41–0.67). Conclusions: LSI values were consistent across knee flexor strength testing modalities, suggesting that symmetry assessment was relatively consistent across different measurement methods in the studied group. In contrast, normalized absolute strength outcomes showed only moderate and variable associations across modalities, indicating that different testing approaches assess related but not interchangeable aspects of muscle strength.