Lower-Limb Strength Symmetry, Postural Control, and Clinical Characteristics in Individuals with ACL Injury and Chronic Ankle Instability

Objective: To investigate the associations between lower-limb muscle strength symmetry and postural control, determine whether lower-limb symmetry explains additional variance in dynamic postural control after adjustment for demographic characteristics and diagnostic group, and compare these outcomes among individuals with anterior cruciate ligament (ACL) injury, chronic ankle instability (CAI), and healthy controls. Methods: This cross-sectional study included 72 participants (mean age, 29.6 ± 5.9 years) equally distributed among the ACL, CAI, and healthy control groups. Isokinetic strength was assessed at 60°/s and higher joint-specific velocities (240°/s for the knee and 180°/s for the ankle), with LSI calculated from peak torque at 60°/s. Static and dynamic postural control were evaluated using the single-leg stance test and a computerized stabilometric platform. Pain intensity (VAS), kinesiophobia (Tampa Scale), physical activity (International Physical Activity Questionnaire–Short Form (IPAQ-SF)), and health-related quality of life (Short Form-36) were also assessed. Between-group comparisons, Spearman correlation, and hierarchical multiple regression analyses were performed. Results: Significant between-group differences were observed in all postural control outcomes, most lower-limb symmetry indices, pain intensity, kinesiophobia, physical activity, and patient-reported outcomes (all p < 0.05). Pairwise comparisons showed better eyes-open static balance in healthy controls than in both clinical groups, poorer eyes-closed static balance in ACL than in the other groups, and better dynamic postural control in controls than in both clinical groups and in CAI than in ACL. Lower-limb symmetry differences involved different pairwise contrasts among controls, CAI, and ACL depending on muscle group and testing velocity. Two nominal associations between lower-limb symmetry and balance did not remain significant after FDR correction. Adding diagnostic group significantly increased the explained variance in dynamic postural control (ΔR2 = 0.581, p < 0.001), whereas adding lower-limb symmetry indices did not (ΔR2 = 0.015, p = 0.583). The final model explained 68.4% of the variance (adjusted R2 = 0.638). Conclusions: Individuals with ACL injury and CAI demonstrated poorer postural control than healthy controls, alongside between-group differences in several lower-limb muscle strength symmetry measures. Associations between lower-limb muscle strength symmetry and postural control were not significant after FDR correction, and symmetry indices did not explain additional variance in dynamic postural control beyond demographic characteristics and diagnostic group.