Functional deficits after anterior cruciate ligament injury are associated with altered neuromuscular control: a cross-sectional study.

Objectives: Persistent muscle weakness after anterior cruciate ligament (ACL) injury may coexist with altered antagonist coactivation, but how these neuromuscular deficits differ according to injury chronicity and relate to patient-reported knee function remains incompletely characterized. This study aimed to compare knee muscle strength and antagonist coactivation among acute and chronic ACL injury groups, and healthy controls, and to evaluate the associations among muscle strength, antagonist coactivation, and Knee injury and Osteoarthritis Outcome Score (KOOS) subscale scores. Methods: This study included 114 patients with unilateral isolated ACL rupture [55 acute (<6 weeks) and 59 chronic (>6 months)] and 57 healthy controls. Maximal voluntary isometric contraction and concentric isokinetic knee extension and flexion at 60°/s were assessed. Surface electromyography quantified hamstring and quadriceps antagonist coactivation. KOOS subscales were collected in the ACL-injured groups. Group and sex comparisons, Spearman correlations, and multiple linear regression analyses were performed. Study design: Cross-sectional study; Level of Evidence: III. Results: Affected-limb extension peak torque was lowest in the acute group and remained lower in the chronic group than in controls for both male participants (2.23 ± 0.64, 3.05 ± 0.60, and 3.48 ± 0.59 N⋅m⋅kg Conclusion: ACL injury was associated with lower knee strength and greater antagonist coactivation, with distinct patterns according to injury chronicity. Strength deficits were most pronounced in patients with acute ACL injury, whereas antagonist coactivation was generally greatest in patients with chronic ACL injury. Greater hamstring coactivation and lower knee extension torque were associated with worse patient-reported knee function.