Muscle performance but not biomechanics associate with second knee injury in a matched cohort of athletes who passed functional return-to-sport criteria after ACL reconstruction

Abstract

Purpose: Athletes remain at risk for a second knee injury after anterior cruciate ligament reconstruction (ACLR), even after passing return-to-sport (RTS) testing. While biomechanical asymmetries have been linked to reinjury, it is unclear whether deficits persist in athletes who meet RTS criteria. This study compared muscle performance and biomechanical function between athletes who sustained a second knee injury after passing RTS testing and matched controls without reinjury. Methods: In this case-control study, 11 athletes who returned to their preinjury sport within one year after ACLR, passed RTS testing, and subsequently sustained a second knee injury were matched to controls without reinjury. Muscle performance and biomechanical assessments were conducted within two weeks of passing RTS testing, prior to second knee injuries. Muscle performance was assessed using isometric and isokinetic strength testing on an electromechanical dynamometer. Biomechanical function was evaluated during bilateral drop vertical jumps, unilateral drop vertical jumps, and single-legged hop for distance using three-dimensional motion analysis with embedded force platforms. Muscle performance and biomechanical function were compared between limb (injured, uninjured) and group (second knee injury, no second knee injury) using 2 × 2 mixed analyses of variance (ANOVA). Results: Eight athletes reinjured the ipsilateral knee and three injured the contralateral knee, with reinjury occurring on average 15.4 ± 5.0 months after ACLR. Compared with controls, the second injury group showed greater quadriceps rate of torque development symmetry during early isometric contraction (0–100 ms; p = 0.008). No other significant differences in muscle performance or biomechanical variables were observed. Conclusion: Greater and more symmetric quadriceps RTD was associated with second knee injury, while biomechanical measures did not differ between groups. These findings suggest that conventional RTS tests may not adequately detect at-risk athletes after ACLR, and broader assessment strategies may be needed to guide targeted injury prevention. Level of Evidence: Level III.