Can relative muscle strength and relative hop performance cut-offs identify patients who sustain a second ACL injury?

Background The aim was to determine cut-off values for relative isokinetic quadriceps and hamstring muscle strength and relative hop performance and evaluate their ability to discriminate between patients who sustained and did not sustain a second anterior cruciate ligament (ACL) injury within 2 years after returning to pre-injury level of sports following ACL reconstruction (ACLR). Methods Data were extracted from a registry in August 2026. Patients aged 15–35 years at index ACLR using an ipsilateral autograft, rated ≥6 on the Tegner Activity Scale who returned to pre-injury level of sport, and had performed muscle strength and hop performance tests at time of return to pre-injury level of sport were included. Relative muscle strength was normalized to body mass and hop performance to body height. Receiver operating characteristic analyses were performed to determine cut-off values and discriminatory ability for second ACL injury within 2 years. Secondary analyses evaluated ipsilateral and contralateral ACL injuries separately. Results 428 patients were included (average age 21.5 ± 4.9 years; 50% female) of whom 50 (11.7%) sustained a second ACL injury within 2 years (31 ipsilateral and 19 contralateral). Relative muscle strength and hop performance demonstrated poor discriminatory ability for any second ACL injury, with AUCs ranging from 0.48 to 0.55. Discriminatory ability remained poor in side-specific analyses of ipsilateral (AUC 0.50–0.55) and contralateral (AUC 0.42–0.55) ACL injuries. Conclusion Calculated cut-off values for relative strength and relative hop performance were unable to differentiate between patients who sustained and did not sustain a second ACL injury. Level of evidence Prospective observational registry-study II. Clinical trial registration Not applicable.