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. 2025 Aug;33(8):2971-2983.
doi: 10.1002/ksa.12718. Epub 2025 Jun 8.

Persistent isokinetic knee flexion strength deficits at the time of return to sport are not associated with a second ACL injury

Axel Sundberg  1   2   3 , Rebecca Hamrin Senorski  2   3   4 , Johan Högberg  2   3   4 , Ramana Piussi  2   3 , Kristian Samuelsson  2   5 , Roland Thomeé  3   4 , Eric Hamrin Senorski  2   3   4   6
Affiliations

Persistent isokinetic knee flexion strength deficits at the time of return to sport are not associated with a second ACL injury

Axel Sundberg et al. Knee Surg Sports Traumatol Arthrosc. 2025 Aug.

Abstract

Purpose: To investigate the rate of a second anterior cruciate ligament (ACL) injury based on different levels of knee flexion strength limb symmetry index (LSI) at the time of return to sport (RTS) after ACL reconstruction with hamstring tendon autograft.

Methods: Data was extracted from a rehabilitation registry for patients aged 15-40 years, who participated in knee-strenuous sports pre-injury (Tegner ≥ 6) and underwent ACL reconstruction with hamstring tendon autograft. Isokinetic knee flexion strength was analysed and reported as LSI. Patients were categorised into three groups (≥90%, 80%-89.9% and <80%) based on their LSI at reported time of RTS. Patients were followed for 2 years after ACL reconstruction to record a second ACL injury, and hazard ratios (HR) were calculated using a Cox proportional hazards model.

Results: A total of 526 patients (48% female, mean age 22 ± 6) were included, with 51 (9.7%) second ACL injuries recorded within 2 years after ACL reconstruction. Among patients with LSI ≥ 90% (71%), 43 second ACL injuries (11.0%) occurred. The LSI 80%-89.9% group had 4 second ACL injuries (4.0%), and the LSI < 80% group had four injuries (8.2%). Persistent knee flexion strength asymmetry did not significantly influence the hazard of a second ACL injury. The LSI 80%-89.9% group had a lower hazard (HR 0.34, confidence interval [CI]: 0.12-0.94), while the LSI < 80% group showed no significant difference (HR 0.70, CI: 0.25-1.97) compared with the LSI ≥ 90% group.

Conclusion: Persistent isokinetic concentric knee flexion strength asymmetry at RTS were not associated with a second ACL injury.

Level of evidence: Level III.

Keywords: ACL reconstruction; hamstring strength; limb symmetry index; second ACL injury.

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Conflict of interest statement

Author Kristian Samuelsson is a board member at Getinge AB.

Figures

Figure 1
Figure 1
Flow‐chart of the patient inclusion and exclusion process. RTS, return to sport.
Figure 2
Figure 2
Frequency of a second anterior cruciate ligament (ACL) injury per month after ACL reconstruction.
Figure 3
Figure 3
Frequency of a second anterior cruciate ligament (ACL) injury per month after return to sport (RTS).
Figure 4
Figure 4
Kaplan–Meier Curve: Second ACL injury following ACL reconstruction analysed between knee flexion strength LSI groups. 1 (green). Knee flexion LSI ≥ 90%. 2 (blue). Knee flexion LSI 80‐89.9%. 3 (red). Knee flexion LSI < 80%. ACL, anterior cruciate ligament; LSI, Limb Symmetry Index.
Figure B1
Figure B1
Kaplan–Meier Curve: Second ACL injury following reported time of RTS analysed between knee flexion strength LSI groups. ACL, anterior cruciate ligament; LSI, Limb Symmetry Index; RTS, return to sport.
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