Eccentric Hamstring Force Asymmetry After Anterior Cruciate Ligament Reconstruction: A Cross-Sectional Comparison of Independent 12- and 24-Month Postoperative Cohorts

Background/Objectives: Prior NordBord studies have documented eccentric knee-flexor asymmetry during the first postoperative year and at later follow-up after hamstring-autograft ACLR. This study directly compared relative and Newton-based eccentric hamstring deficits between independent male-athlete cohorts tested with the same single-center protocol approximately 12 and 24 months after ACLR. Methods: In this single-center cross-sectional comparative study, 60 male athletes were assessed once in an independent 12-month cohort (n = 30) or 24-month cohort (n = 30) after unilateral primary ACLR. Bilateral eccentric peak force (PF) and mean force (MF) during the Nordic hamstring exercise were recorded with a NordBord device at 50 Hz. For PF, the highest value across three maximal trials was retained independently for each limb, so the retained involved- and uninvolved-limb values could originate from different trials. The primary outcome was PF limb symmetry index (LSI; involved/uninvolved × 100); secondary outcomes included MF LSI, bilateral PF values, and PF LSI < 90%. Results: PF LSI was 84.6 ± 9.3% in the 12-month cohort and 93.5 ± 6.8% in the 24-month cohort (mean difference 8.9 percentage points, 95% CI 4.7–13.1; p < 0.001; d = 1.09). Involved-limb PF was 32.5 N higher in the 24-month cohort (95% CI 6.8–58.2; p = 0.014; d = 0.65), whereas uninvolved-limb PF was similar between cohorts (mean difference 2.5 N, 95% CI −21.9 to 26.9; p = 0.838; d = 0.05). PF LSI was <90% in 21/30 athletes (70.0%; 95% CI 52.1–83.3%) in the 12-month cohort and 7/30 (23.3%; 95% CI 11.8–40.9%) in the 24-month cohort (χ2 = 13.13; p < 0.001). Conclusions: The smaller eccentric hamstring asymmetry observed in the independent 24-month cohort was driven predominantly by higher involved-limb force rather than by a difference in uninvolved-limb force, yet nearly one quarter of that cohort remained below the 90% LSI benchmark. Postoperative time alone should therefore not substitute for late-stage objective eccentric knee-flexor assessment.