Objective: To investigate the recovery of knee flexor muscle strength evaluated with a Nordic hamstring eccentric test (NordBord) compared with an isokinetic concentric test (Biodex) during the first year after anterior cruciate ligament (ACL) reconstruction using a hamstring tendon autograft.

Design: Prospective observational registry study; level of evidence, 3.

Setting: Primary care.

Participants: Cross-sectional data of 127 patients (45% women, mean age 24.9 ± 8.1 years) were extracted from a rehabilitation outcome registry at 10 weeks and 4, 8 and 12 months after ACL reconstruction with hamstring tendon autograft.

Main outcome measures: All patients performed a concentric Biodex test, and an eccentric NordBord test on the same occasion or within seven days of the concentric test. The primary outcome was the limb symmetry index between the respective tests.

Results: A greater knee flexor symmetry deficit was observed with the eccentric test compared with the concentric test at all follow-ups with clinically relevant differences at 4 (11.8% ± 12.7% [CI 7.8–15.8%]) and 8 months (13.4 ± 11.9 [CI 9.7–17.2%].

Conclusion: The eccentric NordBord test was able to identify clinically relevant deficits in knee flexor strength symmetry that were not identified by gold standard isokinetic concentric testing during the first year among patients treated with an ACL reconstruction using a hamstring tendon autograft. © 2022 The Authors