Background There is a need for better understanding of how knee flexor strength influence patient-reported outcomes (PROs) after anterior cruciate ligament (ACL) reconstruction. Therefore, the aim was to investigate the relationship between the eccentric NordBord test and the seated concentric Biodex test, with PROs, during the first year of rehabilitation after ACL reconstruction with a hamstring tendon (HT) autograft.

Methods Data of patients with an index ACL reconstruction with an HT-autograft participating in a rehabilitation registry were screened for inclusion. Outcomes of interest were the correlation between absolute (N/kg or Nm/kg) and relative (limb symmetry index) knee flexor strength measured in the NordBord and Biodex with the results of PROs. The significance level was set at p<0.05 and Pearson’s correlation coefficient was used.

Results 137 patients were included (47% women) with a mean age of 24.8±8.4 years. There were non-significant and weak correlations between relative strength for all PROs. Significant and weak correlations between absolute strength in the Biodex with the K-SES18present at 4 and 8 months, and for the ACL-RSI at 12 months was observed, accounting for 8.4–15.7% of the variance. Significant and weak correlations between absolute strength in the Nordbord with the KOOS-Sports at 4 months, the K-SES18present and the ACL-RSI at 8 months were observed, accounting for 9.4–14.4% of the variance.

Conclusion Absolute knee flexor strength relative to bodyweight for both the Biodex and NordBord test appeared to have a stronger relationship with perceived knee function than relative knee flexor strength, although the observed correlations were weak.