Using the Single-Leg Hop Test to Determine Knee Extension Strength After Anterior Cruciate Ligament Reconstruction.

Background: Strength assessment and the single-leg hop test are used to assess an athlete's readiness to return to sports after anterior cruciate ligament (ACL) reconstruction (ACLR). Other factors may confound this relationship, limiting the clinical utility of the single-leg hop test. Purposes: To determine whether the relationship between single-leg hop performance and knee extension/flexion torque is affected by confounding variables and to assess whether the strength of the ACL-reconstructed limb could be predicted using the single-leg hop distance. Study Design: Cross-sectional study; Level of evidence, 3. Methods: Included were 35 men (age, 29.5 ± 6 years; weight, 83.5 ± 8.9 kg) who had undergone ACLR with hamstring tendon autografts. A homogeneous participant population was intentionally recruited to limit variation in outcome measures, increasing the likelihood of successfully predicting strength. The single-leg hop distance and knee extension/flexion strength were assessed. Correlation coefficients were calculated to assess the relationship between single-leg hop test, knee extension/flexion strength, and age. Linear modeling was used to predict knee extension strength, and logistic regression was used to predict which participants met commonly used clinical cutoff values of strength symmetry-measured as the limb symmetry index (LSI). Results: Strength variables were moderately related to knee extension strength and the single-leg hop distance, although these relationships exhibited wide 95% CIs. The mean error when predicting knee extension strength based on the single-leg hop distance, eccentric knee flexion strength, and age was 0.38 N·m/kg (18%). Eight participants (23%) were incorrectly classified as having a knee extension strength LSI of <85% when using logistic regression to determine whether clinical cutoff values for symmetry were met. Conclusion: Although the relationship between the single-leg hop test and knee extension strength was moderate, this relationship was imprecise, and predicting strength based on the single-leg hop test resulted in a clinically meaningful amount of error. Based on study findings, it is unlikely that the single-leg hop test provides clinical utility if the goal is to assess knee extension strength.