Relationship Between Isokinetic Knee Strength and Single-Leg Drop Jump Performance 9 Months After ACL Reconstruction

BACKGROUND: Deficits in knee strength after anterior cruciate ligament reconstruction (ACLR) surgery are common. Deficits in the single-leg drop jump (SLDJ), a test of plyometric ability, are also found. PURPOSE: To examine the relationship between isokinetic knee strength, SLDJ performance, and self-reported knee function 9 months after ACLR. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: Knee isokinetic peak torque, SLDJ jump height, contact time, and reactive strength index (RSI), as well as International Knee Documentation Committee (IKDC) scores were assessed in 116 male, field-sport athletes at 9.2 months after ACLR. SLDJ testing took place in a 3-dimensional biomechanics laboratory. Linear regression models were used to analyze the relationship between the variables. RESULTS: A significant relationship was found between ACLR-limb isokinetic knee extensor strength and SLDJ jump height (P < .001, r (2) = 0.29) and RSI (P < .001, r (2) = 0.33), and between ACLR-limb isokinetic knee flexor strength and SLDJ jump height (P < .001, r (2) = 0.12) and RSI (P < .001, r (2) = 0.15). A significant positive relationship was also found between knee extensor asymmetry and SLDJ jump height asymmetry (P < .001, r (2) = 0.27) and SLDJ reactive strength asymmetry (P < .001, r (2) = 0.18). Combined ACLR-limb jump height and contact time best predicted IKDC scores (P < .001, r (2) = 0.12). CONCLUSION: Isokinetic knee extension strength explained approximately 30% of SLDJ performance, with a much weaker relationship between knee flexion strength and SLDJ performance. Isokinetic strength and SLDJ performance were weak predictors of variation in IKDC scores.