Relationship Between Quadriceps Strength and Knee Joint Power During Jumping After ACLR

Background: Knee joint power is significantly impaired during the propulsive phase of jumping after anterior cruciate ligament reconstruction (ACLR); however, it is currently unknown how quadriceps strength influences knee joint power. Purpose: To (1) evaluate the relationship between quadriceps strength, joint power, and the percentage contribution of the hip, knee, and ankle joints to total limb power during the propulsive phase of jumping and (2) establish a quadriceps strength cutoff value for maximizing the likelihood of having knee joint power characteristics similar to healthy participants. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 75 participants were included in this study—40 patients who underwent ACLR 6 months before (18 females; mean age, 19.3 ± 5.7 years) and 35 healthy controls (HC) (20 females; mean age, 21.5 ± 4.5 years). Participants performed a drop vertical jump and underwent isometric quadriceps strength testing. The peak joint power was calculated as the product of the internal joint moment and joint angular velocity. Pearson product-moment correlations were used to assess the relationship between quadriceps strength and knee joint power. Paired samples t tests were used to quantify differences between limbs. Receiver operating characteristic (ROC) curve analysis was used to determine a quadriceps strength cutoff. Results: The involved limbs of the ACLR cohort (INV) had significantly lower peak knee joint power and percentage contribution from the knee joint during jumping compared with the uninvolved limbs (NON) and limbs of the controls (INV, 2.5 ± 1.2 W/kg; NON, 4.4 ± 1.5 W/kg; HC, 4.3 ± 1.7 W/kg P < .0001). Quadriceps strength was associated with knee joint power in involved limbs and limbs of controls (INV, r = 0.50; HC, r = 0.60). A quadriceps strength cutoff value of 2.07 N·m/kg had an area under the ROC curve of 0.842, indicating good predictive accuracy. Conclusion: Athletes at 6 months after ACLR demonstrated knee-avoidant jumping mechanics and had significant reductions in knee joint power on the involved limb. A quadriceps strength cutoff value of 2.07 N·m/kg can help predict which athletes will display knee joint power characteristics similar to those of healthy controls.