Changes in the Force-Velocity Relationship of Knee Muscles After Anterior Cruciate Ligament Reconstruction Using the Isokinetic 2-Point Model

Purpose: After anterior cruciate ligament reconstruction (ACL-R), knee muscle strength symmetry is used as part of the returnto-sport criteria. However, little is known about the changes in the force-velocity (F-V) relationship, which could affect athletic performance. This study investigated the F-V relationship of knee muscles at 4 and 8 months after ACL-R, using the 2-point method tested by isokinetic dynamometry. Methods: A total of 103 physically trained individuals (24.6 [9.3] y, 59.2% male) who underwent primary ACL-R were included. Demographic information and surgery characteristics were collected at 6 weeks postoperatively. Isokinetic knee flexors' and extensors' peak torques were measured at 60 degrees and 240 degrees per second in the concentric mode at 4 and 8 months postoperative. Peak torques and angular velocities were converted to force and linear velocity for calculating maximum isometric force (F0) and the slope of the regression line (F-V slope). Results: At 4 and 8 months postoperative, F0 was significantly lower and F-V slope was significantly less steep (less negative) on the operated leg compared with the nonoperated leg for knee extensors (P <.001) and flexors (P <.001-.002). The limb symmetry index calculated using F0 was lower than the limb symmetry indexes assessed at 60 degrees and 240 degrees per second, especially for knee flexors (P <.001). The use of patellar tendon grafts was associated with lower F0 and a less steep F-V slope compared with hamstring tendon grafts (P <.010). Conclusion: The isokinetic 2-point model assessing the F-V relationship provides additional and relevant insight for evaluating knee muscle strength after ACL-R.