Effects of Lidocaine into Knee on QF Strength and EMG in Patients with ACL Lesion

Purpose: In our previous study, we found that injection of lidocaine into intact knees reduced the maximal voluntary contraction (MVC) and integrated electromyogram (I-EMG) of the quadriceps femoris (QF). This study was designed to investigate changes in the MVC and I-EMG of the QF in response to lidocaine, in patients with anterior cruciate ligament (ACL) lesion, to evaluate α-motoneuron activity innervating the QF. Methods: The MVC of knee extension and I-EMG of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles were measured in eight patients with ruptured ACL, before and after lidocaine injection into the knee. Results: There were no significant differences between preinjection and postinjection values of MVC (preinjection: 167 ± 49 N·m; postinjection: 164 ± 55 N·m) and I-EMG (preinjection: VL: 0.11 ± 0.06, VM: 0.13 ± 0.10, RF: 0.09 ± 0.04) (postinjection: VL: 0.12 ± 0.07, VM: 0.13 ± 0.10, RF: 0.09 ± 0.05). Conclusion: Our results indicated that hindrance of afferent feedback from the knee in patients with ACL rupture did not significantly change α-motoneuron activity. Lidocaine injection into the knee joint of the subjects in this study only attenuated afferent feedback from mechanoreceptors in the knee joint cavity, but not in the ACL, as afferent feedback from ACL was already lost due to ACL rupture. This indicated that attenuation of afferent feedback from mechanoreceptors in the knee joint cavity other than the ACL did not significantly decrease the activity of α-motoneurons innervating the QF during MVC exertion. Therefore, our findings provide evidence that afferent feedback from the ACL has a major influence on the MVC exertion of the QF.