Gamma loop dysfunction in quadriceps on the contralateral side in patients with ruptured ACL

Purpose: The aim of our study was to test for any neurophysiological abnormality in the gamma loop in the quadriceps femoris muscle on the uninjured side of patients with unilateral rupture of the anterior cruciate ligament (ACL). Methods: Maximal voluntary contraction of knee extension and integrated electromyography (I-EMG) of the vastus medialis, vastus lateralis, and rectus femoris were measured in the uninjured limb of 13 patients with unilateral ACL rupture and 10 normal subjects, before and after 20-min vibration stimulation applied to the infrapatellar tendon. Results: The mean percentage changes of maximal voluntary contraction and I-EMG in quadriceps femoris of the uninjured side of patients with ACL rupture were significantly different from those of the control group. Maximal voluntary contraction and I-EMG after prolonged vibration stimulation did not decrease as much as those of the control group even though the same protocol of vibration stimulation was applied. Conclusion: The abnormal response to prolonged vibration stimulation could represent abnormal gamma loop in the quadriceps femoris muscle of the uninjured side in patients with ACL rupture since the normal response of maximal voluntary contraction and I-EMG to prolonged vibration stimulation could not be evoked without normal function of the gamma loop.