Proprioceptive Mechanisms in the Cruciate Ligaments

The hypothesis of a proprioceptive mechanism arising from mechanoreceptors in the cruciate ligaments and with efferences to the thigh muscles is investigated in an animal model. Electromyographic (EMG) studies of the quadriceps and hamstrings muscles were performed in six sheep anesthetized by intravenous injection of chloralose on applying static and dynamic shear forces to isolated fascicle bundles in the anterior (ACL) and posterior (PCL) cruciate ligaments at 200 to 500 N and 50 to 200 N/sec, respectively. With the help of an external fixator, the knee joint position was varied at 25 to 150 degrees. The relative preload of the anteromedial and posterolateral fascicles in the ACL and the anterior and posterior fascicles in the PCL induced by variation of the joint angle was measured with the help of strain gages. By static and dynamic loading of the anteromedial group of the ACL, EMG activity in the hamstrings increases significantly to 159 +/- 22 and 257 +/- 46 microV seconds, respectively (p < 0.001), with a simultaneous suppression of the quadriceps activity. Static and dynamic loading of the posterolateral ACL group results in a significant excitation of the quadriceps with mean potentials of 142 +/- 29 and 173 +/- 23 microV seconds, respectively (p < 0.001). Mechanical shear applied both to the anterior and posterior fascicles of the PCL induces a significant activation of the ipsilateral quadriceps muscles with a simultaneous inhibition of the hamstrings (p < 0.001). The quality of mechanical loading, i.e., static or dynamic shear, determines the degree of muscular recruitment significantly (p < 0.01). The joint angle contributes to a modulation in the quantitative EMG excitation. Linear regression analysis of the integrated EMG potential values and the amount of passive preload induced by the respective joint position results in a close correlation (r = + 0.78 to + 0.86). The results lead to the conclusion that a proprioceptive mechanism exists that arises from the cruciate ligaments and influences the tone in the thigh muscles. The degree of muscle excitation on mechanical ligament loading is modulated by the amount of preload in the ACL and PCL and the quality of load applied.