Physiologic loading of the anterior cruciate ligament does not activate quadriceps or hamstrings in the anesthetized cat.

We attempted to elicit quadriceps and hamstring electromyographic responses in seven chloralose-anesthetized cats by loading the ACL with controlled anterior displacement of the tibia on the femur using rigid fixation and an MTS testing machine. We did not detect reflex activity in the quadriceps or hamstring muscles of any of the cats in response to anterior tibial displacements of up to 4 mm, with rise times ranging from 1.0 to 0.1 seconds. In four of the cats we loaded the ACL using a wire loop. Loads of up to 125 N (4 to 5 times body weight) produced no reflex activity in any of the four animals, although we consistently observed monosynaptic reflex responses to tendon taps. Whole nerve recordings from the posterior articular nerve revealed substantial activity from afferents in response to tugging on the ACL, although we could not differentiate receptors in the ACL from those in other periarticular tissues. Thus, while traction on the intact ACL causes signals in the afferent nerves, those signals are not translated into direct monosynaptic reflexes.