The Knee Ligament's Connected to the . . . Spinal Cord?Ligaments attach bones to bones. The anterior cruciate ligament (ACL) connects the femur to the tibia. In a way, it also connects the knee to the brain, and to the hamstring muscles of the thigh.
Animal research has shown a nerve pathway that goes from the ACL to the spinal cord and back to the hamstring muscles. This is called the ACL-hamstring reflex arc. Recent studies have shown that the human ACL sends nerve signals to the spinal cord. This study was designed to test whether humans also have the ACL-hamstring reflex arc.
Researchers attached electrodes to the hamstring muscles of nine men with healthy knees. They also inserted electrodes into their ACLs. The muscle reactions were then monitored as the ACL was electrically stimulated. The hamstring muscles of all nine subjects had reflex reactions when the ACL was electrically stimulated.
The ACL was then numbed with an anesthetic, and the tests were repeated. The hamstring muscles showed no reaction after anesthesia. These results support the theory that humans also have an ACL-hamstring reflex arc.
These signals that go from the ACL to the spinal cord help keep the knee in safe alignment. This "sixth sense" is called proprioception. Our sense of proprioception alerts the body about the position of its joints and muscles. Research such as this suggests that proprioception may be one of the most important factors in knee function.
New information like this is important. It can help surgeons develop more effective ACL surgeries, and it can help therapists design better rehabilitation programs for their patients with ACL problems.
Eiichi Tsuda, MD, et al. Direct Evidence of the Anterior Cruciate Ligament-Hamstring Reflex Arc in Humans. In The American Journal of Sports Medicine. January/February 2001. Vol. 29. No. 1. Pp. 83-87.
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