ACL Connected to the . . . QuadricepsResearchers can't always find the answers they need in one study. Sometimes it takes a series of studies performed step-by-step. Scientists at the University of Tokyo in Japan are doing studies on the strength of the quadriceps muscle. The quadriceps muscle covers the front of the thigh. Its job is to straighten the knee.
The first study done by these researchers injected a numbing agent (lidocaine) into the knee cavity of subjects with healthy knees. They measured the strength of the quadriceps before and after the injection. They found that injecting a local anesthetic into the knee joint reduces the strength of muscle contraction.
This tells them that signals coming from the knee joint affect the strength of the quadriceps muscle. It doesn't tell exactly which structure inside the joint is affected. It could be cells lining the joint, ligaments inside the joint, or some other soft tissue.
Next, the study was repeated with one difference. The subjects all had a torn anterior cruciate ligament (ACL). Once again, muscle strength was measured before and after lidocaine was injected into the knee joint. There was no difference in muscle strength. This led the researchers to conclude that a loss of quadriceps strength happens because signals from the ACL somehow affected the muscle.
These findings help explain why patients with a torn ACL have quadriceps muscle weakness when nothing is wrong with the muscle. The ACL has sensors that receive and send signals. These sensors are called mechanoreceptors. The next step may be to find out which mechanoreceptors are affected most by damage to the ACL and show how this results in lost muscle strength. The final step is to find a way to treat it.
Yu Konishi, PhD, ATC, et al. Effects of Lidocaine into Knee on QF Strength and EMG in Patients with ACL Lesion. In Medicine & Science in Sports & Exercise. November 2003. Vol. 35. No. 11. Pp. 1805-1808.
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