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Keeping the Thigh Muscle in the Loop

Injury to the anterior cruciate ligament (ACL) in the knee is really more than just a torn ligament. Along with damage to the ligament comes weakness of the quadriceps muscle. The quadriceps is the large muscle that comes down over the front of the thigh from the hip to the knee.

Scientists are trying to unlock the puzzle of this muscle weakness. The muscle isn't wasting away, a condition called atrophy. So what could be causing this weakness? Is there some way that the ligament signals the muscle to contract in a normal knee? What other changes occur after a ligament is torn or damaged that could affect the muscle?

Researchers at the University of Tokyo in Japan set out to answer these questions. They found signals that form a complete loop from knee joint to ligament to muscle. This is called afferent feedback. Before the experiment, scientists thought that damage to the ACL alters this loop. Decreased signals to the muscle cause the weakness.

To prove this, they used vibration to the tendon of the quadriceps muscle. After vibrating the tendon, they measured muscle strength in patients with a torn ACL and compared this to people with a normal ACL. By looking at the muscle response to vibration, the scientists could measure differences in signal loops with and without a torn ACL.

Sure enough, they found that damage to the ACL leads to weakened signals from the joint to the muscle. This confirms the scientists' belief that there is a complete loop of communication between these structures. Damage to the ligament disrupts the loop and leads to muscle weakness.

References: Yu Konishi, et al. Possible Mechanism of Quadriceps Femoris Weakness in Patients With Ruptured Anterior Cruciate Ligament. In Medicine & Science in Sports & Exercise. September 2002. Vol. 34. No. 9. Pp. 1414-1418.

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