By Dr. Mitchell Hecht
Q: I’ve always found it kind of funny the way my leg jumps when my doctor taps on the knee with his rubber hammer. What causes the knee to jerk like that? — D.H., Menomonie, Wis.
A: It’s a mighty fast reflex, with the time it takes between the hammer tap and the start of the leg kick averaging a blistering five one-hundredths of a second! The biceps tendon, the triceps tendon and the Achilles tendon all do the same thing. The reflex itself is pretty simple: When your doctor strikes the patellar tendon with his rubber hammer, it causes a momentary tension in the tendon
The nerve receptors within the quadriceps thigh muscle detect this and send a message to the spinal cord, which in turn relays a signal to the muscle that it needs to move (contract). But since the tension was momentary, the muscle then quickly relaxes when it realizes that it was all just a false alarm. It’s all happening so fast that the brain, while informed of what’s happening, takes no part in the actual knee-jerk response. Bypassing the brain makes the response quicker.
Amusement aside, we need these lightning-fast reflexes. If you’ve ever burned yourself, you can appreciate the importance of having fast reflexes. Although many reflexes are protective, some are merely to help coordinate complex events such as swallowing a piece of food. Doctors look at the strength and speed of a reflex to determine if there’s a neurological problem.
With some folks, I can’t easily get a reflex. It’s not that they have anything wrong with them; they’re just too focused on what I’m doing to relax. For others, a decreased reflex may indicate a peripheral nerve problem. If a person has leg or arm weakness and pain and a decreased reflex, a compressed nerve root may be the cause. Spinal cord and brain injuries tend to cause exaggerated reflexes. Slow or delayed reflexes suggest a possible underactive thyroid. It’s important to look at the reflexes in the context of a complete medical exam.
Dr. Mitchell Hecht is a physician specializing in internal medicine. Send questions to him at: “Ask Dr. H,” P.O. Box 767787, Roswell, GA 30076. Because of the large volume of mail received, personal replies are not possible.