Published: March 29, 2012
Recently, a new patient came to see me about a painful condition.
I asked if she had undergone any other medical examinations. She responded that she has just received a physical from her primary care doctor.
“My doctor is very thorough,” she said. “She tapped my knee and everything.”
I asked if she knew why the doctor tapped her knee.
She paused for a moment, then responded, “I don’t really know. But, I know that means she checked me over good.”
(I decided I better tap the lady’s knee while she was in my office so she would know I had been thorough too. Just to be sure, I tapped both knees.)
Many patients are familiar with their doctor’s routine in performing a physical exam.
The tests include such maneuvers as listening to the heart, breathing sounds, and the blood vessels in the neck; feeling the pulse at the wrist and the ankle; pressing on the belly; and peering in the ears, nose, mouth and eyes. The patient might be asked to follow the doctor’s finger as it is moved back and forth in front of the eyes and then brought close to the center of the head. They might be instructed to stand on their toes, heels and then one foot at a time.
In many cases, at some point there will be some knee tapping, as well as some tapping inside the elbow, behind the elbow, above the wrist and above the back of the heel.
You may have wondered what all the tapping is about, or maybe like the lady in this story, just assumed that was an indicator that your doctor was being thorough.
When your doctor taps any of those places with the rubber hammer, the test is intended to test the integrity of your spinal reflex arc.
That explanation may lead you to wonder how the doctor is testing something related to your spine by tapping your knee.
When the tendon is tapped in specific points, this causes the muscle attached to that tendon to stretch for a brief instant. It is not much of a stretch, and it doesn’t last long, but it is enough in a healthy person to initiate an immediate response.
When the muscle is stretched for that instant, a nerve signal is immediately sent toward the spinal cord.
The signal communicates that the muscle has been stretched and is no longer maintaining the intended position of the joint that it controls.
Because the body has multiple systems that work together to maintain balance and control, this nerve signal causes an immediate reaction within the spinal cord.
In less than a second a connection in the spinal cord generates a response neurological signal that goes quickly back to the muscle being tested. The secondary nerve signal carries a command to the muscle to instantly contract.
That is why your knee jerks.
Although the knee reflex test has been the subject of thousands of jokes on sitcoms throughout the years, it actually has some serious implications.
If the knee reacts too strongly or too weakly, the doctor may suspect a neurological problem. This simple little test actually communicates quite a bit of useful information to your doctor.
The doctor tests the reflexes of several different muscles because each location can communicate something about the health of different parts of the spinal cord as well as other nerves.
Once, a patient asked me why their doctor performed the test on everyone, even people that appeared to be in obvious good health with no complaints of nerve of muscle issues.
I explained that in some cases, doctors routinely conduct certain tests on everyone, to make sure that they know very clearly what a normal reaction looks like.
If the doctor does not know what is normal, how would they recognize abnormal?
By the way, this doesn’t mean that if your doctor did not tap your knee on your last visit that you were not evaluated thoroughly. Doctors choose specific tests for specific reasons and knee tapping isn’t always necessary.
Next week: A simple medical test everyone should try at home.