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Why Doctors Tap Your Knees

Most routine checkups or pre sports physical will include a doctor, whether that be a medical doctor or chiropractor, tapping on your knee just below your kneecap with a small rubber hammer. This is not a measure of your muscle strength, or even a test directed to your knee. This is a test of your neurological system. The hammer (called a reflex hammer) is an instrument that is used to impart a quick stretch on your patellar tendon. We do this by tapping lightly on the tendon while the muscle is relaxed. This creates a quick stretch in the tendon, which causes certain mechanoreceptors housed within the tendon to fire. These receptors are called muscle spindles. Muscle spindles are special types of mechanoreceptors that respond to quick stretch by tightening up the muscle group attached to the tendon that is being tapped. This is why your leg will often kick out after being tapped. You, or more aptly stated your conscious self, are not exactly responsible for this reaction. What this tap triggers is a closed reflex loop that goes from the tendon, to the peripheral nerves attached to the activated mechanoreceptors, to the back of your spinal cord where. From there it immediately goes to the front of your spinal cord (the area where our motor units are housed) and causes them to activate. This activation then travels down the same peripheral nerve and into the muscle tissue of the quadriceps, telling them to fire. Notice anything missing? Your Brain! Thats right all of this happens in an instance, with our brain and conscious selves only recognizing what has happened after the fact. This is important because it gives us a look into how well your underlying nervous system is performing. People that have damage to their nervous system with have differences in how they respond to the tap on the knee. There are various instruments that doctors use to evaluate this reflex. The typical is a reflex hammer which is shown first. Others include the babinski reflex hammer (the second used in the video) and the Troemner reflex hammer (not shown). In some individuals it is difficult to illicit the reflex, and this is not an indication that there is anything wrong with their neurological system. Oftentimes these individuals are/were athletes including football players, skiers, or other sports requiring high muscle output. We can often make it easier to illicit the response by performing what is called a Jendrassik maneuver. In the Jendrassik maneuver the pt either grits their teeth, or interlocks their fingers while pulling them into opposition. What this does is increase motor output from our cortex, which then increases underlying muscle activity throughout the body, bringing the motor units throughout the body slightly closer to threshold. That way we can get a bigger response with the same stretch imparted on the patellar tendon. We hope you enjoyed this video and its content. Be sure to remember that the content of this video is for educational and entertainment purposes only, and should not be used for medical advice. Be sure to consult a doctor (Dr. google doesn't count) if you have any significant medical concerns.