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Updated 11 April 2016

Why does your knee kick after a tap from a doctor’s hammer?

Ever wondered why your knee gives a random kick when the doctor taps it with that weird looking little rubber mallet while doing a medical check-up? An expert takes a look.

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You might have experienced the popular knee jerk in the doctors room or you may have heard of someone having an automatic 'knee-jerk response' to a situation without thinking. Strange, isn't it? Well, not if you understand why it happens.

Dr Derick van Vuuren, medical physiologist at Stellenbosch University says the spinal cord and not the brain actually controls the common knee jerk (or the patellar tendon) reflex.

Why your knee jerks

“Inside the core of all skeletal muscles are small sensory receptors called muscle spindles, which are sensitive to changes in the length of the muscles. This is important information for the brain so it can keep track of movement and also avoid muscle injury.”

In a lightning-fast version of Physiology 101, Dr van Vuuren says the brain’s job in avoiding muscle injury is closely associated with your stretch reflexes. These reflexes happen involuntarily when certain nerves ‘fire’ and sidestep the conscious parts of your brain. Your stretch reflexes also help keep you standing upright, which you’ll quickly notice if you start tipping to one side as these reflexes automatically correct your position.

Read: Specific training programmes prevent knee injuries

“Basically when the muscle spindle perceives the muscle is being stretched, it initiates a signal, causing the stretched muscle to contract (shorten) forcefully. It does this in order to oppose the stretching from whatever external force is involved,” explains Dr Van Vuuren.

He continues by saying the muscle spindle in the frontal upper leg muscles (the quadriceps) causes the knee jerk reflex, although the actual reflex starts when your doctor taps your knee.

In principle, your doctor is tapping the kneecap (patella) tendon that spans across the front of the knee and keeps the kneecap in position. The patellar tendon connects to the quadriceps muscle via the kneecap (the patella) and the quadriceps tendon.

“When you tap this tendon, it causes a minimal stretching of the muscle spindle in the quadriceps, sending a signal to the spinal cord which then conveys two signals. One stimulates the contraction of the quadriceps muscle (the stretch reflex), while the other signal encourages the muscle at the back of the upper leg (the hamstrings) to relax, resulting in that knee kick.”

Think of the knee kick as similar to what happens when you are training your quads on the leg extension machine at the gym.

Well now we get how and why your leg kicks up when tapped. However, it still doesn’t explain the benefits or why your doctor is actually doing this (aside from possibly simply enjoying whacking patients with a little hammer).

Medical author Dr Janice Rachel Mae explains that doctors routinely use reflex tests to check  if there are any problems in the nervous system involved in movement, nerve functioning or health of the connective tissue in the knee or leg.

For example, if you show no response to a knee tap, it could be a sign of nerve damage or even a more serious condition called cerebellar disease if your knee jerks continually after being tapped, she remarks. “If your muscles did not activate rapidly in response to a stretch, your legs would literally collapse if you unexpectedly place any weight on them.”

So next time your doctor approaches you with that nifty little hammer, just kick and accept it in the name of your health!

Read more:

Why do we laugh when tickled?

Why bumping your funny bone hurts

10 Fun facts about the nervous system

Sources

Dr Derick van Vuuren, Medical physiologist and lecturer, Faculty of Health and Medical Sciences, Stellenbosch University.
Healthguidance.org http://www.healthguidance.org/entry/17200/1/The-Patellar-Reflex-Test--Why-Doctors-Tap-Your-Knee.html

 
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