Watch any cartoon where a character visits the doctor and one of the things you'll see them do is to hit the patient's knee with a tiny little hammer. You may even have experienced this yourself on your own trips to the doctor, though you probably had a slightly more subdued reaction than Tom/Jerry.
So what's happening? Why does your knee kick out like that, and why is this a useful indication of anything?
This process is the result of something known as a 'stretch reflex'. That is a reflex that exists to shorten the muscle when it gets stretched, thereby preventing injury in some cases and helping you remain upright – start tipping and these reflexes will automatically correct your position. That's also why when someone comes along and tries to yank your arm down, you will automatically 'fight them' by keeping your arm locked. Like a seatbelt…
This process is an entirely unconscious one. That is to say that you don't 'set out' to contract the muscle, but rather it happens involuntarily when certain nerves fire and bypass the conscious parts of your brain.
The Patellar Ligament
A doctor wanting to test your reflexes though is unlikely to try yanking your arm against your will. That would be awkward and the fact you knew it was coming would make it tricky for them to get accurate results.
Instead then, doctors tap the patellar ligament with their little hammer (which is actually called a 'reflex hammer'). When they do this, it causes the muscle spindle in the quadriceps muscle to stretch and that in turn triggers the reflex response to pull the muscle back. This in turn then causes the leg to kick out – just as it does on the leg extension which trains the quadriceps.
Why This Test?
That's all good and well then, but what do doctors stand to learn from all this? Do they just enjoy hitting people with tiny hammers? And why don't they play MC Hammer at the same time?
While I can't answer that last question, in general there are two reasons this test is carried out. One is to test the health of the connective tissue in that part of the leg and the function of the nerves.
On the other hand, this test can be used to look for reflex problems – the absence of this reaction can be an early sign of cerebellar disease for instance.
Chances are then that you probably won't encounter this test regularly. The question then is why it remains such a popular image for doctors. And the answer to that one is easy…
It's hitting people with a tiny hammer…
What's not to like?