How does the reflex beneath the knee cap make your leg “swing” when you hit it?
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What you’re talking about it the myotatic reflex – it sounds complicated, but it’s actually one of the simplest phenomena in your body!
There’s a ton of stuff your body does automatically so you don’t have to worry about it, like digesting, keeping your heart beating, breathing, etc. Another thing it does is help your body stay balanced and let you stand up or sit down without having to readjust manually all the time.
To be able to do this, your body has a bunch of sensors, that tell your nervous system what’s going on. In this case, the ones that matter the most are the sensors inside of your muscles that tell the rest of your body how stretched your muscles are.
When the hammer hits your knee, it pulls on a tendon, which is the bit that attaches your muscles to the bone (in that case, the quadriceps in front of your thighs and your kneecap). In turn, that pulls on your muscle, and the sensors light up because the muscle stretched. They send a message through your nerves, all the way to your spine, saying “hey, that muscle extended!”. Now because your body didn’t plan on doing that, it’s gonna try to compensate. So the cell that received the message is gonna send signals to the cells that tell your muscles to contract:
* It’s gonna tell the cell in charge of the quadriceps muscle “hey, you’re extended, go flex”
* It’s also gonna tell the cell in charge of the muscles on the other side (your hamstrings), that do the opposite movement, “hey, something was wrong with the other muscle, stop flexing”
And do your body will react with more force than they should and send your leg flying!
Of course, your body does that sort of operation all the time. It’s the basic process that keeps your body balanced. But when you hit your knee with that hammer, you trick your body into thinking a big stretch is happening, which is why you get that big reflex.
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