ELI5… how do ‘reflex hammers’ work?

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When a doctor or nurse uses a hammer to hit a joint, like a knee, what actually causes the leg to kick? Are patients supposed to make the motion to kick when they feel the knock? Or does the hammer knock onto a nerve, that spasms the muscles into a kick?

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…really? Come on mate.

>In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous movement in response to a stimulus. A reflex is made possible by neural pathways called reflex arcs which can act on an impulse before that impulse reaches the brain. The reflex is then an automatic response to a stimulus that does not receive or need conscious thought.

https://en.wikipedia.org/wiki/Reflex

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>Or does the hammer knock onto a nerve, that spasms the muscles into a kick?

its more like this but not exactly.
the hammer tap in the right spot triggers a signal which travels through the nerves to near the base of the spine; in the spinal cord.
then it immediately loops straight back via a different nerve and tells the relevant knee muscles to contract.

the important thing is that the signal to move your leg never reaches the brain, it short cuts the brain and goes straight back to the knee.
this is what makes it a reflex, and not a conscious decision to move your leg.

The part of the body that is hit with the hammer is the tendon which is attached to a muscle (the reflexes are often called deep tendon reflexes). When you hit the tendon it pulls on and stretches the muscle rapidly but not enough for your body to feel it but enough for tiny stretch receptors to sense it. This sends a signal up through nerves into your spinal cord that there has been a rapid stretch of the muscle. To counteract that stretch, the spinal cord sends a muscle back causing that muscle to contract and your leg to move.

To put this into example in your knee: the patellar tendon which is attached to your quad is hit with a hammer and stretches your quad. Signal to spinal cord and back down to quad causes your quad to contract which causes your knee to extend and you to “kick”