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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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”
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