If countless comedy movies are correct, there's something inherently funny about a man getting kicked in the crotch. But if you've ever been on the receiving end of a flying groin kick, you'll know that there's nothing funny about it, because the pain is enough to even make even the toughest of guys buckle over in agony.
There's something strange about this pain, though: it's displaced. A kick to the groin doesn't cause a man's testicles to hurt - it causes his stomach to churn so badly, he might even have to throw up. But why?
To understand this, let's start at the beginning. When something smashes into a man's groin, a signal travels at a whopping 426 km/h (265 mph) to the brain. This signal, sent by a neurotransmitter called Substance P, tells the brain that something just happened in the groin area, and a pain signal needs to be sent out to alert the body that something is horribly wrong.
This is the whole reason why we experience any pain in any form - it's basically the body's alarm system.
A few other processes might take place as well. Depending on the person, this rush of severe pain and endorphin release can cause inner ear swelling, which can cause dizziness, and could trigger the cervical sympathetic ganglia, which causes people to tear up. In other words, your body makes you into a huge wuss.
Though it may seem more severe with a groin injury, these processes pretty much happen when any part of the body sustains a painful injury. With that in mind, it seems silly that pain, which is meant to alert you of injury, would express itself in a man's abdomen instead of testicles, right? Why would your body mislead you like that?
Well, it actually makes a lot of sense, because a man's testes form in the abdomen. After they descend, nerves and blood vessels are still connected to that area.
When Substance P gets released from a man's testicles, the pain signals have to travel from the abdomen and down into each testicle. Since these pathways are more present in the abdomen, it feels like the pain stops there. This isn't necessarily true, though, because a man's testicles are definitely in pain. It appears that the abdominal pain just distracts us from the pain in the testicles because it's a larger area.
As Anthony Rivas from Medical Daily explains, the vagus nerve - an extremely long and sensitive nerve that runs all the way from your face to your groin - is largely to blame here.
Since the vagus nerve is so long and sensitive, it's one of the pathways along which pain is sent when someone is kicked in the crotch. But its size and length also mean that the pain becomes more generalised when it reaches the abdomen, which is why there's more of a painful feeling there than where the man was hit.
To make matters worse, when the vagus nerve spazzes out, it can also cause nausea, headaches, and a bunch of other symptoms that are no fun for anyone.
In conclusion, a crotch shot is a perfect storm of pain and unpleasantness. Since it's such a small, sensitive area, pain signals seemingly get trapped in the abdomen. Plus, the vagus nerve and a rise of endorphins can cause a person to feel sick, sweaty, and dizzy, which is enough nonsense to make a logical person consider wearing a cup every time they leave the house.