Getting kicked anywhere hurts.

But getting kicked in the groin really hurts. It hurts so much that in most martial arts it’s an illegal move, sometimes called a “low blow”.

And that phrase has come to mean any kind of shameful, unfair attack. Even if you’ve never experienced the immense pain of that low blow yourself, you’re probably at least familiar with it from things like slapstick comedy, self-defense, the protective cups of athletes, or America’s Funniest Home Videos. You might think of this as a male-only issue, but getting kicked in the groin it’s gonna hurt anybody.

There’s a whole collection of factors that come together to make getting hit in the groin the worst thing ever: you have more nerves there, and it’s an area that’s especially exposed and soft. But it gets worse: when you take a shot to the groin, your brain also tends to think you should feel pain in places that weren’t even hit. The first problem is the nerves thing.

You feel pain the same way you sense other things: through sensory neurons. And your groin is packed with them. There are different types of sensory neurons with different receptors.

The ones that sense pain are called nociceptor neurons, and like all neurons, they obey what’s called the all-or-nothing law. That means that a neuron either sends a signal, or it doesn’t. There’s no in-between — it can’t send like a stronger or weaker signal.

So if you feel more pain, that’s because there are more neurons firing. And since your groin has many sensory neurons — including nociceptor neurons — the pain you feel there can get especially intense. But it’s not just the neuron density that makes getting kicked in the groin so awful.

It’s also the way everything down there is positioned. The testicles, especially, are really vulnerable. They’re totally exposed, with no surrounding bones or other support that might help protect them from some of the impact.

It might seem like a bad idea for organs that are so crucial to like the continuation of your genes to be so vulnerable, but they have to be. The best temperature for producing healthy sperm is a degree or two cooler than normal body temperature. So the testicles kinda hang there, and if they’re hit at the right angle, they will absorb the whole blow all by themselves.

Ovaries, meanwhile, are safely inside the body, but there’s still plenty of potential for pain because of the way the vulva and pubic bone are positioned. If the kick is powerful enough, the clitoris can get pinned and compressed between the bone and kicker’s foot. That will cause plenty of pain, too, but it’s less common.

There are just a lot more ways to hurt testicles, since they’re so much more exposed. And it’s not just the groin that hurts. A big part of why getting kicked in the groin is so horrible is that the pain radiates to other places — what’s known as referred pain.

You get kicked in the groin, and suddenly your whole abdomen hurts. And then your like shoulders and neck. It’s like EVERYTHING.

Researchers still aren’t totally sure how referred pain works, but they think it has to do with the way neurons are connected to each other. The neurons in your skin are pretty well-organized. If, say, you get a papercut on your finger, your brain is great at figuring out exactly where the pain is coming from.

But it’s not as good at pinpointing what you’re feeling deeper inside your body. There are fewer receptors in there, and often one more central neuron will detect signals from a bunch of different sources. All your brain knows is that you were hurt somewhere in a certain area of your body, so it just tells you that that whole area hurts.

Referred pain will often spread through the area controlled by the same part of the spinal cord, but it can spread farther than that if the pain is bad enough. Like if someone’s nuts are being crushed by a foot. So getting kicked in the groin really is the worst of both worlds: there are tons of neurons close to the surface ready to fire pain signals, but your brain isn’t equipped to figure out exactly where the pain is coming from when your internal organs are being crushed.

The result is incredibly intense pain, everywhere. So maybe just steer your groin away from other people’s feet. Just no more bar fights for you.

Thanks for sympathetically cringing with me throughout this episode of SciShow. If you would like to learn more about the science of pain, you can check out our video on why we have pain, and how we kill it.