The science behind why a groin kick is so traumatic, and how you can use it to defend yourself.

By Allison Gibbons

Recently while sparing, one of my kicks inadvertently landed square on the tender spot of my partner.  After watching him drop like a sack of potatoes, and spend ten minutes on the ground writhing in pain, clutching his abdomen, gasping for air, crying and vomiting, I couldn’t help but wonder why this strike is so devastating, and why it seems to bring on so many other physical symptoms in addition to the pain inflicted.

That led me to research the science behind what actually happens when one lands a perfect groin strike. Some of the answers surprised me, and actually gave me a little sympathy for the men I’ve brought down sparing and training over the years.

It should come as little surprise that one of the reasons a groin kick is so effective is it targets an area with an exceptional number of sensory nerves in a very exposed, unprotected area.  The testicles are one of the places with the most sensory receptors in the human body.  Unlike other areas that are packed with sensory neurons, they are external to the body, completely exposed with no protection by bone or muscle tissue.

They’re dangling out there without surrounding muscle or other tissue to absorb some of the blow and distribute its force across a wider area.  In fact, they are the only instance of human anatomy where so many sensory neurons are completely unprotected.

That might be nice during a sexual encounter, but it sure seems like a bad thing when sparing at the dojo.

Most men report a strange phenomenon—they actually feel most of the pain in their abdomen even though the blow landed between their legs.

It turns out that’s a byproduct of fetal development.  The testes are formed in the abdomen of a male fetus before dropping into the scrotum later in the development cycle.  That means all the nerves exposed in the testicles actually extend deep into the core of the body.  These stimulated nerves relay pain through entire area they cover which causes much of the pain to be felt throughout the abdomen in addition to the testicles.

Men say the sensation they feel from a bad groin strike pulses pain throughout their body and not just the area directly impacted.  (They call it a bad groin shot while I prefer to call it a good one, but that’s another story.)

It turns out they are not exaggerating.  Large numbers of neurons firing simultaneously create something scientists call referred pain.  Although scientists are unsure of the exact molecular pathway of referred pain, it works similar to crossed wires, where strong currents through many running close together at the same time can jump over to other unrelated neurons, thus causing pain in unrelated parts of the body.

Quite simply all those neurons in the testicles firing simultaneously send such a powerful signal up the spine, that their signals jump to adjoining neurons from other parts of the body that were untouched by the strike, thus giving a sensation of pain radiating throughout the entire body.

That doesn’t sound fun, but it gets even worse for our male brethren.

The Vagus nerve is the largest sensory cranial nerve in the human body. Being a cranial nerve means it is one of twelve sets of nerves that extend directly into the brain.  It is a spider-web-shaped nerve that links many of the body’s core systems to the brain. It courses through the body linking the brain to the pharynx, larynx, heart, kidneys, liver, stomach and intestines before it ends in the testicles.

It’s the magic behind a successful groin strike, because it transmits pain from the event throughout the core of the human body and unleashes a torrent of other symptoms.

Dr. Michael Kelly, the famous UFC fight doctor, explains that sudden trauma to the groin, liver or kidneys can immediately shut down the ability to stand and can quickly drop someone involuntarily.  “I’ve spoken to a number of fighters after they’ve taken a groin or liver shot—they’re not directly connected, but it’s the same type of visceral pain that goes through the abdomen—and they say that when they first received the blow, their legs went out. It’s almost like the nerve signal just stopped instantly. They don’t pass out; they just can’t stand. It’s an interesting phenomenon that we see.”

Thank you, Vagus Nerve. Your link to the testicles makes me feel a little safer walking down dark streets at night.

The Vagus Nerve also regulates functions such as voice, heart and vomiting. That explains the crackling voice and vomiting that often goes hand in hand with a groin strike.

When it functions normally, it stimulates involuntary contractions in the digestive tract, including the esophagus, stomach, and most of the intestines, which allow food to move through the tract.  When it becomes over-stimulated, it creates major problems for the entire digestive track and an overwhelming sense of nausea and paralyzing cramps.

Also, over-stimulation of the Vagus Nerve is the most common cause of passing out.  Although I have never delivered a groin kick that has caused a man to pass out, it is possible to do so because of our friend the Vagus Nerve.

The Vagus Nerve is not acting alone.  Trauma to core functions causes a massive release of a chemical called Substance P.

Substance P exacerbates pain in core areas of the body.  It’s the body’s way of saying something is seriously wrong.

It lowers oxygen levels in the blood, their heart rate and causes immediate dilation of blood vessels, suddenly lowering blood pressure.  This combination severely weakens the subject, his ability to move and typically causes a severe headache and tunnel vision.

Doctors call that phenomena “vasovagal syncope.”  I call it a great way to stop an attack.

In most cases of vasovagal syncope, there is a rapid onset of symptom like dizziness, feeling hot or cold, nausea, pale skin, blurred and “tunnel-like” vision, and profuse sweating. After the episode, symptoms may continue because of continued low blood pressure. Many men will feel extremely weak even after their 15 minutes of intense pain has passed.

Substance P also affects the part of the Ganglia that may induce vomiting and increase their already crippling symptoms of nausea, because it is the primary driver of the human vomiting response.

Substance P often affects the pressure in the inner ear which explains why many men report dizziness as one of the effects of a groin shot.

The Vagus Nerve and its trusty friend Substance P can also cause a melatonin release that lowers the core body temperature, exacerbates their already weakened state, brings on drowsiness and lowers cognitive capacity. This is part of the body’s strategy to protect its core during times of trauma.  While large releases of melatonin may inhibit movement and mental capacity, it is an organ protector for many core functions when the body perceives its core is under duress.

Dr. Michael Kelly further explains, “Any severe blow to these organs can cause a vasovagal reaction, where you have a dilation of blood vessels and a drop in heart rate, so you actually get decreased blood flow going up to the brain. Not everybody will pass out from that, but it does immediately affect the ability to function and greatly weakens muscle function throughout the body.”

Understanding the devastating effects of a well-targeted and powerful groin strike helps explain why groin attacks are so heavily emphasized in Krav Maga and other martial arts.  It is such a central part of this style of training that it’s called “the Krava Maga handshake.”  It’s not just an extremely painful strike, but one that also triggers many other debilitating effects that will promptly end an attack.

My advice for women is to practice your groin strikes heavily.  They are an amazing equalizer when defending yourself.  My advice for men is to wear a cup when you’re at the dojo.