The Physics of Head Stomps, Soccer Kicks, and MMA's Other Taboo Weapons

Fightland Blog

By Jeff Harder

Photo by Josh Hedges/Zuffa LLC

Watching Wanderlei Silva grab the ropes that edge the Pride ring and stomp his poor foe's head into the canvas, it's hard not to feel it in your gut. It's like you're watching the scaffolding of a sport fight and all of its contrivances start to buckle, threatening to fall apart into sheer chaos.

All these years later—and with Silva recently announced for a Pride FC-rules comeback in Rizin Fighting Federation's upcoming open-weight tournament in Japan—head stomps still evoke a visceral reaction. So do other strikes that have been largely weeded over the decades: soccer kicks, knees to the head of a grounded opponent, headbutts, and so on. That appearance of brutality played a large part both in barring these strikes from the Unified Rules of Mixed Martial Arts—the rule set used by the UFC—and as well as their continued appeal and existence on the margins of the MMA world.

But beneath appearances, things get greyer. Fightland spoke with Jason Thalken—a Seattle-based martial artist, a physicist who holds a PhD in condensed matter physics from the University of Southern California, and the author of Fight Like a Physicist: The Incredible Science Behind Martial Arts—about the physics underlying the savagery of MMA's most taboo strikes. (Interview has been edited and condensed.)

Fightland: Let's start with the Wanderlei Silva special: soccer-kicking and stomping the head of a grounded opponent. Beyond the realm of action being different compared to an upright fighter kicking his opponent in the head, what else is of significance here in terms physics?
Jason Thalken:
One important difference is going to be the possible reaction time: how long does it take for a human to react to something? Straight-up reaction time is going to be like 200 to 250 milliseconds, but your choice reaction time—when you see something and decide what to do about it—even at its most basic we're looking to something like 400 milliseconds. That's on par for fighter reaction time. A punch could take between 200 and 400 milliseconds to throw. A kick is going to be longer than that, especially a head kick where you have to shift your weight and throw it. One of the things that I think balances kicks out in standing fighting is that there's a lot more mass behind your leg and it's a lot easier to rotate the head and knock somebody out with a kick, but it takes so much time to get a kick up there. Punches are much easier to sneak in without someone noticing.

With a soccer kick, when you get [one opponent] down on the ground, now kicks require the reaction time of punches: there's going to be much less time to react, and you won't be able to see [a soccer kick] nearly as well either. You've also put the plane of the head in the same plane of motion as a front-back movement of the leg, so instead of having to bring it up and around, you can just use the straight extension of the leg to get your opponent's head moving. It makes it easier for the kicker, but I don’t know if there's necessarily reason to believe that it's much stronger. It's just more advantageous for the person kicking.

Head stomping is interesting because the head is actually supported by the canvas, which means you aren't going to be transferring momentum to the head and getting the head to rotate. It's less likely to result in a knockout from that same motion, but it does start putting things in new precarious situations because there's nowhere for the head to go after impact: you end up putting the head at higher risk for structural damage—broken bones, broken skin, bruises, swelling. I imagine it'd be possible to get a skull fracture. You've shifted the types of injuries you're likely to get from a knockout and a brain injury to localized structural damage. If we're talking about just brain injury, it's like, "Oh you're lucky you've got the canvas there to support your head." But I wouldn’t exactly call that lucky.

Years ago, there was a lot of enthusiasm for allowing knees to the head of a grounded opponent as a way to break up grappling stalemates. If you have one fighter fail on a takedown attempt, he has his hands on the mat, and his opponent sprawls out and starts throwing knees toward his head, how does that differ from when two upright opponents are clinched, pulling on each other's heads and throwing knees to each other's face?
To some degree, I'd say we're talking about the same differences in reaction time for kicks, but there's a difference between kicks and knees that's kind of interesting. With a kick, you've got two extra joints between your [point of] contact and your center of mass just below your belly button. With a knee that's traveling upward, it's traveling in a circular motion around your hip joint. But if it's a forward thrusting knee, you're actually coming in a straight line out from what is effectively your center of mass, or at least close enough. That makes a knee strike the single best strike for transferring momentum to an opponent. Transferring momentum causes your opponent—or parts of your opponent—to move. If you wanted to get your opponent's head rotating and knock that person out, one of the most efficient ways to do so would probably be a forward thrusting knee to the head.

Sometimes a knee will rotate the head and sometimes it will not, depending on the strike and what's going on. If you watch a fight and if the knee does not rotate the head, it's more like a head stomp on the canvas. If it does rotate the head, it's more likely to cause a knockout. Anderson Silva versus Rich Franklin was an example where you saw both in the same fight.

If you've got a person that's trying to take you down and his head's right in front of your knee, it's very likely that the knee to the head that would be coming up at that point would be very easy to knock someone out with. As opposed to standing on their feet, it's not really that different, honestly. Knees are much more precarious [for the person getting hit] than kicks for transferring momentum. Obviously a knee where your head is down low is going to be different than a standing knee, but whether or not your knees are on the mat, I don’t know that that's a big difference.

Let's talk about so-called 12-to-6 elbows, where elbows travel in a direction essentially from the ceiling to the floor. The underlying question of whether 12-to-6 elbows are truly dangerous—relative to lateral elbows or elbows that mimic uppercuts—depends on how the angle at which you're throwing an elbow affects the force you generate and transfer to your opponent.
I've not seen measurements, but the key difference is whether or not you can get your weight falling into it. With an elbow, you're striking with one joint, but your shoulder is turning. It'd be very difficult to situate your elbow in an up-down strike such that it's sort of immovable. It's not like something like a forward knee where it's like a straight connection between your center of mass and point of impact—it's too complicated or difficult a situation to think you can get all of your mass behind it. I don’t know how much of your mass you can get behind a downward elbow, but you can get a lot of your mass behind a sideways elbow. What I'm saying is there's no real reason to believe that a downward elbow is significantly different from other directions.

Headbutts are a relic from so long ago, but tell me how they compare to throwing punches, elbows, kicks, and knees.
There's not a whole lot of different interesting physics that go on with headbutts, but it's where two things collide and the question is where is the damage going to be done. If you have a forehead versus softer, mushier parts of the head, most of the tissue that gets compressed in that impact is wherever it's softer or squishier.

Now, punches to the back of the head—that's an interesting one. Normally, what you have to do to cause a knockout is apply a shear force to the axons in the brain, and that usually requires getting the head turning. But we've all seen fights where there's a punch that's hit the back of the head, and there's really no rotation there, but that will get a guy pretty disoriented pretty quickly. That's an interesting case: the point of rotation of the head when your head moves is the base of the skull, and instead of rotating the head around it, you're actually striking right on that point and applying force to those same axons from the other side.

I know you have some hesitation about using explanations available through physics to make pronouncements about these moves and their correlation to fighter safety. Why is that a fraught thing to do?
One reason is that whether or not something is more damaging or has more force behind it doesn’t necessarily make it more dangerous. It's like what we were talking about before: a kick is way more likely to rotate the head significantly and knock somebody out, but it also takes way longer to get a kick up to the head, you have to shift your weight first, and there's all sorts of things the other fighter can see. That kind of levels the playing field: if you were to just look at the force on impact, you'd say, "Oh my gosh, we should ban kicks!" Then you compare it to another strike that's lower in force but way more dangerous, like an eye poke or something like that.

The other piece is that if we're going to start looking at fighter safety, we really should change the way we think about it and focus mostly on brain injury. I'd say the worst rule we have for fighter safety is the wearing of gloves: the gloves we wear protect the hands and protect the face from cuts and bruises, obviously less so than boxing gloves, but it does nothing to protect the brain. I'd say that because you aren't putting the hands at [as much] risk, you're allowing fighters to punch harder and you're actually putting the brain at greater risk by wearing gloves. If you were asking me to rank which rules are nonsense for fighter safety, I'd put gloves pretty high up on that list.


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