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I have seen many incidents in MMA and similar martial arts where a fighter who throws a roundhouse kick, usually towards the opponent's legs, breaks his leg.

That makes me wonder if this kick directed to opponent's legs is useless, because what's the point if it hurts the attacker more?

However I am not sure why this happens, it could be just a mistake by the attacker, it could be that the opponent's bones are naturally harder it could be a technical issue or perhaps the move is useless.

So I want to ask, what exactly is their mistake and they hurt themselves rather than the opponent?

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  • In many years of karate training I have never seen this. Maybe it has something to do with karateka spending many years practicing their kicks, while MMA fighters have considerably less practice and conditioning? You could argue that maybe it's because MMA fighters only ever throw them full power - but then again, how often do you see this happening in Muay Thai, where the round house leg kick is one of the main techniques?
    – slugster
    May 20 at 13:43
  • Hmm... I feel like you could as well state "why do people throw punches when they keep breaking their hands doing so?" since that also happens fairly often (albeit that modern boxing gloves exist in part to reduce hand injuries on the part of the puncher). May 20 at 14:19
  • @slugster, a quibble: There are the occasional "full power" techniques in MMA, but if you go full power you'll get tired much too soon to win the match. I'd agree with the statement "MMA fighters almost always throw them much harder than amateurs". paradox, I also wouldn't discredit the "it was just a fluke" factor, where they kick someone just so and it's blocked just so, to cause a break. May 20 at 17:01
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There isn't really a "mistake", it's more of an unfortunate confluence of events. Any site that claims it takes X lbs of pressure to break a bone are sort of misleading. There are differences in bone structure from person to person, and it very much depends on the angle, load, time and other factors.

The tibia (shinbone) is sort of prism shaped, which is to say triangular with one of the edges being the leading part of the shin. If you can hit with that leading edge directly, it reduces the surface area of the impact, which in turn increases the amount of pressure/force over the surface area of the strike. The flatter the surface, the more area and the force is lesser. (Think of an axe, if you hit with it flat, not much happens. If you hit with the back of the axe, you dent the wood. You hit with the edge you cut the wood, even if it is dull.)

So the kicking leg needs to have that shin bone angle hitting almost directly into the opponents leg, at a 90 degreeish angle. Any upward or downward angle/deflection will reduce the force applied. The same goes for the blocking leg, it needs to be at the perfect angle, any movement upward/laterally reduces the impact of the strike. If you get both legs at the "correct" angle with sufficient force (And most trained martial artists can easily produce that force with a kick), you run the risk of fracture. Then you start looking at bone health, age of the participant and other factors as well.

Now as for the question of why, most kicks are not directed at the shin. Rather they are aimed at the calf or thigh muscles, so as to deaden them, reduce their effectiveness and/or hit a nerve center to produce a temporary paralysis. You see this when someone gets an effective leg kick and they sort of limp around for a few seconds. Nobody goes into the fight wanting to go shin to shin. Krav Maga calls this "hard on soft, soft on hard", or hard (shin) to hit soft (muscle) targets.

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So far, doing a casual check, I've found two cases of MMA fighters breaking their leg on a checked leg kick, and at least two of people having their legs broken by receiving a leg kick, so the record seem reasonably even, not counting the non-fracture results of leg kicks slowing people down, putting them in pain, etc, so I think there might be more of an observer effect going on where the times it happens stick out in our head more than when it doesn't happen.

So why does it happen?

I'll prefix this by saying I am neither a medical professional nor a professional fighter, so I am not an expert. That said, I think a lot of it comes down to bad luck and the fact that the human body, no matter how well conditioned, can still wear down and break. Ideally, when aiming a strike, the goal is to strike the target in a way that maximizes damage to the target while minimizing damage to yourself. There are all sorts of aphorisms and techniques involved, but it basically comes down to that you want to hit the weaker part of their body with the stronger part of yours. If you are off-target, you may wind up striking with a weaker part of your body or against a harder surface. A classic case is punching without proper bracing or hitting bone, resulting in things like wrist sprains or broken fingers. Or, in the case of kicking, breaking your toes by having them be the point of impact.

Complicating this, of course, is that fighters don't strike in pristine condition. Even aside from damage taken during the fight (say, the previous strikes), they may be carrying injuries from prior fights, training, or even just day to day life. All of that can add up to introduce damage that will make a limb more likely to break. While the bromide is that that which does not kill you only makes you stronger, microfractures in the bone can compound and lead to the bone becoming more fragile. Dr. Chris Raynor has suggested that this was likely a major factor behind MacGregor's broken leg in his July 10, 2021 fight.

So why do leg kicks?

Well, as noted above, these injuries, while gruesome, are fairly rare. A fighter might throw hundreds of leg kicks without a severe injury like this. Compare that to the amount of broken fingers in boxing or grappling And leg kicks, while seldom enough to end a fight, can significantly reduce a fighter's mobility, and in a self-defense situation, may cripple an opponent long enough to make a run for it.

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Incorrect kicking mechanics. I’m a Muay Thai practitioner, and have never noticed a Thai break their leg in Thailand. This might be because in Muay Thai, we are taught to kick from the hip with a straight leg on impact, which is landing with the shin on the opponent. Most of the martial artists that are breaking their legs in the UFC seem to be snapping their kicks, and not throwing their kicks off the hip. This might create the wrong type of tension, and therefore accidents seem to happen if the kick is telegraphed and not set up properly.

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