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What's the reasoning behind checking kicks? Is it true that it's supposed to hurt the kicker when his kicks are checked? The third law of motion dictates that both the kicker and the checker should receive the same amount of force and assuming that the two have the same pain threshold, they should feel the same right?

If that is so, the advantage I can see in checking a kick as opposed to receiving it to the thigh is it reduces the force of the kick (if the checking leg is allowed to move to damp the kick) and it directs the force to somewhere one would feel less pain (the shin). I'm not saying that these aren't enough reasons for me to do them, but are there other benefits to checking kicks?

Also, a very famous MMA fighter just broke his leg after his kick got checked (the leg really snapped). I'm guessing that this famous MMA fighter just got brittler bones than his opponent, but is there any other reason as to why the leg would snap? Another guess is that the part of the kicker's shin that hit the opponent's is somewhere more at the bottom part of the shin which I believe is more thinner than the middle or upper part of the opponent's shin where it hit. Any insight on this would help me understand the technique better.

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7 Answers 7

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There are 2 places where you can check a kick : the knee and the shin. If you check with your own shin bone, you are creating a shin to shin contact and, intuitively, one can expect the damage to be similar for both opponents. However, while the location of the hit will be similar, the results, at least if you want to talk about physics, will be very different. So yes, the same amount of force it applies, but where it goes and what it does can and will vary a lot.

On the kicker's part, you have a lot of momentum going through your leg. Once the hit occurs this momentum translates into a large amount of stress in your shinbone, in the form of a bending moment. If there was enough momentum, this can cause the shinbone to snap, as bones are mostly made to resist compression rather than bending.

On the checker's part, we need to remember that the knee is bent, but not fully. When the kick is received, the impact will cause the leg to bend further, up until the calf potentially collides with the thigh. This bending of the leg acts a bit like a cushion, absorbing part of the hit's energy. As a result, the bending moment on the defender's shin is greatly reduced and thus will rarely result in a snapped tibia. Also, the checker usually checks with the upper shin. Mechanically speaking, this also causes a reduction of the bending moment, resulting in lower stress for the bone.

The other possibility is a check with the knee. In this situation, the kicker's outcome is mostly the same, but the checker's is very different. As was previously mentioned, bones are designed to withstand tremendous compression forces. When checking with the knee, the force is mostly transferred to the femur, which is probably one of the strongest bones in your body (it is also the largest). If done properly (the check done in the UFC fight you mention being a very good example of proper technique), the force can all be absorbed by the femur with a greatly reduced risk of injury on the defender's part.

Why do this instead of receiving the kick on your thigh? The kneecap and shinbone used to check the kick are pretty much sticking out of your body, meaning you will receive the kick straight on the bone, and not on muscle tissue. A strong kick on the back or side of the thigh can cause lots of damage, which will result in a reduced ability to fight (less powerful kicks, reduced balance, impaired mobility, etc.). Checking with a bone reduces the pain (less pain receptors in a bone than in a muscle) and the long term damage that you will endure in the fight, provided that your leg doesn't snap. But as was mentioned, it is more likely for the kicker's leg to break before the defender's does.

TL;DR - Checking the kick redirects the blow to an area that can more easily withstand the hit or absorb part of the damage, at least compared to the kicker. When executed properly, it will likely hurt the kicker more than the defender.

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in ufc, anderson "the spider" silva, broke his leg, throwing a hard lower kick that was checked! so brutal technique! –  tinybyte Jun 18 '14 at 9:09

It seems to me that you're right about the fact that both the kicker and the checker should recieve the same amount of force. There are, however, other factors to take into consideration.

When I practiced Gung Fu, we ofter perfomed exercises with the intent of strenghening our bones and building up protective cartilage. I would assume this is done in other arts as well. To do this, we repeaditly caused our bones and bodies (slight) damage over a long period of time. Our bodies notice that the same areas are getting damaged and thus builds cartilage to protect said area.

Now, if this is done to the shin we get a nice layer of cartilage over the periosteum, which is the real culprit when we're talking about impact pain on shins. If we have this cartilage over the periosteum, the impact is spread over a larger area, thus lessening the pain. We haven't increased our threshold (strictly speaking), but made sure that less pain is caused to us by the same force.

Another factor to bring into the equation is something my old Djeu Gao taught me. If I were to kick someone's thigh, my mind prepares for the impact. Knowing that the thigh is a relatively soft area, we prepare for a rather soft, padded impact. If I were to check a kick shin to shin, however, my mind would prepare for a very hard and stiff impact on a very sensitive area, so we'd prepare for the pain. The attacker, still focused on that padded thigh, will be caught "off guard" and won't be as prepared for the pain.

This difference actually makes a great difference; if our brain knows that pain will come and is ready for it, we can take more of it because we don't need to cause as lound an alarm since we already know, consciously, that the pain is there. Being caught "off guard", as the attacker, our brains would alarm louder since the awareness isn't there, to that extent.

The fact that you as the defender will likely feel less pain than the attacker is another reason for checking kicks; any intense pain will put the victim in a chocked state (perhaps not according to the medical definition..), if only for a short amount of time. This might be enough to gain the upper hand, if you play your cards right.

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Do you actually build up cartilege over the shin? My understanding had been that it built up pain tolerance and helped the bone itself grow denser over time. Cartilege is primarily located in joints. –  TimothyAWiseman Jan 2 '14 at 21:20
Yes, cartalige is primarily located in joints, but it exists in other parts of the body too, such as your nose, ears and rib cage. While it is true that bone gets tougher as is gets damaged, this has nothing to do with pain,since bones have ni connection to the somatosensory system. The periosteum, however, does. –  Psyberion Jan 2 '14 at 22:22
Now that I think of it, one possibility could be that the nerves in the periosteum gets so badly damaged that the sense of pain is dulled, and unfortunately I can't credit a (scientifically) reliable source for my statements, since I've learned them from my former Djeu-Gao and a fellow practioner who is a nurse. –  Psyberion Jan 2 '14 at 22:22

There has been some discussion about the variables at play in this particular leg break, and leg breaks from checked leg kicks in general. It seems that turning the hip over during the kick helps prevent injuring oneself.

There is also a difference between a leg kick being checked against the receivers shin or against their knee. The latter is stronger.

It's also worth noting that Silva threw a power leg kick without a set-up. This is less than ideal:

Folks like Ernesto Hoost and Rob Kaman realized that low kicks are awesome, but kicking people shin on shin (or worse, shin on knee as we will talk about in a moment) is as damaging to the kicker as to the opponent. Ernesto Hoost, in his most recent seminars which you can find on YouTube, is often asked how come he almost never ended up kicking shin on shin.

Hoost explains that it was because he always either a) threw a flurry of punches against his opponent's guard to preoccupy them and keep their feet flat or b) kicked as they were stepping in toward him when their weight was on their lead leg.

...setting up with strikes is what everyone in MMA should be trying to do.

...There was nothing accidental about Silva getting hurt. When a hard kicker kicks a knee, he hurts himself. Sometimes it's enough to finish a fight, sometimes it's just enough to stop him from kicking for a while, but kick a knee Silva did.

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I think that the reason that the checker receives less pain than the kicker is because of what part of the shin the checker uses to block the kick. The checker uses the upper part of the shin, close to the knee. The kicker uses the lower part of the shin, close to the foot.

Due to the great thickness (or density, I'm not sure) of the upper shin, I think that the force is distributed into a larger and thicker area, thereby dissipating the total 'shock' on the body. I think it would be the same principle as two people hitting each others fists while wearing gauntlets. If one person has a ticket gauntlet, I think that they would likely feel less pain.

I really wish I knew more physics to provide a more useful answer. I think the most useful part of my post is: - The Kicker kicks with the lower shin. - The Checker blocks with the upper shin, which is thicker.

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Honestly, it is a choice between getting kicked in the flesh where I get hurt but the guy who kicked me didn't and checking so we both suffer pain. Once I check, he might be less inclined to throw another hard kick, because it hurts him just as much.

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Most of what has been said so far is correct and in your question you asked about reducing the impact. This is also a big factor. If your leg can move when hit, the impact is greatly reduced. If your foot is planted then you absorb the full force. This also places a large side load on your knee. Our legs are designed to take hits from the front, that is why you should turn your shin into the kick and strike with the front. That is also why you should attempt to turn your shin towards the kick when checking.

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Just as you have mentioned in your question

I have noticed when I check a kick that the point of impact and the actual stoping point of the kick are far enough apart to allow a bit of dissipation of power from the kick You are also stoping the kick before full power can be reached.

It is like the monk who throws his stomach into the punch then allows himself to be moved back by the force slightly.

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