The physics of breaking stuff with your fists


iO9 recently ran a story on how martial artists are able to break boards and cement blocks, using their hands rather than mystical powers. I thought it was pretty neat, but then I read an interesting counter-analysis by science journalist (and, significantly, martial arts practitioner) John Rennie.

iO9 is right about the lack of magic powers, he says. But they got the physics wrong. Key slip-up: Assuming martial artists strike like a cobra—fast punch, with a quick pull back at the end—when they have their smashing fun times. iO9's theory was that that movement caused the boards to bend and snap. But that's not how it works, Rennie says. In fact, martial artists are taught to follow through with their punches, aiming not at the board-to-be-broken, but at a point beyond it.

So how's the breaking really done? Rennie quotes an episode of the awesome old PBS show Newton's Apple:

One key to understanding brick breaking is a basic principle of motion: The more momentum an object has, the more force it can generate. When it hit the brick, [karateka Ron] McNair's hand had reached a speed of 11 meters per second (24 miles per hour). At this speed, his hand exerted a whopping force of 3,000 Newton's -or 675 pounds-on the concrete. A slab of concrete could likely support the weight of a few people weighing a total of 675 pounds (306 kilograms). But apply that amount of force concentrated into an area as small as a fist and the concrete slab will break.

The fact that martial artists also pick their materials very carefully doesn't hurt, either.

When breaking wooden boards, you use pine (not oak, not mahogany) that isn't marred by dense knots, cut ¾ inch thick and about 12 inches on the diagonal; you hit them to break along the wood's natural grain. (It's not playing by Hoyle but some breakers have been known to bake their boards in ovens before demonstrations to make them more brittle.) One good board, if held securely so that it won't move on impact, is so easy to break that even those with no training at all can be taught to do it in under five minutes.

P.S.: Rennie's blog, The Gleaming Retort, is part of a new family of science blogs, hosted by the Public Library of Science—a non-profit that publishes open-access science journals. I highly recommend checking out the entire PLoS Blogosphere.


  1. I like the technique of baking the board in an oven to make it more brittle. It brought back the following memory:

    When I was a child, I sent away (an ad in the back of my comic book) for a book of magic tricks. For some reason, they had a chapter devoted to all the fun you could have with mom’s oven. For example, if you sprayed lemon juice on a piece of paper, then baked it, it would “age” and look just like a pirate’s map or the declaration of independence. You could fool your friends into thinking you’d stumbled upon some ancient document.

    Another neat trick was to fool your friends into believing you had super strength. Just leave a standard size, thick telephone book in the oven until all the moisture was baked out of it. The phone book would look normal to your friends, but you could easily tear it in two like a strongman.

  2. My kids took Tae-Kwon-Do classes when they were young. Part of the transition from white belt to yellow belt was to break a board with a kick. The instructors would stress the concept of kicking through the board to break it. When a kid was having trouble with the board, the instructor would count off “one two three KICK” and when he said Kick, he would move the board closer to the kid’s foot. Worked every time.

  3. That’s what I always thought; you don’t try to break the board, you try to break the imaginary board behind the board.

    In the same way, you are not supposed to punch someone in the face, you are supposed to punch someone in the back of the head (eg. punch through the face).

    1. The only reason to punch somebody in the face is to show off in front of a crowd. Dude’s skull is a great big bone, bigger than anything in your hand. You’re more likely to hurt your hand than hurt the target much. Also, a face is a small target, and you’re more likely to miss.

      Hit him in the gut. Hard. Even better, put him on the ground somehow and stomp on him. Come on, nerds, THINK if you have to fight.

  4. We do board breaking demos when our TKD school walks in local parades. Kids < 10 break boards with zero training, just 10 second's instruction. Admittedly, they have been training just in general, but it's not a special skill. The dudes that impress me are the ones that do it not with a fist or a palm strike, but with fingertips. I do not want that guy hitting me in the throat with a spear hand move.

  5. Remember what Miyagi-san says, when asked if he can break a log:

    “Don’t know. Never been attacked by tree.”

  6. “One good board, if held securely so that it won’t move on impact, is so easy to break that even those with no training at all can be taught to do it in under five minutes.”

    True. It is, however, MUCH harder to break a board that is suspended (and swinging freely) from a single point — for instance, a board hanging from a loop of rope suspended in a small hole drilled near the top.) The reason is that most punches take too long to deliver their force — they are too slow and in essence PUSH the board, giving the board a chance to absorb momentum by swinging away.

    Being able to break a suspended board is far more meaningful in terms of fighting practicality; it means that force gets transfered so quickly that there’s no time to “roll” with the punch. Such a skill has been demonstrated by Bruce Lee in the past, as well as (more recently) by Wing Chun Master Samuel Kwok (search Youtube for this.)

  7. There’s really nothing magical about any of the martial arts. I hate it when I hear pop “skeptics” talking about how martial arts (or even acupuncture) are “woo”. The physics is the same. The physiology is the same. The biology is the same. But often the culture from which the martial art comes uses different vocabulary, different terminology.

    When a tai chi master talks about “chi” it’s not some mystical concept. It’s breath plus intent plus energy. In the West we’re not used to talking about those three things together, but it’s a common concept in China.

    So martial arts aren’t magic, they just look that way to unsophisticated and bloody-literal-minded “skeptics”.

  8. If the provided picture is any indication, the secret to breaking stuff with your fists is to use your elbow instead.

  9. lmao @ the accompanying picture… I helped my mom perimeter her garden with those same bricks and she broke one just by picking it up… that karateka is bad-ass, yo!! scarier than Fred Ettisch!!!

  10. When I was a kid, my uncle was showing us his karate board breaking skills. My sister accidentally sat on one of his pine boards that was laying on our squishy couch and it broke in half.

    We were less impressed with him after that.

  11. When my kids started TKD, I had no idea they would really break boards. I thought it was just a TV stunt that real masters (like Miyagi-san) wouldn’t even think about. But it really is an awesome thing. Its a skill that takes practice and talent. When kids (and adults) are testing to get a new belt, it is heartbreaking if they can’t quite break it. But they try again the next time, and it’s a total rush when they do it. My kids have learned a ton because it’s something that’s really hard but it is also doable. Just like a lot of good things in life.

  12. This reminds me of the ‘ladies self defense’ course I took in college – Kick them in the nuts, but aim for the neck.

  13. One of my favorite bits of high school physics class was breaking boards. We calculated the force necessary to break the board, then we swung our hands through light gate timers and calculated how much force we could put out. Everyone in class was able to break them.

  14. Concrete is very weak in tension. Pre-cast blocks have many voids, and are extra weak. Sometimes landscaping blocks like those pictured will break, just from careless handling.

    So, it probably doesn’t matter much where you aim.

    1. Stacks of cement (not concrete) blocks have an interesting dynamic going on where the weight of the top blocks assists in the breaking of the lower blocks. The farther down the stack, the lower the energy required to break them. It would be possible to arrange a stack such that an arbitrary number could be broken, provided the first n blocks could be broken.

  15. K.E. = 1/2 mv^2 — faster isn’t just a little better, it’s better^2.

    I know a guy who breaks baseball bats with his forearm. Still don’t know how he can do it.

  16. The thing about the types of wood is completely true. I am a complete beginner, and I was once taught by the woodshop guy at my camp to break through wood. It’s not hard to do.

  17. while previously thinking stuff like brick-breaking was a scam, it turns out that one of my colleagues has performed the feat as part of taekwondo (TKD hereafter). He told me the “trick” was to punch *through* the brick, not punch *the* brick. They do however use brittle bricks, and apparently one of the tricks they play on TKD noobs is to give them a good old-fashioned house brick to punch through and have a good old larf when their mark ends up in serious pain :)

    as we know, magic does not exist otherwise we’d simply be getting TKD experts to build tunnels/mines for us with their bare hands rather than spending billions of dollarpounds on expensive equipment.

  18. Pine shelving and unbaked bricks will break if you look at them hard.

    Try regular 2x4s cut into 18 inch sections, it’s much more challenging.

  19. Even with special physics to help you, it still helps to do training.

    As part of every martial art, the student do many repetitions of milder versions of the exercise, to build up to the actual break.

    Under repetitive weight bearing loads of this type, your body adds reinforcement to your bones to withstand the strain.


    “This highly efficient process called remodeling ensures that more bone is built in specific locations when it is subjected to heavy-repetitive loads and less is built when it carries lighter loads. In infants, 100 percent of the calcium is exchanged in their bones every year. For people in their 20s, the equivalent of 20 percent of the skeleton is replaced yearly–though high stress areas like inside the head of the upper leg bone may be replaced up to three times per year.”

    For example, in basic training it is not unheard of for raw recruits to break their heel bone on their first 20-mile march with 60 pound packs. It’s called a marching break. You need to work up to it for a while to ensure that your heel bone has gotten naturally reinforced.

    In boxing, one of the purposes of hours of practice with the punching bag is to increase bone density in your hands and arms. Besides lessening the risk of a break, it also increases the effective energy in your punch. And jumping rope not only increases your leg strength but the bone density of your leg and foot bones.

  20. Yeah, the picture above with the edging bricks and little spacers is kinda weak. I wouldn’t even casually lean against such a construct without expecting it to break and dump my ass on the ground.

  21. So…if faced with a real-life threatening opponent, I’m gonna have to make sure he’s brittle, and if not, ask him to sit in an oven before we can fight?

  22. “At this speed, his hand exerted a whopping force of 3,000 Newton’s”

    As hinted by another comment this is nonsense. So the “science journalist” seems clueless.
    F = ma
    This is Newton’s second law – force is not related to speed, it is related to acceleration and mass.

    I recall reading some good analysis on sci.physics USENET group a while back – for anyone interested do a groups search. The physics is a more complicated than just force or kinetic energy.

    Karate-ka generally do strike through though some karate techniques snap back (e.g. back-fist). Different attacks are used against different targets. No magic though.

  23. Striking through the boards is by far the easier method, but it is not the *only* method. Either way works, assuming the board is tightly held.

    Where you’re more likely to see the “quick pull back” method is with “speed breaks.” These are when the board is dangling, and can swing around. I had to do one of those for my black belt test (not too hard, I was 14 at the time). See also the “pizza break,” which I saw one of my instructors do: hold out your hand palm up, rest a board on your fingertips, and break it by striking upwards.

  24. “PopeRatzo • #11 • 11:40 AM Friday, Sep 3, 2010 • Reply

    There’s really nothing magical about any of the martial arts. I hate it when I hear pop “skeptics” talking about how martial arts (or even acupuncture) are “woo”. The physics is the same. The physiology is the same. The biology is the same. But often the culture from which the martial art comes uses different vocabulary, different terminology.

    When a tai chi master talks about “chi” it’s not some mystical concept. It’s breath plus intent plus energy. In the West we’re not used to talking about those three things together, but it’s a common concept in China.

    So martial arts aren’t magic, they just look that way to unsophisticated and bloody-literal-minded “skeptics”.

    That is a really odd thing to say. Of course the Martial arts aren’t magic. That is the point of the skeptics. What you are missing is that when tai chi masters talk of “chi” they **are** talking about mystical energy, as are Chi Gong masters and so forth. The part you are missing is that that mystical nonsense is overlaid over techniques that can benefit from using “chi” as an analogy. Many, but not all, of the Eastern martial arts are steeped in varying degrees of woo.

  25. Board breaking is more of a psychological challenge than a physical one; after all, failure isn’t just humiliating, I’ve seen people get their bones broken. Without committment to the strike, it won’t work, but committing to the strike requires courage, and once someone has succeeded they will always have that extra confidence, so it is a pretty fitting rite of passage for students.

    In a similar sense, the patterns (pumsae, kata, whatever you like) aren’t straightforward rehearsals for combat, and they aren’t simply developing obvious qualities of coordination and technique – they develop the character too, because it takes patience and humility to get them right – qualities the students must to be taught since they are also learning how to hurt people.

    And for what its worth, I used to teach a TKD class for a few years.

  26. Hitting ‘through’ the target also takes advantage of how the perception-action system produces limb movements. Moving a limb from point A to point B entails accelerating from rest to a peak speed and then decelerating to stop at B; this is achieved by manipulating the equilibrium point between antagonist muscles. Peak speed is always reached somewhere in the middle (varying with task, distance, etc), so by producing a movement intended to reach a point through the target, you make it physiologically and biomechanically possible to be moving at high speed.

    This is also why io9’s ‘fast strike’ theory won’t work; maximal force generation can’t happen at the end of a movement.

  27. Part of the magic secret of brick breaking demos by some folks has always been owning one’s own molds for making bricks and/or concrete blocks. By adding more sand to the mixture than one would normally use to make a proper brick or block, the unscrupulous martial artist can make a brick that looks impressive breaking, but is ridiculously easy to break. I’m not saying this practice is super common, but it’s out there.

    I used to break boards when I was a 10-12 year old in Tae Kwon Do classes all the time, and I was never really impressed by it, as you could see how the instructors really went through the piles of boards at Home Depot to find the ones with the weakest looking grain, or that were super dry and easy to break. After a while, I figured that the real reason they had us breaking all those boards wasn’t so much to develop our skills, but to impress the parents who were waiting to pick up their kids and who wrote the checks for more classes.

  28. Also saw a show that had to do with body hardening- helpful when breaking bricks. Basically when training up to do it the martial artist is making small breaks in their bones, which heal up stronger and denser (exactly how body building works with muscles- micro tears in the tissue) meaning technique aside, the average brick breaker’s knuckles are harder than the average internet- surfer’s, and a better tool to break bricks with as a result.

  29. Thanks for the link, Maggie. Glad you enjoyed the post. And thanks, too, for flagging the awesome efforts of all the PLoS BLOGsters.

  30. Many of you miss the point of board breaking. When teaching a beginner how to break a board, you are simply teaching them two concepts.

    1: Correct technique. If you strike incorrectly, you will fail.

    2: Confidence in your own ability.

    Students need to be confident in their own ability in order to apply anything they learn in their class regardless of style. A good way to instill confidence in your student is to have them see the effect of their technique firsthand. Board breaking and later, brick breaking, are the easiest way to instill this confidence and display one’s correct technique.

  31. Martial arts instructor here.

    It’s about kinetic energy and delivery.

    First of all it’s mass times the square of the velocity that determines the kinetic energy being delivered. Consider the fist.

    Generally speaking, the mass is the fist, plus, if the wrist and elbow are held rigid, the mass of the two arm sections and even the shoulder, real experts can hold the shoulder rigid at the time of impact, and so a surprising amount of central body mass can be counted in the formula; from there, add breathing (and yelling) techniques that give you a rigid structure through your chest and stomach musculature, and then locking of the legs through contact. But once you get that far, you’re kind of stuck – because you don’t *have* any more mass.

    Velocity, though, both counts more (because it’s squared) and is easier to boost as compared to your average other person, because we don’t really generally work at anywhere near our potential speed.

    Now, the speed would, you might think, be primarily dependent on the tricep and the chest – extending the forearm and straightening the upper arm. But again, you can do more. If you pivot at the waist, there is a vector addition to the speed from there. If your hips pivot against your feet, there’s a vector addition from there as well. If you drive yourself forward with a leg (or both), that adds speed too. These things do add up, and you can improve them all. Do them all at once, get the vectors to all add together just as you go rigid… that’s a fair speed bump. Squared. And a good deal of mass.

    There’s a third factor too. The article talks about “hitting with the fist.” Bad Idea (note the caps!) Don’t hit with your two smallest knuckles. They’re likely to break, for several reasons; if not them, then the supporting small bones in the hand. or both. You won’t like it. There’s even a name for it: “Boxer’s fracture.” So it’s a bad idea for that reason (damage to you), but for another as well:

    What you hit with is the front side of the first two knuckles. Why? Because the damage delivered to the target is highly dependent upon the size of the area you deliver it to. The smaller that area is, the more damage done. I like to use this explanation with my students:

    Suppose I give you a book, weighs five pounds, and I set it carefully upon your head. Do you anticipate any damage? (students, if thinking, say no. You should too.) Ok. Now lets say I balance a pin, point down, on top of your head (because I’m just that good [laughter]), and *then* set the five pound book, balanced upon the pin. NOW do you anticipate damage? The answer, of course, is yes. Why?

    Because although it’s the same amount of force, loosely described as five pounds of downward pressure, it’s applied to a much smaller area, which cannot possibly oppose it. This is why we typically hit with just two knuckleheads: Whatever force you can generate with your strike, the smaller the area you deliver it to, the harder it is for the target to deal with. Some really advanced strikes use *one* knuckle, but that is extremely dangerous, because the energies are so high that the knuckle often fails – meaning it breaks. I don’t advise trying that without expert supervision, advice, and a *huge* amount of practical experience, deep self-knowledge of just how hard your knuckles are, not to mention the hardness of whatever you’re going to hit, and… well, you get the idea.

    These three things: Speed; mass; tight focus on delivery… they are the keys to striking power that the woo-woo artists attribute to “chi” and similar nonsense. It’s physics, all right, and it’s not that difficult to understand. But it takes practice to get all that stuff working together, and it’s risky, hence you don’t tend to see it from the average person.

    And yes, the same thing applies to the foot, elbow, forehead, or any other strike. Significant breaks are done with a small, hard contact area, moving at high speed, with as much mass as you can arrange in rigid connection at the point of, and through, the strike. It’s risky. Don’t try it at home. But it isn’t magical. There’s no such thing.

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