Martial arts are my hobby and explaining science is my job, so the recent appearance of “How karate chops break concrete blocks” on io9.com naturally caught my eye. Unfortunately, not only did it fall far short of my hopes of offering a lucid explanation, it parroted misleading statements from an article on exactly this same subject that has been annoying me every time I remembered it for 10 years. (Oh wonderful Web, is there no old error you can’t make new again?) Indulge me while I try belatedly to set the record straight. The io9 article starts by asking how the squishy human hands of martial artists can break concrete slabs, wooden boards and other considerably harder objects. Reassuringly, it wastes no time on talk of chi and similar Eastern mysticism but instead goes right to a very loose discussion of the biomechanics of hitting efficiently and striking to vulnerable positions in the target. So far, so good, although the descriptions of what to do are probably too vague to be helpful to readers who don’t already know what they’re doing.
Then the article seems to go off the rails (emphasis added):
It's also important to strike quickly at the surface of the block. Most blows are part connective smack and part push. This delivers the most damage when fighting flesh, but helps protect concrete or wood. Concrete and wood have a good mix of rigidity and elasticity. The materials will bend, and even flex back like a rubber band would, but the limits of their malleability are much lower. Bending and snapping back can do more damage to them than it can to things that flex easier. By making the blow fast and pulling back, the striker hits the block hardest and allows the material to do the maximum amount of bending. A follow-through push will keep the material from snapping back, and snapping itself.
In my experience, and that of every other martial arts practitioner to whom I’ve ever spoken about this subject, that’s just not right.
When you break a board, or concrete, or a Louisville slugger or anything else routinely used these days in demonstrations of tameshiwari (breaking), you have to follow through on the strike. Indeed, advice commonly given to students learning to break is that they should aim at an imaginary target several inches beyond the actual object, for two reasons. First, doing so helps to make sure that the actual strike occurs closer to the movement’s point of peak biomechanical efficiency. Second, it helps to override our natural tendency (partly psychological, partly reflexive) to slow down ballistic movements such as punches and kicks before they reach full extension, which helps to protect the connective tissues around our joints.
This argument is not theoretical for me. I’m not in the same martial arts galaxy as Mas Oyama, and I am anything but a breaking ace. But I have studied karate for 17 years, and during that time I’ve broken my share of boards, concrete and rocks, and helped out with demonstrations by other karateka executing far more powerful breaks. They all followed through.
The reason this faulty description of breaking jumped out at me is because I had read it 10 years ago in Discover magazine’s “The Physics of … Karate,” which is one of the two articles that the io9 piece listed as a source. (The other was an entertaining and apparently blameless piece by The Straight Dope.) The Discover piece was if anything even more confused because it implied that boxers’ punches could not break boards (believe me, they can) and implied that biomechanical efficiency somehow justified its statement that a karate chop “lashes out like a cobra and then withdraws instantly.”
If you’d like more thorough and quantitative explorations of the physics of breaking, you can get them here and here [pdf]. You can also read what are regarded as a couple of classic papers on the subject:
Walker, J. D. “Karate Strikes,” American Journal of Physics, Vol. 43, 10, Oct. 1975
Feld, M. S. et. al. “The Physics of Karate,” Scientific American, pp. 150-158, April 1979
A surprisingly accurate and informative description of the science behind breaking is also available on this page (with video) for the old Newton’s Apple children’s science TV show, which states:
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 io9 article correctly points out that when breaking multiple boards or slabs at once for demonstrations, breakers often put small spacers (wooden pegs, pencils, stacks of coins, etc.) between them at the edges so that they do not sit solidly atop one another. Use of spacers does make breaking a given number of boards much easier (see that Straight Dope article to see how much easier). But then it continues:
Hold a piece of paper by both sides in the air and a knife, applied to the middle, will cut right through it. Put it on a smooth concrete floor, and it will be much, much harder to cut the piece of paper using the same knife.
Like everything else in life, breaking is just a primitive, degenerate form of bending. The paper can't bend, and so it doesn't give. A concrete block works the same way.
Trying to explain breaking by drawing analogies with cutting only confuses matters. And what in the world does "Like everything else in life, breaking is just a primitive, degenerate form of bending" even mean? [Update: Good news, everyone! Allan West in comments points out to me what I should have recognized in the first place: the author is quoting Bender of Futurama. Thanks, Allan; your Planet Express gift certificates are on the way.]
Breaking is filled with practices that, depending on your point of view, are either tricks for fooling observers or techniques for maximizing a strike’s visible effect. You don’t just strike with the power in your arm or leg: you organize the movement of your strike to bring in as much power from your legs, hips and upper body as possible, too. 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. (For some demonstrations, breakers have been known to bake their boards ahead of time 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.
When breaking concrete, you use slabs that are relatively narrow and long, so that the strike can hit at a distance from the supports at their edges for best leverage. With some multiple breaks—involving, say, big slabs of stacked, spaced ice—you can count on the weight of the falling, broken pieces on top to help break slabs lower in the stack. And so on.
All these practices are ultimately demonstrations of simple physics, not magical chi, but doing them well—particularly the more demanding ones—also takes strength, training and concentration. The value of tameshiwari to martial arts training is much debated; it has little or no practical relevance to fighting or whatever else people study martial arts for. Done wrong, breaking is an amazingly efficient way of messing up your limbs, potentially permanently. I don’t do it often and I don’t much miss it. But I have seen (and felt firsthand) how much successfully breaking boards can boost a student’s confidence, so who knows.
In closing, Matt, the Skeptical Teacher, demonstrated karate breaking at The Amaz!ing Meeting a couple of weeks ago in Las Vegas, and even showed that he could teach veteran skeptic Joe Nickell to break in under five minutes to prove the lack of mystical woo involved. Here's a clip of a break he did on the main stage.