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Suppose you have a room full of people. You have access to all their biometrics: the mass of their arms, their body, and so on. You can also make them punch in open air and measure their punching speed or whatever else.

Without making them actually punch a board, how do you determine who has the most breaking power? Is it momentum, kinetic energy, or some other simple physical amount?

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Still "Neither". Here you are looking for "peak pressure" and most systems teach generating it through a kinematic chain where you are attached to the ground. –  dmckee Feb 19 '13 at 22:51
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Me? I'd put a stress transducer under the skin of a ~40 Kg heavy bag and say "Punch this". Better still use an ballistics gelatin model with an articulated softwood skeleton or a crash dummy. If you want to improve it do both speed drills and heavy bag work and get an expert partner to criticize your form over and over again. –  dmckee Feb 19 '13 at 22:57
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In the absence of fancy kit you can armor up (say TKD pads, use two if you're worried) and say "Punch me here" for a rough idea. It can be instructive about how important it is not to let the other guy have time to set up a big lunge punch unless you like trying to breath with the wind knocked out of you. –  dmckee Feb 19 '13 at 23:06
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Google up "inch punch break" to see some breaks done with very little range. That works because you're not throwing your fist at the target: you are pushing it forward with a whole chain of connections through you body and into the ground. It's form and technique that make you hit hard, and that's why you're not going to get a satisfactory answer in terms of momentum or kinetic energy. The measurements you propose won't answer the question. –  dmckee Feb 19 '13 at 23:54
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From a martial arts perspective this is an ok question, but from a physics perspective it is so fundamentally flawed that words fail me... unfortunately my physics side managed to dominate my MA side on this one. –  slugster Feb 20 '13 at 9:50
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3 Answers 3

Answer to the altered question

Still "Neither". Breaking occurs when the localized strain exceeds the elastic limit of the material. That depends on the geometry of the board, it's supports and the location of the impact and on the peak pressure applied.

The geometric dependence is why you want to get a good hit in the center.

Answer to the original question

The most correct answer is "Neither".

Seriously, trying to use physics 101 concepts to understand biological damage in a fight is simply silly. If you insist on picking a single concept that is accessible on that level I'd look at either "peak force" or "peak pressure", but those aren't complete answers either because it also matters what part of the body you are hitting and how you opponent reacts.


As an aside, I have seen a lot of people try to analyze this using physics 101, discrete-body collision kinematics. Usually elastic collisions at that. That's not merely silly but downright stupid as we're not talking about free collisions of rigid objects.

  • Both the impacting bit and the hit bit are connected to extended bodies by a complicated gantry of rigid and tensile bits.
  • Both he the impacting bit and the hit bit may be subjected to active internal forces.
  • Both bodies are likely in active contact with the ground, so they are very much not free.
  • The make-up of human bodies is complicated. Different tissues can tolerate different levels of stress which often differ depending on what kind of stress (compressive, tensile shear) you're talking about.
  • In some cases a win can be scored on the opponents psychology or morale without dealing a traumatic physical injury
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Maybe this is meant to be answered by someone who has gone beyond 101 in physics. –  developer747 Feb 19 '13 at 22:34
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Feel free to check my profile. I have a bit more physics than that. Heck, Ive taught physics 101 as instructor of record at a major university. –  dmckee Feb 19 '13 at 22:35
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The question is fine, even natural, but the underlying assumption that it has a physics 101 answer is simply wrong. Biomechanics is hard. The biology and physics of injury is very hard. One punch|kick|throw|block|sweep|lock is not equivalent to another for these purposes. –  dmckee Feb 19 '13 at 22:45
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I agree with everything in this answer except the condescending attitude. :) It's hard to be nice, but worth the effort. –  Dave Liepmann Feb 20 '13 at 1:40
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Guys, chill out... –  Matt Chan Feb 20 '13 at 3:56
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I believe dmckee is right when it comes to the physics of board breaking. I might not have his physics credentials, but I've still studied a bit of it myself and would have made the same argument. Whatever technique you are using to break that board, if the strain is greater than the local resistance of the board, it will break. This is influenced by the nature of the material as well as it's geometry, and that's pretty much it. No matter if someone has strong punching power or not, as long as he has at least the minimum power required to break the board, he will break it.


In the next part of my answer, I will try to stick to "physics 101", and try to outline where it is insufficient in answering the question accurately. I invite dmckee and other more versed than I am in physics to correct me if I make any mistakes. My only wish is that the spirit of this answer (its reliance on "physics 101" and the outline of it's shortcomings) remains untouched.


Concerning your update... In my opinion, no matter what kind of biometrics you have access to, there is no way to measure someone's punching/board breaking power without actually having them punch/break boards.

This is because the use of momentum in martial arts is way more complex than simply measuring your weight and speed and multiplying them together. Sure, that's the gist of it, but what portion of your total weight do you actually put behind your punch? It can depend on a lot of different factors, such as the quality of your stance or the efficiency of your weight transfer.

As an example, let's say I weigh twice as much as you do. According to you, I'd automatically have double the punching power. This interpretation is somewhat limited. What if I only punched using my arms, without regards to my stance or without shifting weight to my punch? If you had a better technique, you could easily achieve superior punching power. Momentum can indeed help you determine the punching power of an individual but, sadly, it's just about impossible to measure accurately, as there are too many factors to consider.

And even then, as dmckee said, that's not counting on the internal forces that you are applying to your body while punching. Different individuals will have different tolerances allowing for more or less powerful punches. What good is punching at your maximum power if your hand/elbow/joint/etc. breaks just as much as the board? Psychological limitations will also play a part here. Even given superior potential, someone afraid to injure himself (you are attempting to hit wooden boards, after all) will be at a disadvantage compared someone confident in his ability.

And about kinetic energy? Assuming the perfect punch, where you are able to transfer 100% of your energy to your target, kinetic energy becomes a measure of your maximum potential damage on that specific strike. But, like momentum, what mass are you using in your calculations anyways? Only that of your fist? Of your whole body? And once again, the amount of kinetic energy you will be able to transfer as a deformation to your target depends on the target's nature, it's geometry and your own capacity to strike accurately on a location that will maximize the damage. And, not surprisingly, the momentum of your strike is a direct measure of how difficult stopping your punch will be, and thus influences greatly the amount of kinetic energy you will transfer to your target.

Wether you want it or not, both kinetic energy and momentum will matter in some way. Unfortunately, they are pretty much impossible to measure without actually breaking progressively harder boards, as there are too many different factors to consider, some of which even the best doctors probably wouldn't think about.

I hope this shed some light on the situation for some of you, and I do hope I have not made a big fool of myself...

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I guess this does not really answer your question: I wanted to add as a comment, but I reckon that because I have just joined this particular forum, I might not have enough privileges to add a comment.

But here is my view; two-fold approach.

1 - Power Generation From my experience, the Karate guys are the best in this. A good way of assessing a partner's generation of power is getting him or her to perform the punch in slow-motion. If you understand the principles of power-generation you will be able to really appreciate another person's skill when they do so in slow motion: the movements must be controlled, coherent to their positioning and look graceful and effortless.

2 - Focus and Concentration A Martial Arts student who has spent enough time training their mind and emotion (ironically a lot of students and instructors ignore emotions and frame of mind completely) will learn to be focused and really eliminate all distraction. This will lead to the build up of that emotional drive that adds extra speed and resistance to pain for example just like when we get hit by an adrenal bump. The reason why technique is important is that without control, you are just an uncontrolled irrational being. Your emotions to create a predator-like frame of mind while your technique would allow your attack to be accurate.

So that's what I would look for someone in your case scenario. Wooden boards or cardboard is irrelevant... what matters is what the fighter has within themselves.

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I did mention that this wasn't really answering the question: however my point was that it is not by the board itself you measure someone's ability but by other factors which I mentioned. But I admit and accept happily the DownVote :-) –  Lex Feb 26 '13 at 16:43
    
I'd argue that doing the technique in slowmotion will give you an insight as to an individual's skill, but it's not enough to assess board breaking ability. Technique will tell how well the martial artist applies power, not how much power he can apply. You need a benchmark to measure board breaking ability, and that benchmark pretty much has to be a board itself. Preferably, many boards of the same material with similar grain and all progressively thicker that can be calibrated against mechanical tools. Have martial artists attempt to break them, find the hardest one he could break. Voilà! –  Dungarth Feb 26 '13 at 23:05
    
@Dungarth "but it's not enought to assess board breaking ability". I have to admit you are right. I did not see the full picture and perhaps drew a too hasty opinion about it: you have added some new to my day. Thanks for that. –  Lex Feb 28 '13 at 7:43
    
My pleasure! And don't worry too much about the downvotes. I haven't been here so long myself, but I have found that the community is actually pretty fun to hang out with! Just remember to stick to the original question :p –  Dungarth Feb 28 '13 at 14:32
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