Here is an article I wrote on board breaking, how it works and why it is done I hope it gives people some insight on why some arts place any importance on it. Board breaking is common in the martial arts; the most commonly used material is pine. This material is selected because it has approximately the same tensile strength as human rib bone. Bones such as the femur and the humerus are many times stronger than the rib bone however. Boards are always broken along their weakest point which is with the grain. Boards never break across the grain because in that direction they are 5 times stronger, roughly the tensile strength of concrete. Boards, like concrete, are much more resistant to compression than they are in tension which is why they always break from the back of the board. Sometimes to save money boards will be glued back together, this however does not make the board easier to break but does in fact make it harder to break. This is because boards that have been glued together properly almost never break where they are glued and thus have to break at their second weakest point. Some martial artists dispute the need for board breaking but others realize its importance. Breaking boards serves to improve confidence, demonstrate power, speed and/or accuracy. But the primary reason to break boards is to develop technique. Being able to break one board does not necessarily mean that you can break a rib it is more like if you cant break a board then being able to break a rib is much less likely. Often when people, particularly children, fail to break a board with a certain kick they can often break it after they have worked on their technique. Once when I was assisting another instructor there was this child who failed to break with a flying side kick after 6 or 7 attempts. I was asked to work with him one on one on his technique. After 10 minutes helping him with his kick he got to try again and broke the board on his first attempt. When breaking multiple boards there are 2 basic approaches in the way they are stacked; using spacers or not using spacers. When boards break tension is applied to the back side of the board until the magnitude of the tension exceeds the tensile strength of the board and a crack is formed. The crack will then propagate to either end and towards the front of the board. When the crack reaches the ends of the board and all the way to the front the board breaks. It takes twice as much energy for a crack to propagate in 2 directions than it does in 1 so when performing a knife hand strike, for example, it is better to aim over to one side so that the majority of the energy only needs to travel one way. Using spacers Spacers are often used when breaking multiple boards to make the break more visually impressive to potential students or so that practitioners can break a larger number of boards. When 2 or more boards have spacers between them then each board can flex freely and break individually in sequence, to do this the momentum of the strike must be maintained over a long distance, this is not the correct way to apply force with a strike for a martial art. The important physical quantity for breaking boards is Impulse (J). This is the product of force (F) times the time taken (delta t). J = F x (delta t) This means that for a given magnitude of impulse less force is required if it is applied for a longer period instead of a shorter one. It has been calculated, by whom I do not know, that for each spaced board that is added to the stack makes the whole stack 90% more difficult to break due to the momentum of the boards above being transferred to the board below. In other words 2 boards are require 190% of the force required to break 1 board, 3 boards require 280%. What limits the amount of boards that can be broken is the momentum that is lost after hitting each board. The amount of power required to break each additional board is additive. Not using spacers Breaking stacks of boards that are unspaced is considerably more difficult because the amount of power required to break each additional board increases exponentially. It is a common belief that each additional board increases the amount of power required by a factor of 10. In other words 3 boards are 10 times harder to break than 2 and 100 times harder than 1. This is because each board below supports each of the boards above. Because each board breaks at the bottom first the force must be transmitted through the entire stack before any of the boards will break. All of the boards in the stack must break in sequence from the bottom up which would be similar to breaking one board that is a thick as the entire stack, except for the wood fibers between the individual boards that arent connected. This happens over a smaller distance than if the boards were spaced and thus at a faster rate and if the amount of impulse was the same as in the case of the spaced boards breaking the stack would require a much greater force. The amount of power required to break each additional board is multiplicative. Speed breaks These are where boards are held in the fingertips or thrown in the air before breaking. This can only occur when the force is applied over the smallest amount of time and hence the term speed break. The force on the board must exceed its tensile strength before the board can overcome its own inertia. If it does not then the board will just fly off without breaking. Hand held versus using holders In some cases boards will be held in human hands and in others in board holders, for downward strikes towards the floor, boards are almost exclusively held with board holders. It is more difficult to break boards held by human holders than by using board holders. This is because human beings always have some natural give in them while the board holders are more rigid. This means that for board holders only a small fraction of the force is absorbed by the rigid structure allowing more force to be available to break the boards. The use of board holders is good if there are you are on your own but it is more of a challenge to break with people holding the boards. It is difficult for human holders to hold 3 boards by themselves and any technique with the power to break 3 unspaced boards puts a large amount of stress on the wrists of the holders so they must be supported by other people holding the wrists. Holding 4 or more boards is next to impossible unless the holders have giant hands. Breaking boards is a worthwhile endeavor so long as it is used for the right reasons such as developing technique and demonstrating the art. Board breaking for its own sake is counterproductive however as it takes time away that could be better spent elsewhere. Talk amongst yourselves.