Materials that are brittle and whose strength comes mostly while in they are in compression are used in breaking, such things as cinder blocks, bricks, and ice. Other things, such as soft rocks and re-breakable plastic boards, may also be used. However, wood is the most widely used material for breaking.
Breaking materials vary as to consistency. Wood provides the most inconsistent qualities, even when from the same tree. Board sizes may vary but the usual size is 12x12x1, which is always broken with the grain. Long boards, such as 2x4s, are always broken against the grain. Tiles provide the most consistent qualities. Bricks also provide consistent qualities, but only when they come from the same batch at the manufacturer. Ice is extremely consistent as long as is the same temperature throughout, which is hard to maintain. Plastic (as in re-breakable practice boards) provides consistent qualities, but repeated use will quickly degrade in performance, and may give the practitioner a false sense of achievement when moving on to more difficult breaking mediums. Also, the breaking resistance of plastic boards decreases with an increase in temperature.
Also, there is a concept called "spring co-efficient." This co-efficient is different between softer materials like wood and plastic, than in harder materials like brick and tile. If you were to look at a super slow motion video of a material being broken, you will notice that the material does not break at the point of impact, it "gives." The more the "give", the more "bounce" the material has. Softer materials, such as wood, have more give than harder materials, such as tiles. If your technique does not break these softer materials, this bounce is reflected back into the arm or leg and you will feel it. If the amount of force required to break a piece of wood is the same for a tile, you will probably feel more pain from a bad technique into wood than from the tile.
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