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Ron McNair raises his hand and brings it quickly down onto a pile of concrete bricks. As he pulls back his left hand, his right hand smashes through the concrete, sending it crumbling to the ground. An exciting picture, but what is it doing in Scientific American magazine? That powerful karate chop might remind you of the superhuman feats found in Hollywood action movies. But the ability of karate experts to break bricks has an easy explanation in the principles of physics and physiology. In fact, karate experts view breaking bricks and boards as a demonstration of form and concentration. The true nature of karate involves a complete mental and physical discipline that goes far beyond simple hand strikes. 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, 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. Another key to brick breaking lies literally in the palm of your hand. Feel the bone on the edge of your hand, directly below your little finger. This bone (known as the fifth metacarpal) bears the brunt of McNair's hand strike. Human bones can actually resist 40 times more stress than concrete. (Picture a piece of concrete the size of a bone, and imagine how easily it would break.) The natural engineering of the human hand also lessens the severity of the impact. The muscle, tendons, ligaments and other soft tissue in the hand provide a natural cushion, dispersing the impact energy up through the arm. If you attempt brick breaking without proper training, you'll end up with an injured hand and possibly serious nerve damage! You must be instructed by an expert in proper technique. Proper training protects your hand because regularly striking a striking pad or post causes your skin to develop calluses, your muscles to strengthen, and your bones to thicken. Extensive training is also necessary to train your brain and muscles to bring your hand down just right--exactly as it reaches its full speed and right smack in the center, at the brick's weakest point. Have you ever broken a bone? What caused it to break? Why do you think karate practitioners yell "kiai" when they are carrying out a strike? Why do karate practitioners put their hands down several times on the board or brick before attempting to go through it? Why do they pull their other hand back when they strike?