Wolff's law is a theory developed by the German Anatomist/Surgeon Julius Wolff (1835-1902) in the 19th century that states that bone in a healthy person will adapt to the loads under which it is placed. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. The converse is true as well; if the loading on a bone decreases, the bone will become weaker. Over time, bone under stress attracts calcium salts fuses it to surrounding bones as a protective measure to resolve the weakness or stress. Therefore, whatever part of the body receives repeated abnormal stress will be the site of attraction for calcium salts. With repeated forging (training to toughen body parts used for striking), not only does the outer surface of the body part get tougher, the underlying bone gets thicker and stronger.
Bone can withstand 40 times more force than concrete. A cylinder of bone less than an inch in diameter and 2 1/3 inches long can withstand a force of more than 25,000 Newtons. Hands and feet can withstand even more than that, because their skin, muscles, ligaments, tendons, and cartilage absorb a great deal of impact. As a result, a well-kicked foot can absorb about 2,000 times as much force as concrete before breaking.
Boxers also punch quickly and powerfully, they find it difficult to break concrete blocks because of their technique. Boxers throw punches with follow-through to gain maximum momentum (golf and tennis players follow-through for the same reason) so they may knock an opponent down. But the impact itself is diffuse. A Taekwondo punch has no follow-through. It lashes out like a cobra and then withdraws instantly. When a black belt hits a block of concrete, the fist touches the block for fewer than five milliseconds so all the striking forces are transferred to the block.
A well-thrown fist reaches its maximum velocity when the arm is about 80 percent extended. Focus your punch in your imagination so that it terminates inside the block, rather than on the surface. To deliver the maximum power, you want to make contact before the slowdown begins.
Remember the equation, Force = Mass x Velocity2. The moving hand generates force while the stationary brick generates no force; therefore, the hand overpowers the brick. If the hand was stationary and the brick was dropped on it, then the moving brick would injure the hand.