Why so few injuries?

One key to understanding breaking is a basic principle of motion: The more momentum an object has, the more force it can generate. For example, when breaking a brick with a punch, the fist reaches a speed of 11 meters per second (24 miles per hour). At this speed, the hand exerts a whopping force of 3,000 Newtons, or 675 pounds. That amount of force concentrated into an area as small as a fist will break a brick. Human bones can actually resist 40 times more stress than a brick.

In the late 1970s, Stephen Wilk and Ronald McNair, the scientist-astronaut who later died tragically Space Shuttle Challenger exploded, set up a strobe light that flashed either 60 or 120 times per second. They photographed McNair and others throwing various kicks and punches. Once the film was developed, they could calculate the speed of a punch by counting how many times the strobe flashed until the fist  or foot hit its target. They found that beginning students throw a karate punch at about 20 feet per second, enough to break a one-inch board. But a black belt like McNair could punch at 46 feet per second. At that speed, a 1 1/2-pound hand can deliver up to 2,800 Newtons. Splitting a typical concrete slab 1 1/2 inches thick takes on average only 1,900 Newtons.