its a theory based all on constant motion. if you stop dead cold after jumping off a 2 story building, you will snap something, after a fall and you just keep rolling and keep moving your muscles wont alow for the bones to have something hard to snap against. i myself have always loved watching free running. another thing, if you are about to wreck in your car you auto tense up because you know whats coming. If an infant is in the car they dont understand whats goin on or whats coming so they dont tense. They stay lose and thats why we break bones and they dont.