followed by 10 pull-ups, and then an 800 meter run--then repeating the times as possible in 20 set as many minutes. Because the workouts change every day, the body does not easily adapt and is always challenged. Of course, a danger is that some people go overboard and risk injury or at the very least, suffer aches and pains for a few days. But for people who like to push the limits of fitness and strength-there are many police officers, firefighters, and military people who follow it-CrossFit is the key