Your bones have cells, some of which absorb bone, and others that shit out bone. It's a never ending ebb and flow of bone being replaced by bone, and when bone is damaged, more bone and bone with more density is produced. If you beat the shit out of your bones, they will get stronger, and then you can beat the shit out of them even more, so on and so forth. This isn't a permanent thing though, since the bone will eventually go back to how it were when you weren't constantly beating the shit out of them. If this dude quit kicking wrenches to go work a desk job for 5 years, he'd probably break his leg by kicking a wrench after that hiatus.

On one end of the scale you have doctors recommending exercise to people with osteoporosis, and on the other end, you have this dude kicking a wrench.