In many cases it is how you go about conditioning. I have done knuckle conditioning and I do have arthritis in my hands due to the conditioning. Any conditioning you do should be gradual. Pounding on a rock isn't something you do first day. A lot of younger guys get gung ho and start doing things that hurt thier bodies instead of helping it. One specific thread comes to mind where a person was kicking trees with thier shins. All that does is injure your body unless you have gradually worked up to it...oh and they weren't the right kind of trees either
