There’s a good piece on the dangers of makiwara training over at The Uechi-Ryu Journal.  Every kind of body-hardening or conditioning training carries some inherent risk.  You’re subjecting yourself to practices that are specifically intended to change the muscles, tendons, ligaments, and bones that make it possible for you to walk across the street, pour a cup of coffee, or pick up your kid.

Are you sure that what you’re doing will be worth the desired effect, including the possible side effects?  Is it worth the chance of inducing crippling joint pain down the road if all you get out of it the ability to do a kung fu parlor trick?  How many skulls are you planning on cracking with those iron knucks, anyway?