Were you instructed to make your strikes by fully extending the limb and locking the joints? My understanding of how the body works (which is, admittedly, limited,) says that a good full-power strike without doing that won't really strain the joint. It's also pretty silly, considering how easy it is for the arm to be grabbed and snapped like a twig if you lock it.
Edit: I think GKR is best suited for people with no interest in practical application and people in the low teens. I would recommend it as a starting place for people in their low teens, but since I think I damaged some joints striking full power into the air, I can't really do that.