Just ran my knee into Ember's head while playing with her. I'm limping now and she's ready to play more. The knock didn't phase her.
I've got a pit bull mix, she's got a hard head too. Really friendly dog with a lot of energy. She'll run at me and jump, and used to always headbutt me before I wised up. These dogs are not even 100 pounds but they can take a human down, and they're faster.