How is it that a running dog can crash onto a piece of furniture and not seem to feel pain?

I have often noticed my excited dog running around like crazy and spinning around in circles in the sitting room. She crashes into furniture and then not even slightly stunned or in any obvious pain, she continues running around like a bat out of hell. Just minutes ago she crashed and I thought she would have knocked herself out but no, she turned round and started speeding through the house again! No sense of pain?????