What does the extent of the injury have to do with it?
The extent of injury, nothing. The absence of injury, everything. Read carefully what I wrote:
The key question is whether the hitting constitutes force. If it doesn’t cause any injury to the person , and he has been “asking for it” with his behavior, then the case can be made that he had implicitly consented to being hit.
“asking for it”??? implicitly consented??? really???
First of all, for the record, I don’t agree with D’kian’s reckless-driving analogy at all. If you have an accident through your own fault and there is no second party involved, then there is no second party involved, so no potential rights violation to talk about and no one to give consent to.
Normally, random strangers are not given implicit consent to touch you anywhere on your skin or clothes, but in certain situations such touching does customarily happen. For example, suppose that you are standing somewhere and somebody comes from behind you, but you are blocking his path. He says “Excuse me” but you don’t seem to notice–so he gently taps your shoulder. Was this an initiation of force on his part? I would say no, because by being in a public place, you have implicitly consented to things that customarily happen in public places, such as strangers wanting to get past you and tapping your shoulder if you don’t notice them. The shoulder tap is, in effect, a form of communication, in a context where verbal communication fails.
Now, if you seriously insult someone, the insulted party might want to communicate to you that your insult has been successful (so to speak), and he might find that the most appropriate way to communicate this is to slap you on the face. Again, he does not intend to cause any injury, he just wants to tell you that he disapproves of your conduct; he is using the slap on your face as a form of communication. By offering you an insult of a serious kind, the person has implicitly consented to what he knows is often the response to insults of that kind.
Let me stress that I am not saying this is universally applicable, only in certain contexts. I am not positing as a general rule that you can hit whomever has said something you don’t like–just like you can’t walk up to any random stranger and grope him (her?) after saying “Excuse me.” It’s all in the context.