Hmm. I'm annoyed at my lawyer brain, all I can think about is how this would actually be a very interesting case. At least, based on my understanding of U.S. Torts law, which is not my practice area (but which is bar tested and a required law scool course). Don't take any of this as real legal advice.
But, there's a concept known as the eggshell plaintiff rule/doctrine. Basically, it states that if the person you injured is unusually fragile, you're on the hook for their injuries regardless of whether they're a typical result of the action you took. So, here, while the typical result of pulling a finger would be a fart, the puller may be on the hook for the entire damages of a lost arm.
However, undercutting that is the defense of consent. The "victim" here clearly consented to the activity which led to the injury, and should have known that the action may likely result in the loss of an arm, based on the lack of tendons/muscles/skin/everything.
I'm gonna have to save this to show at conventions and see how people think this would play out. I'll totally be the coolest kid in school then. In your face, Mark.