I'm pretty sure the "elfshot" was mainly used as an explanation for when you threw your back out.
I mean, in Swedish the word for that is still "ryggskott" as in "I got shot in the back", and it easily makes sense that when you suddenly get this inexplicable pain in your back, and you can barely more (clearly cursed) then of course it's invisible elves that shot you with some kind of pain needles.