For myself personally, the lower arms have always been a breeze no matter what. Its as if I almost glide into the studio looking forward to those sittings! Outer arm and top of my shoulder where also relatively easy...
Someone once told me that the elbow was really really painful (just incase - really), and so before we started the session that would hit that area I asked my mate who was tattooing me what he thought. He said "it doesn't hurt... it just feels wrong". So an hour or so into it, he's moved my arm into position and I knew what was coming, or at least thought I did... I looked up at him, didn't need to say a word (the weird look on my face would have said much more than words could have) he looked at me and just said "told you". It didn't hurt, it just felt wrong...
Oh and my calf was a breeze, BUT the friend who did that piece has one of the lightest touches I've ever come across