Actually the famous study of nikkyo by Gregory D. Olson and Frank C. Seitz in the early 90s noted it is the touching of the periostium of the ulna and radius that produces the exquisite pain.
And many of us become quite immune to that aspect of it...which they explain as well in that over time it seems there is more of a gap between the bones.
Discussion of nikkyo starts on p. 11
http://issuu.com/csolimpia_slobozia/...al-and-motor-s