I simply stole this reference from another bulliten board, but I agree with it's conclusion; that the extensibility of the nervous tissue within the "stretched" part commonly limits the length of the body segment. Of course, stretching nervous tissue isn't a very good idea.

"Freiwald has done a lot of work on stretching; he has looked at hamstring "stretches" and found that in approximately 70% of people the nerve stops further stretching. (Sportverletz Sportschaden. 1998 Jun;12(2):54-9."