Check out the following link to a usenet discussion of this. The relevant post is the third one. In case the LONG link doesn't work, the gist of the post is, "Regarding neck & shoulder pain, make sure your saddle's level and not pointed down at the nose. A saddle tilted down will shove you forward, and
you spend the entire ride pushing back from the bars to maintain position
(typically without even realizing it's happening). Result? Sore neck &
shoulders. If this is happening, the way to deal with it is with a
different saddle (one that allows you to be comfortable when level) and/or a
change in handlebar positioning so you can be a bit less bent over. "

usenet link