Originally Posted by Mirage-t
This weekend I experienced a similar situation. I'm 27, have never had any problems of this nature and after 20 miles, it felt like someone was driving a nail into my knee cap. It only hurt on the down stoke so I just took some effort off my left leg during the down stroke. The pain seemed to come and go and vary in intensity from hardly noticable to almost unbareable. I haven't made any changes to my bike in the last couple months. The pain actually started about 2 weeks ago when I got my bike back from the LBS, who only adjust my shifting. I can't imagine them adjusting anything ride related. Could this just be an adjustment problem?
I've worked at a bike shop, and I can tell you that often times the wrench will raise the seatpost to clamp it into the work stand. When they're done, sometimes the position isn't returned to exactly where it started previously.
Also, it's possible that over time, your seatpost could have slipped lower without you noticing. Even a couple of millimeters can make enough difference to induce knee problems.
Check your saddle height again, and mark the post somehow when you've found the best position.