Recently I started being more aware of my foot postion while biking, and as such tend to increase my extenstion when pedaling. About this time I started noticing knee pain on long (15 mile plus rides) what I finally realized is that my seat needed to be 3/4" higer to accomidate my change in foot placement.
After I readjuted my seat, I readjusted my sons (because he grew from last week), and I see tons of people with theire seats way to low... Funny how when you figure something out it becomes so obvious- as in "why the hell didnt I figure that out years ago?"