I've been finding for the last couple of months that one of my knees wil get really stiff. At first it only happened when I had my leg really strait while I was sleeping, but now, even though it doesn't seem too much worse, it has been happening if I've been sitting the same way too long, standing stait legged for a while, that kind of thing.

I doesn't hurt, it just feels really stiff, and I have to go for a walk or work it out a little when I wake up. It doesn't hurt when I'm walking, biking, swimming or climbing.

My question is this: I have never had any knee injuries, and this is the first time I've had this happen. I've been doing a lot of mountain biking lately, going out 3 or 4 times a week for a few hours at a time. I also did a bit of hiking on a lot of those weekends (not long hikes, but I did have all my climbing gear on my back). Could this be related to mountain biking, should I ease off a little and concentrate on road riding a little more (taking a total break isn't going to happen, I go back to school in September so I want to get as much biking as I can before I hit the books again).