I've been having knee problems off and on for a while, but they seem much worse lately. Today my knees even bothered me during my ride (usually they don't). Mostly I'm having pain with getting up and down and going up or down stairs, and it's predominantly my left knee for the time being.

Does anyone know of some good knee strengthening exercises? This is starting to hurt to the point I'm afraid it might put me out of commission for a while, and I definitely DO NOT want that to happen! I'm just getting good and strong again following Christmas break.