x-ray and mri shows "mild osteoarthritic changes" in both knees, and small tear in posterior horn of medial meniscus of right knee. My knees feel stiff when I run. Very little pain, but I can only jog slowly due to the stiffness. After completing a jog of 5 miles, the knees begin to hurt a few hours after and it gets worse by the next day. I have to rest for a day or two before I can jog again. Question: Why does rest decrease the knee pain? i.e., If the cartilage that cushions the knee is worn down, shouldn't the knees hurt every time I try to run?