What it feels like: A general pain in the knee joint that increases when running or on stairs; clicking in the knee when you get up from sitting; stiffening in the joint after sitting.
Why it happens: Often the pain comes from the kneecap being pulled off its track, which results in too much or too little pressure on the buffering cartilage. This can happen if you have weak quadriceps muscles or get too ambitious with your mileage and dont build up to long distances gradually. People who pronate (feet roll inward at the arches), have flat feet, or arent flexible have a higher risk for achy knees. And women are more prone than men, because our wider hips create an angle from the hip to the knee that can cause problems.
How to fix it: Take a week off from running and cross-train with activities that are gentler on knees (like swimming or cycling). Then, start running again, but cut your mileage by 30 percent at first. Strengthening your thighs can also help—particularly doing exercises like quadriceps extensions: Sit on a high surface, hang a 3–5 pound purse or weight around your ankle, then extend the leg straight. Hold for 3–5 seconds, then lower and repeat. Do 12 reps with each leg, and progress to 4–5 sets daily. Still in pain? See your doc, or get a referral to an orthopedist.