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Knee pain is the most common sports-related complaint.
Most often, it's caused by repeating the same slightly off-kilter movement over and over. See if one of the following knee-overuse injuries matches your problem. That's your kneecap rubbing against the end of your thighbone. It's called patellofemoral pain, and it's common among people whose feet pronate, or roll to the inside, when they run. This kind of gait causes your lower leg to twist inward, pulling your kneecap inward while your thigh muscles are pulling it outward.