Runner's knee isn't a specific ailment. It's a term used for the pain that occurs around the knee from activities that require bending and cause impact with the ground. It can come from overuse when the tendons attached to the bone become overstretched. There are times that runner's knee can result after a fall too.
The biggest problem with those who get runner's knee is that the knee might be out of alignment. When the bones are out of the correct position, any impact will cause stress and