Running is and remains an unprecedentedly popular sport. Unfortunately, it is also an injury-prone sport. Whether you are a beginning runner or a highly experienced one, injuries are always lurking. We would like to provide you with more information on how knee pain occurs during running and what you can do about it. Read on quickly below. What is knee pain when running? Knee pain during or after running is also called a runner's knee. It is caused by overuse during running. This overuse causes pain on the outside of your knee. The pain is caused by a tendon. This runs down the side of your thigh and attaches to the lower leg just below the knee.