Knee pain while running is considered to be caused by patellar tendonitis, meniscus injury, osteoarthritis, synovitis, ligament injury and other causes. 1. Patellar tendonitis: patellar tendonitis pain is located in the front lower part of the knee, and the pain is aggravated by running or jumping. 2. Meniscus injury: meniscus injury can cause pain when running with the knee caused by the meniscus pulling to the surrounding synovial tissue, flexion and extension activities have a loud sound, and some will have symptoms such as joint interlocking. 3. Osteoarthritis: Osteoarthritis occurs when the subchondral bone is damaged, causing joint pain during running, serious joint deformation, and limitation of flexion and extension activities. 4. Synovitis: When running, the amplitude and volume of activities are relatively large, which will lead to obvious swelling of synovial membrane and joints, limitation of flexion and extension of joints, and pain. 5. Ligament injury: the medial collateral ligament and cruciate ligament will be significantly increased in running, because ligament injury will lead to joint instability, and knee pain while running will occur. Running knee pain may also include fractures and bone tumors and other rare cases, need to go to the hospital in time to check out the cause of further treatment.