Pain in the medial side of the running knee is most often seen in knee injuries, such as medial collateral ligament strains, medial meniscus injuries, and so on. It can also be seen in knee joint inflammation, such as osteoarthritis, rheumatoid arthritis and gouty arthritis. It is necessary to further improve the examination in order to clarify the cause of the disease. 1. Knee joint injury: Since the pain occurs after running, it is considered to be caused by too strenuous exercise or improper posture, which may be due to the strain of the medial collateral ligament of the knee joint or the injury of the medial meniscus. 2. Inflammation of the knee joint: Inflammation of the knee joint may aggravate the pain when running. In particular, osteoarthritis of the knee joint aggravates the wear and tear of the medial side of the knee joint when running, so it causes pain on the medial side of the knee. Gouty arthritis and rheumatoid arthritis can also cause pain on the inside of the knee after running. It is recommended that the patient go to the hospital, complete the auxiliary examination, in order to clarify the cause of the disease, and then the professional physician will give the appropriate treatment for the cause of the disease.