Soreness in the hamstring behind the knee can be caused by muscle or ligament injuries, popliteal cysts, and osteoarthritis of the knee. 1. Muscle or ligament injury: muscle or ligament injury behind the knee joint, or chronic strain injury and short-term high-load activities, will cause pain, soreness and other discomforts near the popliteal fossa at the back of the knee joint, there is no obvious structural damage, and the symptoms can be basically disappeared after resting for 1~2 weeks. 2. Popliteal cysts: most popliteal cysts have no obvious discomfort. In some cases, excessive knee activity or load stimulation of popliteal cysts may cause localized pain or even enlargement of the cysts. MRI of the knee can clarify the diagnosis. If the pain caused by popliteal cyst is serious, surgery is recommended to remove the cyst. 3. Knee osteoarthritis: Knee osteoarthritis is characterized by pain in the medial side of the knee joint, and in some cases, there may be lateral pain or pain in the popliteal fossa at the back of the knee joint, which is aggravated by activities, and becomes more and more significant with the progress of the disease. An X-ray of the knee can assist in the diagnosis. Physical therapy and heat may be considered. If the osteoarthritis is severe, treatment such as artificial joint replacement may be considered. It is recommended to consult the hospital for the cause of the pain in the hamstring behind the knee joint, and carry out appropriate treatment to avoid the aggravation of the condition that affects the function of knee joint activities.