The tendon at the back of the knee hurts when the knee is straightened may be caused by localized tendon contracture, ligament injury, meniscus injury, cyst in the popliteal fossa, and so on. Different causes, different treatment methods, common treatment methods are physical therapy, drug therapy, surgical treatment. 1. Local tendon contracture: Keeping the knee joint bent for a long time will produce local tendon contracture, so it is easy to have pain in the process of straightening. Generally, it can be relieved on its own, and you should pay attention not to keep a position for too long. 2. Ligament injury: when the popliteal ligament, coronary ligament, etc. behind the knee is strained or injured, the ligaments are overly stretched when straightening the leg, which will lead to pain behind the knee. Mild ligament injuries can be treated by immobilizing the knee and avoiding stress on the knee, while severe injuries require surgery to repair the ligaments. Non-steroidal anti-inflammatory drugs such as ibuprofen can be used to reduce inflammation and pain, and mannitol can be used to reduce joint swelling. 3. Meniscus injury: When straightening the leg, the injured meniscus will be squeezed, resulting in increased pressure inside the knee joint, which will lead to pain at the back of the knee. It is necessary to brake the knee joint and actively exercise the quadriceps muscle after the acute stage, and rehabilitation exercises can be carried out under the guidance of the rehabilitator. Ineffective physical therapy or severe meniscus rupture should be operated as early as possible. Non-steroidal anti-inflammatory drugs such as ibuprofen can be used to reduce inflammation and pain. 4. Cysts in the popliteal fossa: The pressure in the popliteal fossa increases when the leg is straightened, which stimulates the ligamentous joint capsule in the popliteal fossa and leads to pain at the back of the knee. If there is no symptom, it can be left untreated. Larger popliteal cysts should be removed surgically. If the rest can not be relieved, it is recommended to go to a regular hospital in time, by the doctor to clarify the cause of the disease, to give targeted treatment. The use of drugs need to follow the doctor’s advice.