The tendon pain at the back of the leg may be caused by the following reasons: 1. It may be caused by muscle or tendon strain, which is often seen when patients do intense running or jumping without warming up, which leads to muscle or tendon strain and pain. 2. It may be caused by peripheral nerve lesions, such as in severe lumbar disc herniation or sciatica. The pain may be caused by varicose veins, which can easily appear in the lower extremities, and once this disease occurs, there may be some venous bulbs protruding from the skin on the back side of the lower extremities. In addition, once the venous bulb becomes inflamed, it may cause pain and cause the patient to mistake it for tendon pain.4. If it appears on the back side of the knee joint, which is the popliteal fossa, it may be caused by popliteal cysts. This type of disease is often due to excessive fluid accumulation inside the knee joint, which protrudes into the popliteal fossa from the weak part of the joint capsule on the back side of the knee, which then leads to pain from pressure on the nerves in the popliteal fossa.