Swelling and weakness of legs below the knees may be caused by prolonged standing, drug factors, nephritis, lumbar disc herniation, lower extremity venous thrombosis and other reasons. 1. Standing for a long time: If the patient stands for a long time, it may lead to congestion of the lower limbs, causing poor blood circulation, so the patient will have the symptom of swelling and weakness below the knees. 2. Drugs: Drugs may also cause swelling and weakness of the legs, if the patient is taking amlodipine, nifedipine and other drugs, it may cause lower limb edema and other side effects, it is recommended that the patient can stop taking or replace the drug under the guidance of the doctor. 3. Nephritis: If the patient suffers from nephritis, the function of the kidney to excrete water will be decreased, which will lead to the gathering of water in the body for a long time, so the patient will have the symptom of swelling and weakness of the legs below the knee. 4. Lumbar disc herniation: If the patient suffers from lumbar disc herniation, the lumbar disc herniation may compress the nerves and cause the symptoms of swelling and weakness of legs below the knees. 5. Lower extremity venous thrombosis: Lower extremity venous thrombosis usually blocks the venous blood return channel of the lower extremity, which may cause restricted return in the long term, resulting in symptoms of swelling and weakness of the legs below the knee. There are many other reasons for the swelling and weakness of legs below the knee, so it is recommended that patients go to the hospital for relevant examinations and make targeted treatment according to the causes.