Pain in the back of the thigh is a warning against pyriformis syndrome, muscle strain, lumbar disc herniation, myofasciitis and other diseases. 1. Piriformis syndrome: Piriformis syndrome can lead to sciatic nerve compression in the region of the piriformis muscle. Patients will experience pain in the buttocks, radiating pain from the buttocks through the back of the thigh, so patients will experience pain in the back of the thigh. 2. Muscle strain: Muscle strain may be caused by overstretching of muscle fibers and muscle rupture during heavy lifting, sprinting or jumping. Patients may show muscle pain, swelling, muscle stiffness or weakness. Therefore, if a patient has a muscle strain on the back of the thigh, it will manifest as pain on the back of the thigh. 3. Lumbar disc herniation: The causes of lumbar disc herniation include disc degeneration, lumbar spine injury, lumbar spine developmental abnormality and so on. Patients will have lumbar pain, radiating pain in the lower limbs, numbness and weakness of the lower limbs and so on, so the patients will have the symptom of pain in the back side of the thigh. 4. Myofasciitis: Myofasciitis is caused by acute injury to muscles or fascia, and prolonged over-activity. Patients will have symptoms such as localized muscle swelling and pain, limited activity, localized muscle bulging or spasm. If the patient has myofasciitis on the back of the thigh, he or she will have the symptom of pain on the back of the thigh. In addition, the back of the thigh pain may also be caused by the accumulation of lactic acid in the thigh muscles after strenuous exercise. In addition, patients with back of the thigh pain should also be alert to femur fracture, lumbar spondylolisthesis and other diseases. Patients with pain in the back of the thigh should consult a doctor in time to identify the cause of the disease, and cooperate with the doctor for standardized treatment.