A tendon on the outside of the thigh is very painful to consider iliotibial tract syndrome, muscle strain, femoral head necrosis, lumbar disc herniation. 1. Iliotibial fascia syndrome: mainly caused by repeated friction of the local iliotibial fascia chronic injury, a very painful lateral thigh tendon, pain aggravated by strenuous exercise. 2. Muscle strain: lateral femoral muscle or broad fascia tensor muscle injury can involve a tendon pain in the lateral thigh. 3. Necrosis of the femoral head: it can cause hip pain, walking limp, limitation of hip joint activities, and occasionally involve the pain of the lateral thigh. 4. Lumbar disc herniation: the herniated nucleus pulposus compresses the nerves and can radiate to the thigh causing pain and numbness. A tendon on the outside of the thigh is very painful, if the rest does not get better, you need to consult a doctor in time to clarify the diagnosis and further treatment.