A tendon under the left rib cage may be a subcutaneous vein, or it may be caused by intercostal neuralgia or costochondritis. 1. A tendon under the left rib cage may be a subcutaneous vein, which is usually not easy to observe, but if the body is thin and the subcutaneous fat is relatively small, it will make the vein more obvious, or in case of anger, the pressure on the vein is larger, which will be easier to show, which is a normal phenomenon, and you don’t need to worry about it too much, and you just need to maintain sufficient nutrition. 2. Intercostal neuralgia: Due to the damage or stimulation of intercostal nerves, chest pain may occur, and the pain is mostly located in the area innervated by the intercostal nerves, and you may feel a tendon under the left side of the ribs, and the pain will be aggravated by the exercise. 3. Costochondritis: due to bacterial infection, endocrine abnormality may occur costochondritis, accompanied by costochondral tingling, swelling and pain. There may be a tendon under the left side of the rib cage. If there is a tendon under the left side of the rib cage, it is recommended to replenish the body nutrition, abnormalities, timely medical treatment, and follow the doctor’s instructions for symptomatic treatment.