Left undershirt pain, caused by a variety of reasons, as follows: 1, back muscle strain: left undershirt pain, the most common reason is the back muscle strain, mostly due to long hours of work, often bending over, play with the phone, bend down to do housework, which can lead to the back muscles are in a long time in the load state. Over time there will be aseptic inflammation of the back muscles and muscle spasms, which can be manifested as pain and stiffness in the back.2, supraspinous ligamentitis of the back: due to frequent bending labor, such as bending the supraspinous ligament will be subjected to tension, there will be gradual emergence of supraspinous ligament aseptic inflammation, manifested in the center of the back pain.3, the use of women’s rear buckle bras: the buckle will compress the supraspinous ligament, and over time, the buckle and the supraspinous ligament, the ligament and the supraspinatus ligament, and the supraspinatus ligament. Over time, the buckle and the ligament will rub against each other and cause inflammation, which will also manifest as pain in the left undershirt.