Pain under the shoulder, the specific causes are as follows: first, frozen shoulder: frozen shoulder has a wide range of pain, it can be in the neck and shoulder, or on the outside of the shoulder joint, or it can occur under the shoulder joint, that is, on the outside of the upper arm or in the underarm area. In the case of frozen shoulder, there is often pain in the shoulder joint, accompanied by limited movement of the shoulder joint, which mostly occurs in people around the age of 50, so it is also called fifty shoulder or frozen shoulder. With the development of frozen shoulder, the joint capsule will be found to be adhered to, so in the later stage, there will mostly be the loss of active and or passive movement function of the shoulder joint. Secondly, muscle strain: it is considered to be the strain of deltoid muscle or upper arm muscle, which is mostly seen in people who play sports or do fitness and exercise, and there will be soreness and pain below the shoulder when there is the strain of the above muscles.