Most of the symptoms of pain inside the left rib cage due to breathing are caused by the following diseases: 1. The most common disease that can lead to breathing pain is a strained muscle in the chest. There are pectoralis major, pectoralis minor and intercostal muscles in the chest, and the intercostal muscles mainly control the breathing movement of the thorax. Sudden violent coughing, sneezing or overworking the body and strenuous exercise can lead to damage to the intercostal muscles. Once the intercostal muscle is damaged, it is easy to breathe and cause pain inside the ribs. 2, prolonged thoracic back posture fixed and poor posture will lead to misalignment of the small joints of the thoracic vertebrae, once the small joints of the thoracic vertebrae position relationship is disturbed will cause the intercostal nerve compression symptoms. When breathing, the thoracic vertebral joints will be relatively active, and the intercostal nerve stimulation will change with breathing, resulting in painful symptoms inside the chest ribs.