There are many causes of muscle pain in the upper left arm, which are mostly seen in trauma, adverse drug reaction, infection, cervical spondylosis, frozen shoulder, and related cardiac diseases, and it is recommended to improve all the examinations in order to clarify the etiology. 1. Trauma: there is a clear history of trauma, such as contusion and strain of the upper arm muscles of the left arm, with clear pressure points at the site of the strain, and the pain is aggravated by activities. 2. Adverse drug reactions: most often seen in the left deltoid muscle after drug injection, the deltoid muscle can appear hard lumps, pain and other symptoms, will disappear on its own. 3. Infection: there are mostly local skin breaks, improper handling of the case of bacterial invasion, which in turn causes pain in the muscles above the left arm. 4. Cervical spondylosis: Cervical spondylosis causes compression of the nerve roots, which can lead to radiating pain in the muscles above the left arm that it innervates. 5. Frozen shoulder: In patients with left-sided frozen shoulder, because of the aseptic inflammatory reaction in the left shoulder joint and muscle adhesions, which can lead to limited activities of the left upper limb and pain in the muscles above the left arm. 6. Heart disease: more common angina pectoris, myocardial infarction and other ischemic cardiomyopathies, in addition to the left precordial pain, but also the pain of the left upper limb. It is recommended that patients go to the hospital to improve the auxiliary examination, in order to clarify the cause of the disease.