Muscle pain in the chest is most common for several reasons: 1. Because of injury from sports or labor. The pain is usually caused by a strain on the chest muscles after heavy lifting or strenuous exercise. 2. It may be caused by a strain injury. People who often lift heavy things or often do shoulder joint inward labor can easily lead to strain on the pectoral muscles, resulting in chronic strain pain or local aseptic inflammation. 3, it may be a local inflammation, such as infection in other parts of the body, and then spread to the chest muscles, resulting in local muscle infection and redness, swelling, heat and pain. However, it must be noted that if there is pain in the left side of the chest, in addition to muscle pain will be accompanied by chest tightness, chest pain, palpitations, etc., it must be considered whether it is an acute disease of the heart, such as angina pectoris or myocardial infarction. If it is either of these situations, it is life-threatening and should be immediately treated urgently in a hospital.