The chest pain that occurs when you walk fast is caused by breathlessness. The main reason for this is respiratory muscle spasm, that is, the respiratory muscles will have a transient ischemia and hypoxia and the formation of pain. Because the human body activities or walking fast, the limbs of the skeletal muscles will consume more oxygen, this time leading to visceral muscles and respiratory muscles, including the diaphragm and intercostal muscles will appear transient ischemia and hypoxia, and the performance of chest pain as well as bilateral subcostal pain. Therefore, when in pain, you should immediately rest, stop activities, and try to switch to abdominal breathing, you can also use your hands to gently massage the lower part of the chest, which can be effective in relieving.