Accessory muscles in the neck and chest are the last group of inspiratory muscles, used only for deep and heavy breathing, such as during intense exercise or respiratory failure. During expiration, the diaphragm relaxes, decreasing thoracic cavity size and causing the lungs to deflate. With normal breathing, expiration is purely passive. But with exercise or forced expiration, expiratory muscles (including the abdominal wall and internal intercostal muscles) become active. These important muscles are necessary for coughing.