During inhalation, you breathe in and this contracts the diaphragm and moves downwards. This increments the chest cavity space which means the lungs are expanding. The intercostal muscles or the muscles in between the ribs also aids in the enlargement of the chest cavity. Both muscles contract to pull your rib cage upward and outward when you inhale. As your lungs expand, air is sucked through your nose and mouth. It then travels down to the windpipe and into the lungs to the bronchus, bronchioles and eventually in the alveoli where air exchange between carbon dioxide and oxygen happens.
The additional accessory muscles of respiration are typically used only under conditions that are of high metabolic demand or respiratory dysfunction. However, in instances where these muscles become stiff and hard, expansion of the rib cage can be quite restricted. The accessory muscles of respiration include sternocleidomastoid and the scalene muscles namely anterior, middle and posterior scalene. Both aid in elevating the rib cage. However, their involvement seems to depend on the degree of respiratory effort. During quiet breathing, the scalenes are consistently active at certain phases while the sternocleidomastoid is quite.
