Final answer:
The external intercostal muscles contract during inspiration to lift the rib cage and relax during normal expiration, allowing passive recoil of the lungs. The internal intercostal muscles contract during forced expiration to actively push air out.
Step-by-step explanation:
The intercostal muscles play a crucial role in the mechanism of breathing, specifically in the processes of inspiration and expiration. During inspiration, the external intercostal muscles contract, which lifts the rib cage and increases the volume of the thoracic cavity. This action creates a lower pressure within the lungs compared to the atmospheric pressure, causing air to flow into the lungs.
In the case of expiration, during normal breathing, these muscles relax, allowing the lungs and thorax to recoil and air to be expelled passively due to the elastic properties of the lungs and chest wall. However, during forced expiration, such as in vigorous activities or respiratory illness, the internal intercostal muscles contract to depress the ribs, actively reducing the thoracic volume and pushing air out of the lungs more forcefully. The innermost intercostal muscles also function as synergists to the internal intercostals during forced expiration.