Final answer:
The muscles involved in expiration are the internal intercostals and the abdominal muscles, with the internal intercostals drawing the ribs together to constrict the rib cage and the abdominal muscles increasing pressure in the thoracic cavity. The diaphragm relaxes but does not actively contribute to expiration.
Step-by-step explanation:
Expiration Muscles
During the process of expiration, several muscles are involved in forcing air out of the lungs. The internal intercostals are key muscles that function during expiration by drawing the ribs together and constricting the rib cage. Moreover, during forced exhalation, the abdominal muscles play a significant role in actively pushing air out by increasing the pressure in the thoracic cavity. Conversely, the diaphragm relaxes and moves upward, reducing the thoracic volume and passively aiding in expiration.
The external intercostals are not involved in normal expiration; they are used during inspiration. The internal intercostals, by contrast, assist in expiration. The internal intercostals are directly involved in the process by helping to push the ribcage inward which increases the pressure inside the thoracic cavity, facilitating the expulsion of air from the lungs. The abdominal muscles can be particularly active during more forceful expiration, such as when you blow out a candle or during physical exertion.