Which muscles are primarily involved in expiration?

The muscles primarily involved in expiration are the internal intercostals. During expiration, these muscles contract to decrease the volume of the thoracic cavity, facilitating the expulsion of air from the lungs. The internal intercostals play a significant role, especially during forced expiration, by pulling the ribs downward and inward, which increases the pressure in the thoracic cavity and helps push the air out more forcefully.

The other muscle groups have different primary functions during respiration. The external intercostals are primarily responsible for inhalation by elevating the ribs and expanding the thoracic cavity, while the scalene muscles assist in inhalation by raising the first two ribs. The diaphragm, which is the main muscle of respiration, is primarily involved in inhalation, as it contracts and flattens to increase thoracic volume. During normal expiration, the diaphragm relaxes, but active expiration primarily engages the internal intercostals.

Therefore, the internal intercostals are specifically adapted for the work of forcing air out of the lungs, making them key players in the expiration process.