Which accessory muscle is primarily engaged during forced expiration?

During forced expiration, the internal intercostals play a crucial role. These muscles are located between the ribs and contract during the expiratory phase of breathing, specifically when a person is expelling air forcefully, such as during vigorous exercise or when blowing out air.

The function of the internal intercostals is to pull the ribs downward and inward, decreasing the volume of the thoracic cavity and thereby increasing the pressure within the lungs, which helps to push air out more rapidly. This is in contrast to normal expiration, where passive muscular relaxation is often sufficient.

While the scalene and trapezius muscles are important for other aspects of respiration and shoulder movements, respectively, they are not primarily engaged during forced expiration. The transverse abdominal muscles can assist in expiration as well but they act more by compressing abdominal contents, which indirectly aids in expelling air rather than being specifically engaged in the mechanics alongside the internal intercostals. Thus, the internal intercostals are the primary accessory muscles involved in this process.