Which intercostal muscles function both as expiratory and inspiratory muscles?

The internal intercostal muscles are essential for both expiration and inspiration, making them unique among the intercostal muscles. During forced expiration, these muscles contract to pull the ribs downward and inward, reducing the volume of the thoracic cavity and expelling air from the lungs. In contrast, during inspiration, especially under conditions of increased respiratory demand, the internal intercostal muscles can also assist by stabilizing the rib cage and helping to pull the ribs together to facilitate the expansion of the chest and the inflow of air.

In comparing the choices, the external intercostal muscles primarily function to elevate the ribs during inhalation, contributing mainly to inspiratory efforts, while the transversus thoracis mainly aids in expiration by pulling the ribs downward and is not involved in inspiration. The serratus anterior, while significant for scapular movement, does not directly contribute to the mechanics of breathing. Thus, the internal intercostal muscles are the key muscles that serve both phases of respiration effectively.