Which muscles contract during forced expiration?

During forced expiration, the internal intercostals play a significant role in the process of exhalation. These muscles are located between the ribs and are responsible for pulling the ribs down and inward during expiration, which decreases the volume of the thoracic cavity. This action increases the intra-abdominal pressure and helps to expel air more forcefully from the lungs.

While the external intercostals, diaphragm, and parasternal muscles are primarily involved in inhalation or passive expiration, they do not contribute significantly during forced expiration. The external intercostals facilitate inhalation by expanding the thoracic cavity, and the diaphragm contracts downward to create negative pressure for inspiration. In contrast, during forced expiration, the focus shifts to the internal intercostals and sometimes the abdominal muscles, which actively assist in expelling air from the lungs through a coordinated effort to decrease thoracic volume.