Final answer:
The internal intercostal muscles contribute to forced expiration by drawing the ribs together to constrict the rib cage, thereby increasing the pressure within the pleural cavities and forcing air out of the lungs.
Step-by-step explanation:
The internal intercostal muscles play a crucial role in the process of forced expiration. They are one of the three sets of intercostal muscles located between the ribs. Unlike the external intercostal muscles, which raise the rib cage to assist in inspiration, the internal intercostal muscles are responsible for drawing the ribs together, thereby constricting the rib cage during expiration.
This action is especially prominent during forced expiration when additional effort is required to push air out of the lungs. The contraction of these muscles decreases the volume of the thoracic cavity, increasing the pressure inside the pleural cavities and forcing air out of the lungs. Additionally, these muscles work synergistically with the diaphragm and abdominal muscles during the process of pulmonary ventilation.