As a respiratory physiologist, I can tell you that the innermost intercostal muscles play a crucial role in the mechanics of breathing. These muscles are located between the ribs and are involved in both inhalation and exhalation. During inhalation, the innermost intercostal muscles, along with the external intercostal muscles, contract and lift the rib cage up and out. This action increases the volume of the thoracic cavity, drawing air into the lungs. During exhalation, the innermost intercostal muscles, which are also known as expiratory muscles, contract and help to narrow the intercostal spaces by adducting the upper ribs towards the lower ribs. This action decreases the volume of the thoracic cavity, which helps to push air out of the lungs. It's important to note that the innermost intercostal muscles are not the primary muscles used for forceful exhalation; that role is primarily played by the abdominal muscles. However, they do contribute to the process, especially during quiet or resting exhalation. read more >>

Innermost intercostal muscle. Through adduction of upper ribs to the lower ribs the muscles narrow intercostal slits and diminish thorax volume, therefore the muscles represent expiratory muscles. ... lateral muscle slips laterally (which cross a single intercostal space)read more >>