2.4.3. Innermost Intercostal Muscles: Subcostalis
This muscle consists of various individual bands of muscle fibres that lie in the posterior part of the thoracic wall, as shown in the accompanying diagram. When the fibres contract during forced expiration, they depress (move down) the lower six ribs.
Origin: internal aspect of ribs 6 - 10/11.
Insertion: internal aspect of the rib two or three levels down from the origin.
Fibre direction: inferomedial.
Nerves: intercostal nerves (ventral rami of T1 - T11).
Action: depresses lower ribs during expiration.
As you can see from the past few sections, the innermost intercostal muscle groups tend to have 'assisting' or 'supporting' functions in forced respiration.