The innermost intercostal muscles are the deepest of the three intercostal muscle groups within each intercostal space. They are separated from the internal intercostal muscles by the neurovascular bundles of each space.
Each innermost intercostal arises from the internal, inferior surface of the rib that lies superiorly within each interspace. It inserts into the internal, superior surface of the rib that lies immediately inferiorly.
The innermost intercostals are innervated by collateral branches of the first to eleventh intercostal nerves. They act to fix the position of the ribs during respiration.