Proximal Attachment
The sixth intercostal intimus muscles lies deep to the sixth intercostal internus in the middle two-quarters of the intercostal space.  It is a more substantial muscle than those of the upper intercostal spaces and arises from the internal aspect of the sixth rib.

Distal Attachment
Anteriorly the fibers pass infero-laterally, while laterally they pass infero-medially to insert onto the internal aspect of the seventh rib.  

Nerve Supply
The sixth intercostal intimus muscles is supplied by the sixth intercostal nerve.

Action
The sixth intercostal intimus muscle is most active during expiration, but its primary role is with the sixth intercostal externus to stiffen the chest wall during respiration.