The transversus abdominis is a deep abdominal muscle that extends horizontally around the waist. It is the innermost of the three layers of abdominal muscles and runs transverse to the fibers of the external and internal obliques. Its main function is to stabilize the trunk and compress the internal organs. The transversus abdominis is also involved in respiratory mechanics, particularly during expiration, as when it contracts it reduces the volume of the abdomen and helps to force air out of the lungs.