the abdominal viscera and play a role in increasing intra- thoracic pressure during forced expiration and coughing. The primary mover of trunk fl exion is the anteriorly located rectus abdominis. The muscle is divided into sec- tions by fascia, covered by a fascial sheath, and separated into right and left sides by a fi brous band of connective tissue called the linea alba. The muscle attaches superi- orly to the cartilages of ribs 5 through 7 and the xiphoid process of the sternum. Its fi bers are oriented longitudi- nally and attach to the pubis. Contraction of the rectus abdominis fl exes the thoracic and upper lumbar spine and posteriorly tilts the pelvis.
The external and internal oblique muscles originate posterolaterally on the trunk and run obliquely toward
the midline as they merge with the linea alba. The super- fi cial large external oblique muscle runs from a superior lateral to a medial inferior point. Deep to the exter- nal oblique is the internal oblique muscle, whose fi bers run in the opposite direction from the external oblique. The obliques are trunk rotators: The external oblique on one side and the internal oblique on the contralateral side work together to rotate the trunk. Together, the two muscle groups produce a diagonal rotational force. Figure 5.33 displays these superfi cial muscles and their relationship to each other.
The transversus abdominis, the deepest of the abdom- inal muscles, compresses the abdomen and helps sta- bilize the lumbar area through its attachments to the
