Whether you have a six-pack, a four-pack or a whole keg, the abdominal muscles are a focal point of human anatomy from both a cosmetic and functional point of view. Beyond turning heads on the beach, your awesome abs perform an important role in movement, trunk stability and injury prevention.
Muscles of the Abdominal Wall
Your abdominal wall is composed of a complex of six muscles: the rectus abodminis, the internal and external obliques, the transversus abdominis, the pyramidalis and the cremaster. The pyramidalis and cremaster are located deep in the pelvic floor and do not contribute to movement. The other four muscles are involved in movement and stabilization of the trunk and pelvis.
The rectus abdominis attaches to the cartilage of the fifth, sixth and seventh ribs and runs vertically along the trunk to the pubis. The linea alba is a tendinous sheath that attaches the right and left sides of the muscle and three tendinous attachments divide the muscle into six segments that create the "six-pack". The rectus abdominis functions to flex the trunk, shortening the distance between the ribs and the pelvis. It also participates in lateral flexion with the oblique muscles. The rectus abdominis is a powerful stabilizer that protects the spine and influences pelvic alignment, an important component of posture.
External and Internal Oblique Muscles
The muscle fiber insertion of the external oblique is spread across the lower eight ribs and descends in a "V" shape to attach at the iliac crest of the pelvis. The internal oblique runs at a right angle to the external oblique, from the lower three ribs to the pelvis and low back. Together, the internal and external oblique muscles hug the trunk like a girdle or corset and are involved in trunk rotation and forward and lateral flexion, or bending to the side. Toned obliques cause the waistline to taper inward.
The transversus abdominis muscle does not contribute significantly to movement but works as a pelvic stabilizer, compressing the abdomen and holding vital organs in place. The transversus abdominis also plays a role in respiration, facilitating expiration of air from the lungs. A toned transversus abdominis helps flatten the lower abdominal wall. In an MRI study reported in the March 16, 2006, issue of "Spine," researchers concluded that the transversus abdominis contracts bilaterally during a "drawing in" of the abdomen, forming a musculofascial band that tightens like a corset, improving lumbopelvic stability.