All About Abs 

Anatomy and Physiology of the Abdominal Muscles 

Did you know that there are four muscles that make up a tight tummy? And did you know that those “six-pack abs” are really one muscle called the rectus abdominis, the most recognized abdominal muscle. The transverse abdominis, external and internal obliques constitute the rest of the abdominal wall muscles. It’s important to understand how these muscles work and their functions so you can focus on engaging them during exercise and get the most out of your hard work.  

Rectus Abdominis 
The rectus abominis (rectus = straight, abdom = abdomen) , as its name suggests, is a long, flat, superficial (near the surface) muscle of the abdomen. It starts at the pubic bone and attaches to ribs 5-7 and is responsible for spinal flexion, stabilizing the pelvis during walking, and increasing intraabdominal pressure. In exercise it is used for sit-ups and curls and also for stabilization and posture in all other exercises. To engage the rectus abdominis during exercise, focus on bringing your belly button to your spine as you bring your shoulders off the mat for a crunch.
Transverse Abdominis
The transverse abdominis is the deepest muscle of the abdominal wall located beneath the rectus abdominis and the oblique muscles, and it has horizontally running fibers. The transverse abdominis originates at the inguinal ligament (as seen in the picture above), the lumbar fascia (running along spine), the last six ribs and the iliac crest (frontal hip bones) and it inserts in the linea alba (see above) and pubic crest. It is the muscle responsible for “drawing in” the abdominal wall. (This is the muscle used to “suck in your gut”) It functions in abdominal compression, vigorous exhalation and expulsion. The transverse abdominis is our built in “Spanx” or girdle. Exercises emphasizing the transverse abdominis are reverse curls, planks and other core stability exercises with little to no spinal flexion. To engage the transverse abdominis, think about the way you compress your tummy to fit into those jeans that accidentally got shoved in the dryer.
External and Internal Obliques
The external oblique is made of superficial diagonally running fibers (oblique = running at an angle). The fibers run at the same angle as your fingers do when they are put into your pants pockets. The external obliques originate on the outer surfaces of the lower 8 ribs and insert into the linea alba and pubic crest. The external obliques (like the internal obliques) serve in spinal flexion and rotation and are used in oblique curls and bicycles.
The internal oblique, like the external oblique, has diagonally running fibers, but in the opposite direction (running perpendicular to external oblique). The internal oblique is a deep muscle (farther from the surface of the body) and originates in the lumbar facia (near spine). It inserts in the linea alba, pubic crest and the last 3-4 ribs. It has the same function as the external obliques and is exercised the same way.
To engage the oblique muscles try sitting in a chair with arms with your forearms parallel to the arms of the chair. Now alternate pushing your forearm to the arm of the chair. The contraction you feel is your oblique muscles. You can also engage your obliques by sitting straight up in a chair and turning your torso from side to side.
Remember your abdominal muscles are necessary not just for a flat tummy but also for the stabilization of your body during all exercises, so they need to be strong!
Hoehn, Katja and Marieb, Elaine. “The Muscular System.” Human Anatomy and Physiology. Eighth edition, 2010. p. 342-343.
Yoke, Mary, MA. “Anatomy and Kinesiology.”  Personal Fitness Training and Practice. First edition, 2006. p. 56-57.