An Anatomical Study of Our Abdominal Muscles

By Elizabeth A. Murray, Mount St. Joseph University

The abdominal wall muscles or abs—comprise of the external oblique, internal oblique, transversus abdominis, and rectus abdominis. Anatomically, and developmentally, these are a continuation of the pattern seen in the intercostal muscles. In fact, the fiber directions of the external and internal oblique muscles in the abdominal wall match those of external and internal intercostals.

Origin from the Intercostals

Like the transversus thoracis of the innermost layer of the chest wall, the deepest layer in the abdominal wall is the transversus abdominis. It’s as if the pattern continues—we just run out of ribs. But in places where they both occur, the abdominal wall muscles superficially overlap the intercostals.

The external oblique muscles originate on the lower eight ribs, and like the external intercostals, they also have their fibers down and in, like hands in pockets. The internal oblique muscles have their fibers perpendicular, or down and out, you might say, similar to the internal intercostals.

The internal oblique and transversus abdominis both originate on the thoracolumbar fascia of the back—the hip bones—and from an inferior structure called the inguinal ligament, which can be thought of as the rolled hem of the abdominal wall muscles.

The transverse abdominis also takes part of its origin off the costal cartilages of the lower ribs, and as its name implies, it has muscles fibers in a transverse direction, like a belt at the waist.

This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.

Linea Alba

We have no anterior bones from the sternum to the pubic bone, so the external oblique, internal oblique, and transversus abdominis muscles all insert on their companion muscle of the other side. This reinforced midline is called the linea alba—Latin for the “white line”.

These three more laterally placed abdominal wall muscles reach that midline linea alba by large, flat tendons called aponeuroses; they only have muscle fibers laterally in the body wall.

Rectus Abdominis

The final muscle of the anterolateral abdominal wall, the rectus abdominis, is actually encased in the aponeuroses of the other three paired abdominal muscles, forming the so-called rectus sheath around the rectus abdominis muscle.

The fibers of the rectus abdominis are vertically oriented, and this rectus sheath aids the rectus abdominis in its key role of flexing the spine, as in a forward bend. This is partly because the anterior part of the rectus sheath—made of the aponeuroses of the other abdominal muscles—is attached to the rectus abdominis muscle by a series of transverse bands of connective tissue.

The Eight-packs

These are called the tendinous intersections of the rectus abdominis muscle and are what give well-muscled people that so-called six-pack look to their anterior abdomen. Most people have three or four of these tendinous intersections in each rectus muscle.

So, if anything, it’s really an eight-pack or a 10-pack. The fitness buffs know that exercises like sit-ups, crunches, and side-bends—with or without the aid of a weight machine—can strengthen the muscles of the abdominal wall to get that six-pack.

Functions of the Abdominal Muscles

In addition to flexing the spine, the rectus abdominis shares one function with the other three abdominal wall muscles, and that is compression of the abdomen. This action aids in childbirth, defecation, or forced exhalation, since compressing the abdomen can force the abdominal cavity contents inferiorly or push the diaphragm more superiorly into the thorax.

But compression of the abdominal wall is the only function that all four abdominal muscles share, and it is the only function of the transversus abdominis, the deepest of these.

The External and Internal Oblique

Due to its orientation and midline insertion, the external oblique can also cause contralateral rotation of the spine. This means when the left muscle is used, it turns the right side of the torso, including the right shoulder, towards the midline. On the other hand, the internal oblique engages in ipsilateral rotation, meaning you use the right one to turn the right side of the body towards the midline.

Looking at the fibers of the left external oblique, we can compare them to the fiber direction of the right internal oblique. The directionality is the same, so when we turn our right shoulder to the midline, we are using our left external oblique along with our right internal oblique.

Direction of Muscle Fibers Is Important

It needs to be remembered that the direction of a muscle’s fibers—its grain—tells you a lot about its function—and the left external oblique’s direction matches that of the right internal oblique.

And what about the innervation of these abdominal wall muscles? They are primarily supplied by intercostal nerves, the ventral rami of thoracic nerves. As we get to the most inferior reaches of the abdominal wall musculature, down by the inguinal ligament, the first lumbar nerve also gets involved.

Common Questions about Our Abdominal Muscles

Keep Reading
The Different Types of Muscle Tissues
The Role of the Nervous System in Muscle Movement
Skeletal Muscles: How the Structure Determines Function