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Your Abdominal Muscles: More Than Just A Six-Pack

Abdominal Muscles

When we say the word abdominals, people often think of the superficial layer of muscles that those folks in swimsuit ads are always showing off. But the muscles that surround our abdomen are far more than that. They are essential for structural support and healthy breathing. In this article, we’ll take a closer look at which muscles make up the abdominal group, where they’re located, and what they do.

What muscles make up the abdominal group?

From superficial to deep, the four main muscles that are usually considered the abdominal group include:

  • Rectus abdominis 
  • External oblique
  • Internal oblique
  • Transverse abdominis

(There is also a small muscle called pyramidalis which is sometimes considered an abdominal muscle, but which we don’t cover in this article.)

Muscle attachments

These four muscles are arranged in interwoven layers of muscle tissue and associated fascia. They all span broad areas and for that reason have many points of attachment. It’s important to remember when talking about muscle attachments that humans are variable! We describe the common attachments for each of the four abdominal muscles, but there is some variability from person to person.

Rectus abdominis

Rectus abdominis is the most superficial layer, however, at its inferior attachment, it actually weaves through the transverse abdominis to end up as the deepest layer of tissue at that point. Rectus abdominis attaches at its superior end to the xiphoid process and on the costal cartilage of the 5th, 6th, and 7th ribs. Its fibers run vertically on either side of the fascia that divides the sections of this muscle, called the linea alba. At its inferior end, rectus abdominis attaches to the crest of the pubic bone and to the cartilaginous joint where the two sides of the pubis meet called the pubic symphysis.

External oblique

The next layer underneath the rectus abdominis, are the two sides of the external oblique. It attaches at the superior end on the external surface of the 5th through the 12th rib. Then, on both our right and left sides, the fibers of this muscle run at an oblique angle (hence the name) to insert into the linea alba and attach onto the anterior iliac crest and pubic tubercle.

Internal oblique

Deep to the external oblique muscles are the internal oblique muscles. Like the external obliques, we have a right and left side of these muscles. Both sides attach at the superior end onto the lower three or four ribs, the linea alba, and the pubic crest. Both sides of the internal obliques then run at an oblique angle to attach at the inferior end onto the anterior iliac crest, the lateral portion of the inguinal ligament, and the thoracolumbar fascia.

Transverse abdominis

The transverse abdominis is the deepest layer of musculature that is part of the abdominal group. The fibers of transverse abdominis run roughly horizontally as its name suggests. It wraps around the lower part of our abdomen from the thoracolumbar fascia to the linea alba. More specifically, it attaches to the internal surface of the costal cartilage of ribs 7-12, the anterior iliac crest, the lateral portion of the inguinal ligament, the thoracolumbar fascia, the linea alba, and the pubic crest. 

Muscle actions

Between the four layers of abdominal muscles, we have muscle fibers wrapping around our abdomen going in every direction. This arrangement helps support the main actions of all four of the abdominals. All four muscles work to compress the abdominal contents. The actions of the whole abdominal group are also essential for stabilizing the trunk and spine in both static posture and movement. In particular, research indicates the internal obliques, external obliques, and transverse abdominis are more active in standing and sitting postures, but are less active in a supine, lying-down position (Mesquita Montes et al, 2016; Snijders et al., 1995; De Troyer, 1983).

Each of the individual muscles also has additional actions that they contribute to. The muscle fiber direction of each of the specific layers of abdominals defines their additional actions. Rectus abdominis has fibers that run from superior to inferior vertically. That makes it well-placed to flex the spine

The external obliques, internal obliques, and transverse abdominis each have two sets of fibers, one on each side of the abdominal aponeurosis. For that reason, they have actions that they do unilaterally (on one side) and bilaterally (both sides working together). Bilaterally, the external and internal obliques work with the rectus abdominis to flex the spine. The external and internal obliques both unilaterally do lateral flexion on the same side of the body as the muscles that are contracting. However, because of their fiber direction, when the external obliques unilaterally contract, they rotate the body in the opposite direction. When the internal obliques unilaterally contract, they rotate the body in the same direction as the muscles that are contracting. 

Finally, the abdominal muscles are an important part of breathing. All four abdominal muscles contribute to forced expiration. That’s a more technical way to refer to actions like coughing. These muscles are sometimes considered “secondary respiratory muscles” because they work in addition to our main muscle of respiration, the diaphragm. They’re particularly important when we increase the stress on our breathing system, like when we’re exercising. A recent study confirmed that the external obliques and rectus abdominis are active during our exhale generally, but the activity of both of those muscles increased as the intensity of exercise increased during the study (Abraham et al., 2002). 

Are abdominal muscles part of “the core”?

How are the abdominal muscles related to the idea of “the core”? There is no official definition of the term “the core.” What is true is that none of the abdominal muscles cross or act directly on the hip joint. They all attach to some part of the pelvis, so the tension in these muscles is part of the relationship between the position of the ribcage and the position of the pelvis. But, they don’t connect the legs to the torso. So while the abdominal muscles certainly contribute to stabilizing the spinal column, their primary actions affect the spine and the abdominal contents.

Conclusion

The abdominal muscles are a unique group with multiple functions. They cover a large portion of our torso. They fill in much of the space between the top of our pelvis and the bottom of our ribcage. Their location and interwoven arrangement make them an essential part of both stabilizing and moving our torso. They are also important for healthy breathing.

References

Abraham K.A., H. Feingold, D.D. Fuller, M. Jenkins, J.H. Mateika, R.F. Fregosi. 2002. Respiratory-related activation of human abdominal muscles during exercise. J Physiol. 541(Pt 2):653-63.

Mesquita Montes A., J. Baptista, C. Crasto, C.A. de Melo, R. Santos, J.P. Vilas-Boas. 2016 Abdominal muscle activity during breathing with and without inspiratory and expiratory loads in healthy subjects. J Electromyogr Kinesiol. 30:143-50.

Snijders, C.J., M.P. Bakker, A. Vleeming, R. Stoeckart, H.J. Stam. 1995. Oblique abdominal muscle activity in standing and in sitting on hard and soft seats. Clinical Biomechanics. 10(2):73-78.

De Troyer, A. 1983. Mechanical role of the abdominal muscles in relation to posture. Respir Physiol. 53(3):341-53.

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