The transversus abdominis, also known as the transverse abdominis, is the deepest of the four muscles collectively referred to as the abdominals. Wrapping the midsection like a corset, this muscle is not visible like the “six-pack” of the rectus abdominis muscle, nor as well known. It is, however, essential both to maintaining upright posture and to breathing properly and therefore is significant to the function of the core.
Found beneath the rectus abdominis, external obliques, and internal obliques, respectively, the transversus abdominis is broad and flat with fibers running horizontally. Spanning the abdomen from the ribcage to the hipbones, the uppermost fibers of this muscle originate on the ribs, the intermediate fibers on the sides of the abdomen, and the bottom-most fibers on the hip bones. Specifically, the transversus abdominis finds its origin along the inguinal ligament, which runs diagonally across the hip from the pubic bone up and out to the anterior superior iliac spine on the top front part of the hipbone. It also arises from the inside of the iliac crest, the bony prominence felt on the front of the hip, from the insides of the bottom six ribs, and from the lumbodorsal fascia, connective tissue wrapping the sides of the waist.
The fibers of this muscle insert along a broad layer of connective tissue running down the middle of the abdomen known as an aponeurosis. From the sternum or breastbone in the chest to the pubic bone, the aponeurosis lies to either side of the linea alba, the midline of the abdominals that is visible in a “six-pack.” Its fibers also run horizontally, bordered medially, or to the inside, by the linea alba, and laterally, or to the outside, by the fibers of the transversus abdominis.
Unlike the rectus abdominis and the obliques, whose job is to move the torso either by flexion, or bending forward or sideways, or by rotation, or twisting, the transversus abdominis is responsible for compression and stabilization. Compression is the action of drawing the muscle inward, applying pressure to the ribs and internal organs, as a means of stabilizing the core and pelvic region. Simply put, this helps to reduce downward pressure on the spine and strain on the low back, particularly during heavy lifting movements, as well as helps to maintain erect posture. The action of this muscle also assists pregnant women in pushing during labor and delivery.
Another important action of the transversus abdominis is its role in breathing. The diaphragm, an umbrella-shaped muscle filling the bottom of the ribcage under the lungs, makes inhalation possible. When it contracts, it reduces pressure on the thoracic cavity and allows the lungs to expand, drawing air in. Conversely, the abdominals, particularly the transversus abdominis, assist in exhalation by placing pressure on the thoracic cavity, opposing the action of the diaphragm with every breath. This relationship of opposing actions between the two muscle groups is known in anatomy as antagonistic.