This is an excerpt from Core Assessment and Training by Human Kinetics.
When someone says “abs,” the first thing that comes to mind for many people is the six-pack. For many, including fitness clients, the abdomen has been marginalized to include just one muscle—the rectus abdominis. However, the abdominal region is composed of several key muscles that contribute to core function. The abdomen is the region lying between the proximal chest and the distal pelvis. This region is served by several muscles that contribute to spine stability in a variety of postures, providing the ability to flex, side bend, and rotate the trunk. These muscles also serve to protect the abdominal organs. Four muscles provide shape and movement to the anterior abdominal wall (figure 2.9). Three of these muscles are described as flat muscles (the obliques and the transversus abdominis), and one is described as being straplike (the rectus abdominis).
The rectus abdominis (RA)—the muscle made famous in movies and television—provides both core stability and trunk mobility (figure 2.9). The RA is a trunk flexor. This muscle arises from the xiphoid process and adjacent costal cartilages, and it attaches distally into the pubic bone at the crest and symphysis. The RA muscle is trained when an individual performs an exercise such as the crunch.
The transversus abdominis (TA) is the deepest of the three flat abdominal muscles. The TA originates from the lower six costal cartilages, the thoracolumbar fascia, and the iliac crest; this muscle attaches medially at the linea alba (figure 2.9). The TA is reported to play a significant role in core stabilization, especially during rehabilitation (Richardson et al. 1999).
The external and internal oblique muscles rotate and side bend the trunk. These muscles also contribute to spinal stability.
The external oblique is the most superficial muscle of the three flat abdominal muscles (the external oblique, internal oblique, and transversus abdominis). The external oblique arises from the front lateral portion of the lower seven ribs, and it inserts into the linea alba, the pubic tubercle, and the anterior portion of the iliac crest (figure 2.9). Acting alone, the external oblique can flex the trunk, side bend the torso toward the same side (i.e., the side of the contracting muscle), and rotate the trunk toward the opposite side.
The internal oblique originates from the thoracolumbar fascia, the inguinal ligament, and the anterior iliac crest. The internal oblique also functions to provide spine stability, and it flexes and rotates the trunk toward the same side (table 2.4).