Aesthetically, the abs are seen as one of the most desirable muscles to possess. To the majority of people, abs consist of only the “6 pack,” however, this is only one part of the abdominals. They encompass far more important functions than many people give them credit for including being largely responsible for stabilization of the spine, which has obvious applications to daily life as well as athletic activities.
There are four muscles that make up the abdominal muscle group: the rectus abdominis, the external oblique, the internal oblique, and the transverse abdominis.
Commonly referred to as the “six pack,” the rectus abdominis is the muscle most people think of when they hear the word abs.
The rectus abdominis is one sheet of muscle that originates from the front lower part of the pelvis (just above the penis for males and the vulva for females) and inserts into the costal (middle) cartilage of ribs 5-7.
The lines in the rectus abdominis that form the '6-pack look' are actually tendons.
The rectus abdominis, as well as the tendons that traverse the rectus abdominis, make bending over (spinal flexion) possible.
The transverse abdominis originates at the pelvis and the lower 6 ribs, and inserts into the linea alba (the group of tendons that separate the left and right side of the 6 pack) and the pubic crest. The transverse abdominis sits underneath the other abdominal muscles.
The transverse abdominis is responsible for: hollowing of the abdomen (sucking in your stomach), support of the abdominal wall, raising intra-abdominal pressure (the creation of pressure within the abdominal wall), and stabilization of the spine and pelvis.
The transverse abdominis has applications to basic everyday functions like standing and walking, but also has applications to rigorous physical activity like heavy weight lifting.
The External Oblique
The external oblique is visible at low body fat percentages. The muscle appears as a series of diagonal lines connecting to the six-pack.
The external oblique originates from the 5th – 8th ribs, and insert at several points (linea alba, iliac spine, pubic crest, and the iliac crest).
The functions of the external oblique are to aid in rotation (chopping down a tree), flexion (bending over), and lateral flexion of the spine (bending to the side).
The Internal Oblique
The internal oblique sits between the transverse abdominis and the external oblique. The origin is from the iliac crest, and the lower part of the pelvis. The insertion is at the linea alba, pubic crest, and the lower 3-4 ribs.
The internal oblique serves the exact same functions as the external oblique – aiding rotation, flexion and lateral flexion of the spine.