Anatomy of Abdominal Muscles

The deepest layer of abdominal muscles is called the "transversus abdominis." The transverse abdominal muscle wraps around the torso from front to back and from the ribs to the pelvis. The transverse abdominis runs horizontally, in the transverse plane of the body (trans = across). A simple way to think of it is that it acts like a belt, drawing your entire core in more tightly to your vertical midline. And that's exactly what you want in order to give your waist a narrow appearance.

The internal oblique muscles are a pair of deep muscles that are just below the external oblique muscles. The internal and external obliques are at right angles to each other.

The internal obliques attach from the lower three ribs to the linea alba and from the the inguinal ligament to the iliac crest and then to the the lower back (erector spinae). The lower muscle fibers of the internal obliques run nearly horizontally.

Along with the external obliques, the internal obliques are involved in flexing the spinal column, sideways bending, trunk rotation and compressing the abdomen.

Because of their unique alignment, at right angles to each other, the internal and external obliques are referred to as opposite-side rotators. When the trunk rotates left, the external obliques (on the right) contract. When the trunk rotates to the right, the external oblique fibers (on the left) activate the movement.

External Obliques muscles are located on each side of the rectus abdominis. The muscle fibers of the external obliques run diagonally downward and inward from the lower ribs to the pelvis, forming the letter V. You can locate them by putting your hands in your coat pocket.
The external obliques originate at the fifth to twelfth ribs and insert into the iliac crest, the inguinal ligament, and the linea alba of the rectus abdominis.

The external oblique muscles allows flexion of the spine, rotation of the torso, sideways bending and compression of the abdomen.

The rectus abdominis muscle is a paired muscle running vertically on each side of the anterior wall of the human abdomen (and in some other animals). There are two parallel muscles, separated by a midline band of connective tissue called the linea alba (white line).

The rectus abdominis is an important postural muscle. It is responsible for flexing the lumbar spine, as when doing a "crunch". The rectus abdominis assists with breathing and plays an important role in respiration in the event the patient is short of breath.