This is an excerpt from Dynamic Alignment Through Imagery - 2nd Edition by Eric Franklin.
Musculature of the Abdomen and Back
Often muscles seem to be running in discrete lines and directions, with insertions and origins being discernable. In reality, there are more interconnections between muscles through the fascial network than commonly depicted. Not only are muscles interrelated but also the layering of muscles down to the sarcomeres are in a complex fascial dialogue. When you observe the muscles of the back, organizational principles are not apparent at first. It is therefore best to understand these muscles as patterns and relationships.
The long erector spinae muscles at the back of the torso—spinalis, longissimus, and iliocostalis (from the center out)—lie in three strands parallel to the vertebral column (figure 12.37). These muscles are involved in dynamic stabilization and movement of the spine, torso, and head as well as breathing. The lowest part arises from a strong aponeurosis that is attached to the sacrum and iliac crest. From here, discrete bands run up to the ribs (iliocostalis), transverse processes and ribs (longissimus), and spinous processes (spinalis). The longissimus is the most developed of the three and the iliocostalis the most lateral. All of these muscles can extend the spine, but the more lateral strand also can flex the spine to the side.
Below the erector muscles, several groups of short muscles lie deep within the gutter next to the spinous processes. These so-called transversospinalis muscles are the multifidi, semispinalis, and rotators. They all run from the transverse processes to spinous processes but at different angles. The most oblique are the rotators because they cross only one or two vertebrae. The multifidi cross two to four vertebrae and are therefore more angled upward. The semispinalis is the most oblique of all, crossing four to six vertebrae. Together, they look like layers of triangles with ever-larger and longer sides and narrower peaks. The semispinalis is the largest and most medial of the lumbar back muscles. It has wide attachments at the back of the pelvis and lower spine, such as the lumbosacral ligaments, posterior surface of the sacrum, posterior iliac spine, and the even sacrotuberous ligament. The deep fibers of the multifidi are in the perfect position to control shear and torsion and seem to have differing neural control from the superficial part. The deep layers play an important role in the dynamic stabilization of the low back and contract in anticipation of movement. The superficial layer is more able to extend the low back and control lordosis (Moseley, Hodges, and Gandevia 2002).