3.3.5 Posterior abdominal muscles 1: erector spinae, quadratus
Now we’ll move on to look at the muscles that make up the wall that surrounds the abdominal cavity. We’ll begin with the muscles which, along with the vertebral column and the diaphragm, form the posterior part of the abdominal wall. We’ll look at the erector spinae muscles and their surrounding fascia, then at quadratus lumborum, then at psoas major and iliacus.
We're looking from behind. Here’s the iliac crest; here’s the twelfth rib; here’s the mid-line. Here are the muscles known collectively as the erector spinae. We looked at them in the first section of this tape. They arise from the sacrum and from this part of the iliac crest. They’re inserted on the thoracic vertebrae, and on the the backs of the ribs.
The erector spinae muscles are enclosed on the front and on the back by this envelope of tendinous tissue called the thoraco-lumbar fascia. The layer on the back of the erector spinae arises from the spinous processes; the layer on the front arises from the transverse processes.
The two layers of thoraco-lumbar fascia fuse into one thick layer along the border of erector spinae. The thoraco lumbar fascia is an important attachment for muscles of the abdominal wall, as we’ll see shortly.
Next we’ll add quadratus lumborum to the picture. Here’s quadratus lumborum. It lies in front of the erector spinae muscles and their investing fascia.
Quadratus lumborum arises from the twelfth rib, and the transverse processes of the upper three lumbar vertebrae. Its inserted on the most medial part of the iliac crest, and the transverse process of L5. Quadratus lumborum assists in producing lateral flexion of the lumbar spine.