The muscle indicated is the iliocostalis lumborum muscle, one of the erector spinae group of back muscles.

The back muscles can be divided into three groups of muscles:

• Superficial
• Intermediate
• Deep group

The erector spinae muscles belong to the deep (intrinsic) group of back muscles and are the largest group of intrinsic back muscles. There are three main erector spinae muscles, from lateral to medial:

• Iliocostalis
• Longissimus
• Spinalis

Each of these three muscles is further subdivided into separate parts:

• Lumborum
• Thoracis
• Cervicis
• Capitis

Origin: sacrum, iliac crest, spinous processes of lumbar vertebrae and thoracic vertebrae T11 and T12

Insertion: Inferior borders of lower 6/7 ribs

Innervation: Posterior branch of spinal nerve

Action: Unilateral contraction – flexes head to same side. Bilateral contraction – extension of vertebral column

Learn all about the anatomy of the superficial back muscles and the intermediate and deep back muscles.