True back muscles receive their motor innervation from the dorsal rami of appropriate spinal nerves. They originate on the hip bones and the inferior end of the vertebral column and insert more superiorly on the vertebral column, ribs and skull. They extend the vertebral column and bend it side to side.

The study of the back involves several muscles that affect the upper extremities. These muscles are found on the back, however they are not "True Back Muscles". They include the latissimus dorsi, rhomboids, trapezius and levator scapulae. During embryonic development, these muscles migrated from the anterior and lateral body wall. All but the trapezius have ventral rami motor innervation. The trapezius is innervated by cranial nerve XI. These muscles will be studied in Exercise 10.

The back muscles and vertebral column form a strong upright support for the skull, ribs and upper extremities. They occupy superficial, intermediate and deep layers.

Figure 1-10
Superficial muscles of the back.

Figure 1-11
Intermediate and deep back muscles. Inset shows the deep transversospinalis muscle group.

The splenius capitis and splenius cervicis are superficial back muscles in the cervical area. (Figure 1-10)

The intermediate muscle layer, the sacrospinalis (erector spinae), is a long vertically running bilateral muscle bundle on each side of the vertebral column. Ascending from the sacrum and ilium, this muscle splits into three columns: (Figure 1-11)

• Lateral: iliocostalis originating on the ilium and inserting on the ribs.
• Intermediate: longissimus originating on the ilium and inserting on vertebrae, transverse process to transverse process.
• Medial: spinalis originating on the ilium and inserting on vertebrae, spinous process to spinous process.

The deepest group of back muscles are the transversospinalis muscles: (Figure 1-11)

• rotatores: originates on the transverse process of one vertebra and inserts at the base of the spinous process of the vertebra above.
• multifidus: originates on the transverse process of one vertebra and inserts on a distant superior spinous process of another vertebra.
• semispinalis: originates on the transverse process of one vertebra and inserts on yet a more distant superior spinous process.

The fibers of these muscles take on an oblique direction and sometimes are called the deep oblique back muscles. (Figure 1-11, inset; Figure 1-24)

All back muscles extend, laterally flex or rotate the vertebral column. They are also critically important in stabilizing the column.