Muscles of the Back

We now consider muscles of the back that extend, rotate, and abduct the vertebral column (figs. 10.17-10.19). Back muscles that act on the pectoral girdle and arm are considered later. The muscles associated with the vertebral column moderate your motion when you bend forward and contract to return the trunk to the erect position. They are classified into two groups—a superficial group, which extends from the vertebrae to the ribs, and a deep group, which connects the vertebrae to each other.

In the superficial group, the prime mover of spinal extension is the erector spinae. You use this muscle to maintain your posture and to stand up straight after bending at the waist. It is divided into three "columns"—the iliocostalis, longissimus, and spinalis. These are complex, multipart muscles with cervical, thoracic, and lumbar portions. Some portions move the head and have already been discussed, while those that act on cervical and lower parts of the vertebral column are described in table 10.7. Most of the lower back (lumbar) muscles are in the longissimus group. Two serratus posterior muscles—one superior and one inferior—overlie the erector spinae and act to move the ribs.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Erector Spinae

Pectoralis minor

Serratus anterior

Rectus sheath -

Internal abdominal -oblique

Internal abdominal -oblique

Rectus Sheath

Subclavius Pectoralis minor (cut)

Internal intercostals

External intercostals Rectus abdominis (cut)

External abdominal oblique (cut)

Internal abdominal oblique (cut)

Transversus abdominis (cut)

Figure 10.15 Thoracic and Abdominal Muscles. (a) Superficial muscles. The left rectus sheath is cut away to expose the rectus abdominis muscle. (b) Deep muscles. On the anatomical right, the external abdominal oblique has been removed to expose the internal abdominal oblique and the pectoralis major has been removed to expose the pectoralis minor. On the anatomical left, the internal abdominal oblique has been cut to expose the transversus abdominis, and the rectus abdominis has been cut to expose the posterior rectus sheath.

Subclavius Pectoralis minor (cut)

Internal intercostals

External intercostals Rectus abdominis (cut)

External abdominal oblique (cut)

Internal abdominal oblique (cut)

Transversus abdominis (cut)

Posterior wall of rectus sheath (rectus abdominis removed)

Figure 10.15 Thoracic and Abdominal Muscles. (a) Superficial muscles. The left rectus sheath is cut away to expose the rectus abdominis muscle. (b) Deep muscles. On the anatomical right, the external abdominal oblique has been removed to expose the internal abdominal oblique and the pectoralis major has been removed to expose the pectoralis minor. On the anatomical left, the internal abdominal oblique has been cut to expose the transversus abdominis, and the rectus abdominis has been cut to expose the posterior rectus sheath.

Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 10 The Muscular System 349

Pectoralis major

Pectoralis major

External abdominal oblique

Tendinous intersection Linea alba

Rectus abdominis

Rectus sheath Internal abdominal oblique

Inferior Oblique Physiology
4

Figure 10.16 Thoracic and Abdominal Muscles of the Cadaver.

The major deep thoracic muscle is the semi-spinalis. This is divided into three parts, the semi-spinalis capitis, which we have already studied (see table 10.4), the semispinalis cervicis,40and semispinalis thoracis,41 in that order from superior to inferior. In the lumbar region, the major deep muscle is the quadratus42 lumborum. The erector spinae and quadratus lumborum are enclosed in a fibrous sheath called the thoracolum-bar fascia, which is the origin of some of the abdominal and lumbar muscles. The multifidus43 muscle deep to

40cervicis = of the neck

41 thoracis = of the thorax

42quadrat = four-sided

43multi = many + fid = split, sectioned this connects the vertebrae to each other from the cervical to the lumbar region and acts to extend and rotate the vertebral column.

Insight 10.2 Clinical Application

Heavy Lifting and Back Injuries

When you are fully bent over forward, as in touching your toes, the erector spinae is fully stretched. Because of the length-tension relationship explained in chapter 11, muscles that are stretched to such extremes cannot contract very effectively. Standing up from such a position is therefore initiated by the hamstring muscles on the back of the thigh and the gluteus maximus of the buttocks. The erector spinae joins in the action when it is partially contracted.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

350 Part Two Support and Movement

Superficial muscles

Deep muscles

Sternocleidomastoid Trapezius

Deltoid

Sternocleidomastoid Trapezius

Deltoid

Latissimus dorsi

Extensor Muscles The Back

Gluteus maximus

Figure 10.17 Neck, Back, and Gluteal Muscles. The most superficial muscles are shown on the left, and the next deeper layer on the right.

Latissimus dorsi

External abdominal oblique

Thoracolumbar fascia

Semispinalis capitis

Splenius capitis Levator scapulae Rhomboideus minor Rhomboideus major Supraspinatus Infraspinatus Teres minor Teres major

Serratus anterior

Serratus posterior inferior

External abdominal oblique

Internal abdominal oblique

Erector spinae Gluteus medius

Gluteus maximus

Figure 10.17 Neck, Back, and Gluteal Muscles. The most superficial muscles are shown on the left, and the next deeper layer on the right.

Standing too suddenly or improperly lifting a heavy weight, however, can strain the erector spinae, cause painful muscle spasms, tear tendons and ligaments of the lower back, and rupture intervertebral discs. The lumbar muscles are adapted for maintaining posture, not for lifting. This is why it is important, in heavy lifting, to kneel and use the powerful extensor muscles of the thighs and buttocks to lift the load.

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Responses

  • hiwet
    What back muscle is deep to the erector spinae?
    6 years ago
  • Luca
    What are oblique muscles?
    6 years ago
  • ermias
    Is the rectus abdominis deep or superficial muscle?
    6 years ago

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