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The muscles are the power sources of the spine. They can be categorized as intrinsic, such as the paravertebral muscles, which are made up of short bundles that act directly on individual spinal segments with short lever arms, and extrinsic, which act via long lever arms and play a role in stabilizing spinal movement.

Figures 6.1 to 6.4 depict the superficial and deep muscles of the back.

PARASPINAL MUSCLES

The paraspinal muscles have been categorized by C. Gillot according to their positions relative to the planes of the transverse processes. He describes them as follows:

Muscles Located Anterior to the Plane of the Transverse Process

Muscles located anterior to the plane of the transverse process are situated along the lateral surface of the verte-bral body and the anterior surface of the transverse pro-cesses. They are well developed in the lumbar and cervical levels and are absent from T4 to T11.

• At the cervicothoracic level are the longus colli and the rectus capitis anterior muscles. These muscles are flexors and accessory rotators. They are com-pleted by the rectus capitis lateralis, a flexor and lateral flexor of the occiput.

• In the lumbar spine, the psoas muscle acts as a flexor of the hip (or of the trunk, depending on whether it acts in an open or closed kinetic chain) as well as an external rotator of the hip.

Figure 6.1 Superficial muscles of the trunk: 1, trapezius; 2, deltoid; 3, latissimus dorsi.

Figure 6.2 Deep muscles of the trunk: 1, serratus posterior superior; 2, iliocostalis; 3, serratus posterior inferior; 4, sacrospinalis.

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Muscles Located between the Transverse Processes

These muscles are situated in the frontal plane as follows:

• In the cervical spine, the scalenus anterior, medius, and posterior. These muscles are ipsilateral latero-flexors and assist in contralateral rotation.

• In the lumbar spine, the quadratus lumborum is a lateral flexor of the trunk, or, when the spine is fixed,

it elevates the ipsilateral hemipelvis toward the spine.

• At both the lumbar and cervical spines are small intertransverse (intertransversaria) muscles that are replaced by ligaments in the thoracic spine. These muscles assist in lateral flexion (Gillot).

• In the thorax, Delmas and Gillot consider the inter-costal muscles as paravertebral muscles.

Figure 6.3 Deep muscles of the trunk: 1, levator scapulae;

2, rhomboideus; 3, serratus anterior; 4, quadratus lumborum.

Figure 6.4 Deep muscles of the trunk: 1, iliocostalis; 2, long-issimus thoracis; 3, multifidus.

Figure 6.5 Muscles of the abdominal wall (seen in section): 1, rectus abdominis; 2, obliquus externus.

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All of the above muscles, with the exception of the intertransverse muscles, are innervated by branches of the anterior primary rami.

Muscles Located Posterior to the Plane of the Transverse Processes

These are the muscles of the paravertebral sulcus.

According to Gillot, they can be categorized as:

• The small deep muscles situated at the craniocervi-cal junction

• The erector spinae muscles within the vertebral sulcus Small Deep Muscles of the Neck Situated at the Craniocervical Junction

There are four small deep muscles situated at the cran-iocervical junction (Fig. 6.7):

• Rectus capitis posterior

• Rectus capitis anterior

• Obliquus capitis superior

• Obliquus capitis inferior

These are the “vernier muscles” of Kapandji that offer fine tuning to the movements of the upper cervical spine.

The obliquus capitis superior is a rotator; the rectus capitis posterior is an extensor and assists in rotation; the rectus capitis major acts as a rotator; the obliquus capitis inferior acts as an extensor and an accessory rotator. These muscles

are innervated by the posterior ramus of Cl. The obliquus superior also receives some fibers from C2.

Erector Spinae Muscles within the Vertebral Sulcus

These muscles extend from C3 to the sacrum, and their descriptions are very complex. They are:

• Multifidus, the deepest layer

• Iliocostalis, located most laterally

Along with the longissimus thoraces, they form a group of spinal muscles whose inferior insertion on the sacrum constitutes a common mass.

In the cervical spine are the splenius and semispinalis muscles. The semispinalis capitis maintains the cervical lordosis and is essential in maintaining the head in the upright position. The splenius, which lies more superficial, is a rotator and acts as an antagonist to the sterno-cleidomastoid.

The right sternocleidomastoid rotates the head to the left, whereas the right splenius rotates it to the right. Both muscles, however, are synergists in lateral flexion to the ipsilateral side. All these muscles are innervated by branches of the dorsal primary rami.

ABDOMINAL MUSCLES

The abdominal muscles play a very important role as flexors of the thoracic and lumbar spine, in maintaining vertebral balance and in reducing strain on the lumbosac-ral spine. They are, latelumbosac-rally, the obliquus externus abdom-inis, the obliquus internus abdomabdom-inis, and transversus abdominis, all located on both sides of the midline.

The rectus abdominis is located on either side of the linea alba. Unilateral contraction of the abdominal muscles

Figure 6.6 Posterior muscles of the neck: 1, splenius; 2, semispinalis capitis; 3, levator scapulae.

Figure 6.7 Muscles of the occipital region. 1, Obliquus capi-tis superior; 2, rectus capicapi-tis posterior minor; 3, rectus capicapi-tis posterior major; 4, obliquus capitis inferior; 5, interspinal lig-ament; 6, supraspinal ligament.

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results in rotation and lateral flexion of the trunk; rotation is contralateral for the external oblique and ipsilateral for the internal oblique.

MUSCLES OF THE NECK AND BACK The trapezius muscle acts as an extensor of the head (Fig. 6.1).

The sternocleidomastoid muscle, when contracted uni-laterally, acts as a powerful lateral flexor of the cervical spine with contralateral rotation. When both sternocleido-mastoid muscles contract simultaneously, they act as flex-ors of the inferior cervical spine.

During cervical rotation, the sternocleidomastoid acts synergistically with the contralateral splenius capitis muscle.

In the upper cervical spine, the obliquus is the principal rotator.

MUSCLES OF THE LUMBAR REGION The lumbar spine is supported by powerful muscles. It looks like a central column surrounded by four thick mus-cular pillars: the two psoas muscles anteriorly and the erector spinae masses posteriorly, form a “composite beam” composed of bone and muscle (Rabischong, Dolto). See Fig. 6.5.

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