Trapezius Muscle.

The trapezius muscle is the most superficial and superior back muscle (see Fig. 4-1). It is a large, strong muscle that is innervated by the accessory nerve (cranial nerve XI). In addition to its innervation from the accessory nerve, the trapezius muscle receives some proprioceptive fibers from the third and fourth cervical ventral rami. Because the trapezius muscle is so large, it originates from and inserts on many structures. This muscle originates from the superior nuchal line, external occipital protuberance, ligamentum nuchae of the posterior neck, spinous processes of C7 to T12, and supraspinous ligament between C7 and T12. It inserts onto the spine of the scapula, acromion, and distal third of the clavicle. Detailed morphometric measurements have been performed on this muscle (Kamibayashi and Richmond, 1998).

Because of its size and the many locations of its attachments, the trapezius muscle also has many actions. Most of these actions result in movement of the neck and scapula (i.e., the “shoulder girdle” as a whole). The function of the trapezius depends on which region of the muscle is contracting (upper, middle, or lower). The middle portion retracts the scapula, whereas the lower portion depresses the scapula and at the same time rotates the scapula so that its lateral angle moves superiorly (i.e., rotates the point of the shoulder up). The actions of the upper part of the trapezius also depend on whether the head or neck or the scapula is stabilized. When moving the head and neck, the actions of the upper trapezius also are determined by whether the muscle is contracting unilaterally or bilaterally. There is some conflicting evidence as to whether or not the upper fibers of the trapezius muscle help in elevation of the scapula. Based upon information gathered from cadaveric dissections of the attachment points and directions of the upper fibers of the trapezius muscle, Johnson et al. (1994) concluded that these fibers do not aid in elevation of the scapula. On the other hand there is good electromyographic evidence that the upper fibers of the trapezius muscle are active during elevation of the shoulder (Campos et al., 1994; Guazzelli et al., 1994). Table 4-1 summarizes the actions of the trapezius muscle.

Table 4-1 Functions of Trapezius Muscle

Latissimus Dorsi Muscle.

The latissimus dorsi muscle is the most inferior and lateral of the two muscles that make up the first layer of back muscles (see Fig. 4-1). This large muscle has an extensive origin. The origin of the latissimus dorsi includes the following:

The latissimus dorsi muscle derives much of its origin from the thoracolumbar fascia. This is a tough and extensive aponeurosis (see the following discussion). The latissimus dorsi muscle passes superiorly and laterally to insert into the floor of the intertubercular groove of the humerus, between the laterally located attachment of the pectoralis major muscle and the medially located attachment of the teres major muscle. Contraction of the latissimus dorsi results in adduction, medial rotation, and extension of the humerus.

The latissimus dorsi is innervated by the thoracodorsal nerve, which is a branch of the posterior cord of the brachial plexus. The thoracodorsal nerve is derived from the anterior primary divisions of the sixth through eighth cervical nerves.

Thoracolumbar (or Lumbodorsal) Fascia.

Because of its clinical significance, the anatomy of the thoracolumbar fascia deserves further discussion. This fascia extends from the thoracic region to the sacrum. It forms a thin covering over the erector spinae muscles in the thoracic region, whereas in the lumbar region the thoracolumbar fascia is strong and is composed of three layers. The posterior layer attaches to the lumbar spinous processes, interspinous ligaments between these processes, and median sacral crest. This layer has its own superficial and deep laminae (Macintosh and Bogduk, 1987a). The middle layer attaches to the tips of the lumbar transverse processes and intertransverse ligaments and extends superiorly from the iliac crest to the twelfth rib. The anterior layer covers the anterior aspect of the quadratus lumborum muscle and attaches to the anterior surfaces of the lumbar transverse processes. Superiorly the anterior layer forms the lateral arcuate ligament (see Diaphragm). The anterior layer continues inferiorly to the ilium and iliolumbar ligament. The posterior and middle layers surround the erector spinae muscles posteriorly and anteriorly, respectively (see Fifth Layer), and meet at the lateral edge of the erector spinae, where these two layers are joined by the anterior layer (Williams et al., 1995). Barker and Briggs (1999) have demonstrated in a cadaveric study that the thoracolumbar fascia may continue more superiorly than classically described. They found that the superficial lamina of the posterior layer ran superiorly to be continuous with the rhomboid muscles, whereas the deep lamina of the posterior layer was continuous superiorly with the splenius muscles. These superior extensions of the fascia were of variable thickness, but seemed thick enough to transmit tension. This means that the thoracolumbar fascia may have a more global influence on biomechanics than thought previously.

The lateral union of the three principal layers of the thoracolumbar fascia serves as a posterior aponeurosis for origin of the transversus abdominis muscle. The direction of the fibers within each lamina of the posterior layer makes the thoracolumbar fascia stronger along its lines of greatest stress. When the thoracolumbar fascia is tractioned laterally by the action of the abdominal muscles, the distinct direction of fibers of the posterior layer’s two laminae aids in extension of the spine and the maintenance of an erect posture.

Some investigators believe that because the posterior and middle layers surround the erector spinae muscles, injury to these muscles at times may lead to a “compartment” type of syndrome within these two layers of the thoracolumbar fascia (Peck et al., 1986). This syndrome results from edema within the erector spinae muscles. The edema stems from injury and increases the pressure in the relatively closed compartment composed of the erector spinae muscles wrapped within the posterior and middle layers of the thoracolumbar fascia. This may result in increased pain and straightening of the lumbar lordosis (Peck et al., 1986). However, further research is necessary to determine the best approach available to diagnose this condition and the frequency with which it occurs.

Rhomboid Major and Minor Muscles.

The rhomboid major muscle originates by tendinous fibers from the spinous processes of T2 through T5 and the supraspinous ligament at those levels. The muscle fibers run in an inferolateral direction and insert via a tendinous band on the medial border of the scapula between the root of the spine and the inferior angle.

The rhomboid minor muscle is located immediately superior to the rhomboid major; its fibers also run in an inferolateral direction. Beginning from the lower portion of the ligamentum nuchae and the spinous processes of C7 and T1, the rhomboid minor ends on the scapula’s medial border at the level of the spinal root.

Both rhomboid muscles are innervated by the dorsal scapular nerve, which arises from the anterior primary division of the C5 spinal nerve.

Because the fibers of these two muscles are parallel, their actions are similar. In addition to the other muscles that insert on the scapula, they help stabilize its position and movement during active use of the upper extremity. Specifically, the rhomboids retract and rotate the scapula such that the lateral angle moves inferiorly (i.e., rotates the point of the shoulder down).

Levator Scapulae Muscle.

The levator scapulae muscle arises by tendinous slips from the transverse processes of the atlas and axis and the posterior tubercles of the transverse processes of C3 and C4. Its fibers descend and insert onto the scapula’s medial border between the spinal root and the superior angle. It is innervated by branches from the ventral rami of the C3 and C4 spinal nerves and the dorsal scapular nerve (C5). If the cervical spine is fixed, the levator scapulae helps in elevating and rotating the scapula such that the lateral angle moves inferiorly (i.e., rotates the point of the shoulder down). When the scapula is stabilized, contraction of this muscle laterally flexes and rotates the neck to the same side. Bilateral contraction helps in extension of the cervical spine.

Serratus Posterior Superior and Serratus Posterior Inferior.

The serratus posterior superior muscle originates from the spinous processes of C7 through T3 and the supraspinous ligament that runs between them. It inserts onto the posterior and superior aspect of the second through the fifth ribs. This muscle is innervated by the anterior primary divisions (ventral rami) of the second through the fifth thoracic nerves (intercostal nerves).

The serratus posterior inferior originates from the spinous processes and intervening supraspinous ligament of T11 to L2. It inserts onto the posterior and inferior surfaces of the lower four ribs and is innervated by the lower three intercostal nerves (T9 to T11) and the subcostal nerve. These nerves are all anterior primary divisions of their respective spinal nerves. The serratus posterior superior and inferior muscles may help with respiration. More specifically, the serratus posterior superior raises the second through fourth ribs, which may aid with inspiration. The serratus posterior inferior lowers the ninth through twelfth ribs, which may help with forced expiration. However, electromyographic evidence refutes a respiratory function for these muscles (Vilensky et al., 2001); they may function primarily in proprioception.

Splenius Capitis and Splenius Cervicis Muscles.

The splenius capitis muscle begins from the lower half of the ligamentum nuchae and the spinous processes of C7 through T3 or T4. It attaches superiorly to the mastoid process of the temporal bone and to the occiput just inferior to the lateral third of the superior nuchal line.

The splenius cervicis muscle originates from the spinous processes of T3 through T6 and inserts onto the transverse processes of the atlas and axis and the posterior tubercles of the transverse processes of C3 and sometimes C4. These insertions are deep to the origins of the levator scapulae. The splenius capitis and cervicis are innervated by lateral branches of the posterior primary divisions (dorsal rami) of the midcervical and lower cervical spinal nerves, respectively. When the splenius muscles of both sides act together, they extend the head and neck. When the muscles of one side contract, they laterally flex the head and neck and slightly rotate the face toward the side of contraction.

Iliocostalis Muscles.

The iliocostalis (iliocostocervicalis) group of muscles is subdivided into lumborum, thoracis, and cervicis muscles (see Fig. 4-3, A and B). Inferiorly the iliocostalis muscles derive from the common origin of the erector spinae muscles.

Longissimus Muscles.

The longissimus muscles are located medial to the iliocostalis group. The longissimus group is made up of thoracis, cervicis, and capitis divisions. The lateral branches of the posterior primary divisions (dorsal rami) of the spinal nerves exit the thorax and then course laterally and posteriorly between the iliocostalis muscles and longissimus thoracis muscle (see Fig. 4-3, A). This fact is used in the gross anatomy laboratory not only to quickly find the lateral branches of the posterior primary divisions, but also to demonstrate the separation between these two large muscle masses. After providing motor and sensory innervation to the sacrospinalis muscle, the lateral branches continue to the skin of the back, providing cutaneous sensory innervation.

Spinalis Muscles.

The spinalis muscle group originates from spinous processes and inserts onto spinous processes, except for the spinalis capitis muscle. This muscle group is made up of thoracis, cervicis, and capitis divisions.