The muscles of the back are separated into extrinsic and intrinsic components, which are based on their function in movement and embryological origin. The extrinsic muscles include the trapezius, latissimus dorsi, rhomboid major and minor, levator scapulae and the serratus posterior superior and inferior muscles. The principal function of the extrinsic muscles of the back is to move the upper extremity by controlling the movement of the scapula and humerus.
As a group, the extrinsic muscles of the back move the upper extremities and neck. Each muscle is listed below with its specific motor function:
The trapezius muscle works in conjunction with other muscles to move the arm and maintain shoulder stability. When it works with the levator scapulae, it elevates the scapula. If it works with the serratus anterior, it rotates the scapula superiorly allowing for the arm to elevate above the head. Working with the rhomboids, it retracts the scapula to the midline of the back, stabilizing the shoulders as they move. It may also help move the head and neck back and laterally.
When the latissimus dorsi contracts, it works to adduct, extend, and medially rotate the humerus. With the arm is raised above the head, it helps pull the torso up and forward. It also is active when forcible expiration takes place such as sneezing, coughing, or playing a musical instrument.
The levator scapulae muscle works to stabilize the movements of the scapula in conjunction with other muscles. When it works with pectoralis major, it rotates the scapula, depressing the shoulder. It acts with the trapezius to elevate the scapula.
The rhomboids (major and minor) work together to retract the scapula, moving it medially and superiorly to square the shoulders.
The posterior serratus muscles are noted to be accessory breathing muscles; the superior pair help with inspiration, causing expansion of the chest cavity and the inferior pair help with forced expiration; however, definitive evidence to support this claim is lacking.
Embryologically these muscles are derived from the ventral portion of the somite that originates out of paraxial mesenchyme of the primitive streak during primary and secondary gastrulation. As such, the nerve innervation of these muscles is predominantly from the ventral primary rami of the spinal nerves.
The trapezius receives its blood supply from the transverse cervical artery off the thyrocervical trunk; however, each fiber segment gets blood supply from different arterial branches including the dorsal scapular artery, which traverses the trapezius on its way to the rhomboids. The levator scapulae have the same blood supply as the trapezius muscle, the transverse cervical artery.
This latissimus dorsi receives blood from a single artery known as the thoracodorsal artery, which is the continuation of the subscapular artery which branches directly off of axillary artery.
Blood supply to both rhomboid major and minor is most commonly from the dorsal scapular artery; however, sometimes it can get blood from a deep branch off the transverse cervical artery.
The posterior serratus muscles receive blood from the intercostal arteries.
Innervation of the latissimus dorsi is by the thoracodorsal nerve (also known as the middle subscapular nerve). This nerve is one of the branches of the posterior cord of the brachial plexus.
Motor innervation of the levator scapulae comes from branches of the third and fourth cervical spinal nerves and the dorsal scapular nerve.
Innervation of both rhomboid muscles comes from the dorsal scapular nerve of C4 and C5.
The serratus muscles are innervated by the intercostal nerves of the ribs that they bypass.
There are seven extrinsic muscles of the back: the trapezius, the latissimus dorsi, the levator scapulae, the rhomboids (major and minor) and the serratus posterior (superior and inferior):
The trapezius is the most superficial muscle of the back is made up of two triangles that together make the shape of a trapezoid, thus its name. This muscle originates from the center of the spinal column and skull; the fibers connect to the middle portion of the superior nuchal line, the external occipital protuberance, the ligamentum nuchae, and the apices of the spinous processes of C7-T12. The fibers run laterally toward the shoulder where they attach. The superior fibers descend to connect to the posterior border of the distal 1/3 of the clavicle; the middle fibers run horizontally to connect to the superior portion of the scapular spine; the inferior fibers ascend toward the apex of the scapula.
Latissimus dorsi is the largest extrinsic back muscle, spanning across the entire lateral side of the back. It originates from the lateral sides of the spinous processes of T7 – T12, the thoracodorsal fascia, which connects to the spinous processes of the lumbar and sacral vertebrae, the posterior portion of the iliac crest, and proximal 1/3 of ribs 9 – 12. The fibers ascend in a superolateral fashion towards its insertion point, the floor of the bicipital groove of the humerus.
Levator scapulae is a thin muscle that descends from the neck to the scapula. It originates from the transverse processes of C1 (atlas), C2 (axis), C3, and C4. The fibers run diagonally from these points to the medial scapular border just above where the spine of the scapula connects. Origination points of this muscle can vary in the neck and posterior skull.
The rhomboids: minor and major, these two muscles originate from the spines and supraspinous ligaments of the cervical and thoracic vertebrae. Rhomboideus minor is the smaller of the two muscles and located superiorly. It originates from the seventh cervical vertebrae and first thoracic vertebrae and attaches to the medial border of the scapula at the point where the spine connects. Rhomboid major takes its origin from the second to the fifth thoracic vertebrae and attaches just below the rhomboid minor on the medial border of the scapula.
The serratus posterior muscles, separated into superior and inferior are attached to the spinous processes of the vertebrae and the ribs. The superior muscle originates from spinous processes of C7-T3 and attach to ribs two through five. The inferior muscle originates from T11-L1 and attaches to ribs 9-12.
The extrinsic muscles of the back have multiple surgical considerations. Latissimus dorsi has utility as a flap for a multitude of reconstructive surgeries. One of the most studied surgical considerations for latissimus dorsi is flap breast reconstruction post-mastectomy, quadrantectomy, or lumpectomy. The quantity of tissue, reliable vasculature, texture, and autogenous transfer makes the latissimus dorsi a good flap choice for breast reconstruction.
Injury to the accessory nerve for trapezius in greater than 80 percent of cases is unrepairable. Often times that surgery is forgone. However, similar to the latissimus dorsi, the trapezius muscle serves a role in reconstructive surgery. The trapezius muscle is used in flap procedures for lateral craniofacial and lateral basilar skull defects. Lower trapezius musculocutaneous (LTMC) flap based on the dorsal scapular artery has been used to reconstruct head and neck defects in head and neck cancer patients. The posterior trapezius muscle is a premium choice for myocutaneous flaps in scalp repairs due to trauma or malignancy.
In the past, for patients with inadequate fluid intake or when the rapid establishment of IV access was not possible, the subcutaneous tissue and extrinsic muscles of the back were sites for "clysis" hydration, which is the placement of a line directly into the large muscles of the back and rapidly administer isotonic fluid.
Normally, the trapezius muscle works in conjunction with the serratus anterior to rotate the scapula superiorly. This allows for the arm to elevate above the head. Any injury to the long thoracic nerve or serratus anterior muscle can compromise this action. The long thoracic nerve originates from nerve roots C5 to C7. The nerve rest on the outer surface of the serratus anterior. The physical exam of a damaged long thoracic nerve or serratus anterior will show lateral protrusion of the scapula, and medial winging on abduction. 
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