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.[1]

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.[12]  

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.[13]

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. 

Very few physiological variants occur in this muscle group. Most of the time these variations will occur in the origin and insertion points of the muscle fibers listed above.[14][10]

The extrinsic muscles of the back have multiple surgical considerations. Latissimus dorsi has utility as a flap for a multitude of reconstructive surgeries.[15] 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.[16]

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.[17] 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.[18] The posterior trapezius muscle is a premium choice for myocutaneous flaps in scalp repairs due to trauma or malignancy.[19] 

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.[20]

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. [21][22]