The anatomy of the back refers to the muscles of the back, as well as the bones of the scapulae, ribcage, and spine. Covering an expanse from the neck to the tailbone, the back muscles are responsible for a broad range of functions, from extending the spine to shrugging the shoulders. These muscles facilitate movement by attaching to one or more bones of the back, either to the spinous processes of the vertebrae, the ribs, or the shoulder blades. Many also attach to the posterior pelvis. A discussion of the anatomy of the back may also make mention of the organs in proximity to the back, such as the kidneys and lungs, but as the key structures found here relate to movement and weight-bearing functions, references to anatomy of the back tend to focus on the bones and muscles.
As the spine is what determines the shape of the back, as well as the structure that gives rise to a large number of the back’s muscles, it is perhaps the most important component of the anatomy of the back. Extending from the base of the skull into the pelvis, it consists of 33 stacked bones known as vertebrae. These include the cervical vertebrae in the neck, the thoracic vertebrae of the ribcage in the upper and middle back, the lumbar vertebrae in the lower back, and the vertebrae that are part of the pelvis. The body of each vertebra is separated from the ones above and below by a shock-absorbing intervertebral disc, and each vertebra features several bony protrusions known as spinous processes projecting backward and to either side from the body. It is to these processes that the muscles that move the spine attach.
Other bones included in the anatomy of the back are the 12 ribs and the paired shoulder blades, or scapulae. The ribs attach to the 12 thoracic vertebrae, each of which features a rounded facet on either side of the vertebral body for articulation with an individual rib. While the ribcage as a whole functions to protect the organs of the thoracic cavity and facilitate breathing by the action of the thoracic diaphragm muscle, it also serves as a site of attachment for many of the muscles of the erector spinae group, which extend, rotate, and laterally flex or side bend the spine. Similarly, though the shoulder blades are considered part of the shoulder girdle and therefore are closely linked to the movements of the arms of the shoulder joint, many of these movements are initiated by muscles of the back, muscles that attach directly to the scapulae like the trapezius and the rotator cuff group.
The muscles accounted for in the anatomy of the back are many, but they can be classified and located according to their function. Those found predominantly in the lower back, where they act primarily to extend or straighten the spine, are the muscles of the erector spinae group: the iliocostalis, longissimus, and spinalis muscles. These muscles extend all the way up the spine, where they may also contribute to movements like rotation and lateral flexion in the thoracic and cervical regions. The large muscles of the middle back like the latissimus dorsi and lower trapezius tend to pull downward on the arms and scapulae, while those found between the scapulae like the rhomboids and middle trapezius pull the shoulder blades back and together in a motion known as retraction. In the upper back, muscles like the upper trapezius and rotator cuff muscles function mostly to elevate or stabilize the shoulder girdle or arms, or to rotate the arms in the shoulder joint.