Human back

The human back, or dorsum, is the large rear surface of the body, extending from the top of the buttocks up to the back of the neck. It sits opposite the chest and abdomen, with the vertebral column running down its center and forming a shallow groove. The back’s width is defined by the shoulders at the top and the pelvis at the bottom. Back pain is a frequent medical issue and is usually not serious.

The backbone is the back’s central structure, specifically the section from the top of the thoracic vertebrae to the bottom of the lumbar vertebrae. This column houses the spinal cord in its spinal canal and typically has curves that shape the back. The ribcage attaches to the spine at the upper back, starting at the T1 vertebra, and extends more than halfway down, leaving a less protected area between the lower ribs and the hips. The upper back’s width is set by the scapulae, the broad, flat shoulder bones.

Back muscles fall into three groups: superficial, intermediate, and deep. The superficial group, also called the appendicular group, moves the limbs and includes the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae. These muscles are supplied by the anterior rami of spinal nerves, reflecting their origin outside the back. The intermediate group, or respiratory group, may aid breathing and consists of the serratus posterior superior and serratus posterior inferior. It is also innervated by anterior rami. The deep group, or intrinsic group, originates in the back and has four subdivisions: spinotransversales (splenius capitis and splenius cervicis), erector spinae (iliocostalis, longissimus, and spinalis), transversospinales (semispinalis, multifidus, and rotatores), and segmental muscles (levatores costarum, interspinales, and intertransversarii). The deep group is supplied by the posterior rami of spinal nerves.

Near the back, the lungs lie within the ribcage and extend to its rear, allowing them to be listened to through the back. The kidneys sit below the muscles just under the end of the ribcage, loosely attached to the peritoneum. A blow to the lower back can injure them.

The skin on the back is thicker and has fewer nerve endings than elsewhere on the torso. It usually has less hair than a man’s chest, with some exceptions. The upper-middle back is the one area a typical person cannot normally reach to touch. This skin is innervated by the dorsal cutaneous branches and the lateral abdominal cutaneous branches of the intercostal nerves.

The back’s complex structure supports the head and trunk, provides trunk strength, and allows flexibility and movement. The upper back, with ribs firmly attached to each thoracic vertebra, has very limited movement and the most structural support. The lower back, or lumbar vertebrae, allows bending backward (extension) and forward (flexion) but does not permit twisting.

The back contains interconnecting nerves, bones, muscles, ligaments, and tendons, all potential sources of pain. Back pain is the second most common pain in adults, after headaches. Muscle strain is by far the most frequent cause; back muscles usually heal within a few weeks, though the pain can be severe. Other common causes include disc problems like degenerative disc disease or a lumbar disc herniation, fractures such as spondylolisthesis or osteoporotic fractures, and osteoarthritis.

The curve of a woman’s back often appears in paintings, as many cultures allow the back to be shown nude—implying full nudity without displaying it. Exposing the lower back has been done for centuries. Clothing like halter tops and backless dresses is designed to reveal the back. Many women’s shirts, even conservative ones, expose the lower back because the lower waist is a pivot point that lengthens and arches when sitting or bending, and women’s fashion often favors waist-length tops that leave the back bare during movement. The back also serves as the largest canvas for body art on the human body. Its size and relative lack of hair make it ideal for tattoos, especially lower back tattoos among young women. Some people have tattoos covering the entire back, while others have smaller ones at places like the shoulder blade or lower back. The part of the back that cannot be reached to scratch is sometimes called acnestis. An itch there can be irritating, leading to the use of backscratchers. Many English idioms refer to the back.