In human anatomy, the loin refers to an area extending vertically from just below the waist, or more specifically from the bottom of the ribcage, to just below the pelvis. The term is somewhat inexact, and is most often used to describe the sides of the body in the area below the ribs, sometimes also called the flanks, and the parts of the lower back on either side of the spine. Sometimes the term also includes the genitals, the groin, the hips, and the lower abdomen. Loin is used to describe this part of the anatomy in humans, primates, and many species of four-legged mammals, including cattle and horses. The word loins is sometimes used as a euphemism for both the male and female reproductive organs, and the original Latin word lumbus, from which loin originated, referred both to the general loin area and the genitals.
Loin is not a term commonly used by doctors or in medical science. In the field of medicine, the term lumbar region is more commonly used to describe a similar, though not identical, area of the body. This part of the body includes the major psoas muscle, a large muscle that is used for example when one is sitting up or bending down and is also involved in hip movement. The lumbar vertebrae, numbered as L1-L5, are also located in the loin area. Back pain is often located in this part of the spine because it carries most of the body's weight.
Even though the word loin is not commonly used as an anatomical term in medicine, the word is included in the names of some disorders, for example loin pain hematuria syndrome. The two main symptoms of this syndrome are pain in the loins and blood in the urine. It is not fully understood what causes the pain associated with this syndrome, but it is thought to be related to the kidneys.
Pain in the loins is commonly caused by problems related to the kidneys, such as kidney stones and renal colic, or by problems related to the ureter, meaning the tubes that carry urine from the kidneys to the bladder. The reason for this is that the nerves from the ureter enter the spinal cord in the lumbar region. Other causes of pain in the loins are injuries to the muscles or vertebrae in the lumbar region, herpes zoster, and radiculitis, a type of nerve pain that often affects the sciatic nerve.