Back pain is one of the most common medical conditions in the United States, with between 50 and 80 percent of all adults experiencing back pain at some time, according to the Arthritis Foundation. About 10 percent of all Americans will experience back pain or related symptoms (such as limited mobility or stiffness) in any given year.

A person’s back is composed of the bones, muscles and other tissues that form the posterior part of the trunk, which runs from the neck to the pelvis. The back is held upright by muscles and ligaments that are attached to the bones of the back. These bones, called vertebrae, are stacked on top of one another. Together, these bones are known as the spine, spinal column or vertebral column.

The spine supports the upper body’s weight. It also houses and protects the spinal cord, which transmits electrical signals between the brain and the nerves in the legs, arms and other areas of the body. The spinal cord extends from the base of the brain to just below the rib cage. Small nerves enter and emerge from the spinal cord through spaces between the vertebrae.

The vertebrae are separated by intervertebral discs, also known as spinal discs. These are made up of a strong outer fibrous covering (annulus fibrosis) that surrounds a soft, jelly-like center (nucleus pulposus). The discs cushion the vertebrae and keep them from wearing out.

Several factors can contribute to back pain. Poor posture and ergonomics are common factors. A spasm, sprain or strain in a muscle or ligament can occur when a person lifts an object or overstretches. If the spine becomes overly strained or compressed, this trauma can cause a disc to bulge or rupture. A bulging or ruptured disc may put pressure on one of the dozens of nerves rooted to the spinal canal that control body movements and transmit signals from the body to the brain. Irritated, compressed or pinched nerves cause back pain.

Developmental abnormalities may lead to back pain. Examples include scoliosis (sideways curve of the spine), kyphosis (exaggerated forward curve in the upper back) and lordosis (exaggerated inward curve in the lower back).

Several conditions associated with aging can cause back pain. These include:

• Degenerative disc disease, deterioration of the spinal discs
  
  
• Compression fractures due to the bone-thinning disease osteoporosis
  
  
• Spinal stenosis, in which spinal passages narrow
  
  
• Osteoarthritis, a gradual breakdown of cartilage in joints
  
  
• Ankylosing spondylitis, a form of arthritis that can inflame spinal joints

Back pain can also result from chronic disorders ranging from lupus to sickle cell anemia. However, in many other cases, the source of back pain can be difficult to determine. Many people develop back pain for no apparent reason.