Back pain is pain felt in the back that usually originates from the muscles, nerves, bones, joints or other structures in the spine. Back pain may have a sudden onset or can be a chronic pain; it can be constant or intermittent, stay in one place or radiate to other areas. It may be a dull ache, or a sharp or piercing or burning sensation. The pain may radiate into the arms and hands as well as the legs or feet, and may include symptoms other than pain. These symptoms may include tingling, weakness or numbness. Back pain is one of humanity's most frequent complaints. In the U.S., acute low back pain is the fifth most common reason for physician visits. About nine out of ten adults experience back pain at some point in their life, and five out of ten working adults have back pain every year. Low back pain causes 40% of missed days of work in the United States. Additionally, it is the single leading cause of disability worldwide. The spine is a complex interconnecting network of nerves, joints, muscles, tendons and ligaments, all of which are capable of producing pain. Large nerves that originate in the spine and go to the legs and arms can make pain radiate to the extremities.