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By Louis Neipris, M.D., HealthAtoZ writer
Almost every American has experienced a bout of back pain at one time or another. Back pain is the second most common diagnosis in the outpatient setting, after headaches. Whether the cause is degenerative or traumatic, back pain leads to missed days of work and temporary disability in 3 percent to 4 percent of the American population each year.
The spine through the ages
Our backs weren't designed for the modern sedentary lifestyle. Anthropologists believe that early humans were bipeds (walked upright), but also spent time on all four limbs. After examining the skeletons of prehistoric species, they think the spinal structure changed as human beings stood for longer periods. The lower spine curved more and more over the eons. That curve, the lordosis, supports upper body weight better than the straight spine of much earlier primates that walked on all fours.
Does a more curved lower spine help us deal better with life as an upright but often seated creature? In fact, just the opposite is the case. While increased curvature of upper or lower spine does handle the weight of the upper body on the pelvis, the benefits of a slightly curved spine end at a certain point. Complications of poor posture and obesity tend to offset the stabilizing design. Perhaps further modifications over eons will help us to live life seated. No one is sure. Nonetheless, maintenance of the natural "S" curve is essential to good back health.
S.O.S: Save our "S"
The spine is naturally configured in an "S" curve from the side. A slight concave curve of the cervical spine (neck) gently changes to the longer convex thoracic (chest) area ending in a concave curve of the lumbar region. The neutral "S" curving cervical, thoracic and lumbar spine is also the most stress-free state.
Yet, due to everyday demands, not even from lifting heavy objects, but just by driving, standing or sitting at a desk for hours at a time, our spine strays from the comforting "S." The neck and mid-back suffer as we slouch forward or bend the wrong way repeatedly. One day, our back is abused just once too often. Picking up a crumpled piece of paper that didn't make it into the trash brings on a sudden twinge of pain. Our back reminds us that it is there and demands our attention.
The back is the spinal vault
Like the brain protected by the skull (a fusion of several bones), bones called vertebrae surround the spinal cord. The highway from brain to limbs, the spinal cord carries information about our world and our intentions - both voluntary and automatic. The index finger tip touches the stove, senses that it's hot and generates a signal that travels up the median nerve to the brachial plexus (grouping of nerves near the axilla or armpit) to the cervical spine and then to the brain. Our sense of touch is processed and the brain generates an impulse that travels down the spine from nerve-to-nerve, and in a lightning flash, we withdraw the finger.
A vulnerable vault
The back does a fine job of protecting the delicate spinal cord. Why is it prone to injury? Unlike the skull, the back must maintain flexibility while providing bony protection for the spinal cord. Bending and twisting of bone over bone is only possible with specialized structures called vertebrae and their supporting muscles and ligaments. These components work together, but each is subject to its own share of problems.
Muscles and ligaments
Back muscles are the most frequently injured structures because they are already working very hard to maintain the "S" curve. Bending too far, or the tendency to overextend with a poor posture, puts more strain on back muscles. Eventually, whole regions of back muscles tighten. Continued demand on tight, tired muscles leads to the injury and pain of a muscle strain, or even a torn ligament when the attachment of muscle to bone gives way.
Vertebrae
Each vertebra is made up of a body and arch. The vertebral body houses a spongy central disc that allows for movement of the vertebra above and below. Behind each vertebral body, the spinous process contains a portion of the spinal cord, which runs continuously above, and below, opening up into nerve roots that converge again to form major peripheral nerves.
In spite of their complex design, the vertebrae cannot handle life's forces without the help of the back muscles and ligaments. When an area of muscle weakens, the stress is transferred to the closest vertebrae at the ligament, which surrounds the disc. Eventually, the ligament gives way and the disc slips forward onto the spine or nerve roots.
The spinal cord
So why make us carry a "vault" on our back? All the armor of the vertebrae (bones), muscles and ligaments serves to protect the spinal cord, a delicate, whitish gray vertical tube. The spinal cord is composed mainly of myelin, a fatty substance that insulates nerve fibers. Nerve cells and other nutrient-supplying cells gather in clusters to receive and transmit impulses from the brain and out to the rest of the body and back.
This article was reviewed and updated June 2007.
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