Slide show: Causes of back pain

Your backbone (vertebral column) is actually a stack of more than 30 bones called vertebrae. Together they create a bony canal that surrounds and protects your spinal cord. Small nerves enter and exit the spinal cord through spaces in your vertebrae.

The vertebrae are held together by muscles, tendons and ligaments. Between the vertebrae are intervertebral disks, which act as "shock absorbers" by preventing the vertebrae from hitting one another when you walk, run or jump. The intervertebral disks also allow your spine to twist, bend and extend.

The lower back — which carries most of your weight — is the site of most back pain. Here are some of the most common culprits.

The most common cause of back pain is an injury to a muscle (strain) or a ligament (sprain). Strains and sprains can occur for many reasons, including improper lifting, excess body weight and poor posture. Strains and sprains can also develop from hauling around a heavy handbag or sleeping at an awkward angle. As a pivot point for turning at the waist, the lower back is especially vulnerable to muscle strains (shown here).

Sometimes a strain or sprain causes immediate back pain. In other cases, soreness and stiffness come later. An injured muscle may also "knot up." This muscle spasm is your body's way of immobilizing the affected area to prevent further damage.

Sometimes back pain is related to osteoarthritis of the spine, a degenerative joint condition that causes slow deterioration of the disks between the bones that make up the backbone. Without the cushioning that these disks normally provide, the joints (facets) between vertebrae press tightly against each other. This can cause back pain and stiffness. Your body may try to compensate for these changes by building new bone (spurs) to support the area where loading pressure is increased.

Because osteoarthritis usually develops over many years of physical activity, it's known as "wear-and-tear" arthritis. Obesity and injury to a joint are other risk factors for osteoarthritis.

Normal wear and tear over time can cause one of the disks in your spine to rupture (herniate). Exceptional strain or traumatic injury can have the same effect. Many people describe this as a "slipped" disk. Back pain results when the herniated disk pinches one of the 50 nerves that come out of the spinal cord (illustration on the left).

The sciatic nerve (illustration on the right), which runs from your spinal cord to your leg, is most likely to be affected. Compression or inflammation of this nerve causes sciatica — a sharp, shooting pain in the lower back, buttocks and leg.

As you age, the amount of calcium in your bones decreases. This lowers the density or mass of your bones, making them porous and brittle — a condition known as osteoporosis.

But what does osteoporosis have to do with back pain? Plenty. If you have osteoporosis, daily lifting and other routine activities can cause low back pain by fracturing the front part of the weakened bones. These are known as compression fractures. A fall can have the same effect.

Fibromyalgia is a chronic condition characterized by fatigue and widespread pain in the muscles, ligaments and tendons — including the lower back. The diagnosis of fibromyalgia usually includes a history of at least three months of widespread muscle pain accompanied by pain and tenderness in at least 11 of the 18 "tender points" shown here.

There are other causes of back pain. Rarely, back pain may indicate a more serious underlying problem, such as an infection, diabetes, kidney disease or cancer. If you're struggling with back pain, consult your doctor. He or she can identify what's causing the pain and help you get back on track.