The key to a healthy backbone is the muscles that support it. Particularly important are the abdominal muscles, which do the heavy work of lifting when the back flexes and its own muscles relax.

If the abdominal muscles are flabby -- as in the case of a potbelly -- something has to give, and all too often it's the back.

The best way to strengthen the abdominal muscles is by doing situps -- the right kind, with the knees bent. (Straight-leg situps use the leg muscles, not the abdominal ones, and increase pressure on the lower disks.)

Exercise can help prevent back problems, but not all exercise is good for the back. Weight lifting strengthens muscles but can be dangerous unless done with proper supervision. Running is great for the cardiovascular system and weight loss, but over time it can take a toll on the knees and back -- especially running on asphalt surfaces that don't cushion each step.

Long-distance running on asphalt can lead to the "runner's disk," a painful compression of the vulnerable disk between the fourth and fifth lumbar (lower back) vertebrae.

"One of the biggest boons to neurosurgeons and orthopedic surgeons," says Dr. Michael Dennis, chief of neurosurgery at the Washington Hospital Center, "has been the running craze."

Walking, bicycling, swimming or rowing are more likely to stretch and strengthen the back without harming it, Dennis says.

Here are some other tips for keeping your back in shape: Lifting: Lift with your legs. Bend your knees, not your back, and don't twist while lifting. Hold the object as close as possible to your body. For heavy items, get help from another person, or a machine.Posture: Stand up straight, but not military-stiff: shoulders back, abdomen pulled in. Avoid standing in the same position or place for long periods of time. Rest one foot on a rail or stool. Sitting: Try not to spend so much time sitting, but when you can't avoid it, choose a firm chair and don't slouch. Sit firmly against the back of the chair. Avoid leaning forward and arching your back. Use a chair low enough to keep both feet on the floor. A fat wallet in the hip pocket can distort your posture during a long sitting and press on the sciatic nerve, sending shooting pains down the leg. Sleeping: Use a firm mattress that doesn't sag. Sleep on your side with knees bent, or on your back. Sleeping on your stomach increases the curve of the lower back, compressing the disks and causing a swayback.Driving: Adjust the car seat so you don't have to stretch for the pedals. Sit straight, with your back against the seat. On long trips, take a break every couple of hours at least.