It is a capricious pain that disrupts the lives of nearly eight out of every 10 Americans. The malady is not only fairly common but can immobilize people for days.

Lower back pain accounts for one half to two days of work absences annually per worker and is the chief cause of activity restrictions in people under 45, according to Lawren Daltroy, an instructor in behavioral sciences at the Harvard School of Public Health.

Backaches can be as frustrating as sitting in a traffic jam. Patients know their backs hurt and may seek a medical opinion to determine why only to discover that their doctors are as perplexed as they are. "The trouble with back pain is figuring out the exact source of discomfort," said Bernard Pfeifer, an orthopedic surgeon at the Lahey Clinic in Burlington, Mass. "X-rays will tell you if there is a structural cause for the pain."

This may mean that the bones in the spine are deteriorating or a disk has ruptured, putting pressure on the spinal nerves. Disks are circular structures made of cartilage that act as shock absorbers to cushion the vertebrae -- the bones that form the spine. Most back problems, however, are caused by general stresses and strains. Lifting a box that is too heavy or lifting it in the wrong way -- without bending the knees -- can injure the back. Poor posture may be another cause. "These stresses accumulate over the years and when the back finally goes out, it may be because of some trivial thing like bending over to pick up something," said Harvard's Daltroy.

Damage to the ligaments, muscles and tendons -- soft tissue areas -- can frequently trigger back pain. Such injuries make diagnosis especially difficult because the damage does not show up on an X-ray. Soft tissue injuries cause swelling, tightening muscles and creating pain.

About 90 percent of back injuries are resolved within two months, according to Daltroy. A quick recovery, he said, may be related to lower back strength and overall fitness. Exercising the quadriceps and hamstrings -- muscles at the front and back of the thigh -- and the abdominals can help strengthen the lower back.

Daltroy recommends the following back-strengthening exercises to be done five to 10 times daily:

Pelvic tilts. Lie on your back with the knees bent and both feet on the floor. Push the small of the back toward the floor, hold for 10 seconds and relax.

Knee to chest. Lying in the same position, bring the right knee to the chest, grasp it with both hands and pull it as close as possible. Hold for five seconds; repeat with left leg.

Sit-ups. Again lying in the same position, begin with a pelvic tilt. Grasp the knees with both hands, tuck the chin and raise the shoulder blades off the floor as far as you can. Hold for five seconds.

Spending less time in bed -- about two days -- is better than prolonged bed rest for a bad back, doctors said. "In the old days, we would put people in bed for a couple of weeks," said Lahey's Pfeifer. "But you decondition in bed and it will be harder to get back into shape."