A personality conflict with your boss can be just as hard on your back as heavy lifting, concludes the director of the largest study of back pain ever undertaken.

"The job itself does not seem to matter as much as how well you get along with your supervisor," said Dr. Stanley J. Bigos, an orthopedist and director of the Spine Resource Clinic at the University of Washington in Seattle.

Bigos heads a study that has looked at 31,200 Boeing Co. workers over the past three years, including their habits, jobs, medical records and history of back health.

Back pain, the most common cause of disability in Americans under 45, remains something of a medical mystery. In 88 percent of cases, Bigos said, doctors cannot pinpoint anatomical reasons for back pain.

Of the 78 jobs in the Boeing study, none stood out as a clear risk for back pain. Sitting at a desk all day proved to be just as tough on the back as heavy lifting.

"The first thing you should avoid is sitting, which puts 40 to 80 percent more stress on your back than standing does," Bigos said.

The Boeing study found a higher incidence of back problems in workers who had had a negative job evaluation within the past six months. Back trouble was also common among smokers -- probably because smoking saps endurance -- and among people who drive more than an hour to and from work each day.

Bigos' advice on lifting: "Keep everything you lift close to your belly button."