WASHINGTON -- At work, Pam O'Donnell types at her computer, talks on the phone and, every so often, bounces a bit on a big blue ball.

O'Donnell has an office chair, but this doesn't mean she uses it. She prefers to sit on an exercise ball.

Although some posture experts are leery of the practice, she and others say sitting on the ball lets them add a little workout into their work time -- and strengthen their legs, abs and back muscles.

The inflatable balls, typically the size of a big beach ball but made of tougher plastic, have migrated to offices from health clubs and physical therapy clinics.

The lack of armrests, back support or other attributes of a chair, except for a spot on which to sit, makes her pay more attention to posture, said O'Donnell, director of member service for the International Health, Racquet and Sportsclub Association, a Boston-based trade group.

"It makes me sit up straight," which gives a ball some advantage over a chair, she said. "When you are sitting at your desk, especially at a computer or keyboard, you tend to hunch over."

"What we are trying to promote is active sitting versus passive sitting," said physical therapist Cheryl Soleway, of Vernon, British Columbia, a consultant to Ball Dynamics of Longmont, Colo., which sells the products.

"You get a low level of neuromuscular activity -- your abdominal and trunk muscles are contracting to some degree," Soleway said. "Without that activity, you would fall off."

To Soleway, there is even an upside to the fear of falling down. The natural urge not to slide off the ball should help to train the balance system, reducing the risk of falls, she said.