Glued to your desk at work? Cross that off the list of excuses for not having the time to exercise.

A growing number of Americans are standing, walking and even cycling their way through the workday at treadmill desks, standup desks or other moving workstations. Others are forgoing chairs in favor of giant exercise balls to stay fit.

Walking on a treadmill while making phone calls and sorting through emails means "being productive on two fronts," said Andrew Lockerbie, senior vice president of benefits at Brown & Brown, a global insurance consulting firm.

Lockerbie can burn 350 calories a day walking 3 to 4 miles on one of two treadmill desks that his company's Indianapolis office purchased earlier this year.

"I'm in meetings and at my desk and on the phone all day," he said. "It's great to be able to have an option at my work to get some physical activity while I'm actually doing office stuff. You feel better, you get your blood moving, you think clearly."

Treadmill desks designed for the workplace are normally set to move at 1 to 2 mph, enough to get the heart rate up but not too fast to distract from reading or talking on the phone comfortably.

It's been a decade since scientific studies began to show that too much sitting can lead to obesity and increase the risk of developing diabetes, high blood pressure and heart disease. Even going to the gym three times a week doesn't offset the harm of being sedentary for hours at a time, said Dr. James Levine, an endocrinologist at the Mayo Clinic.

"There's a glob of information that sitting is killing us," Levine said. "You're basically sitting yourself into a coffin."

Levine said he was at first skeptical that a standup desk would offer improvements in health comparable to treadmill desks or other moving workstations.

"It appears I was completely wrong," he said. "Once you're off your bottom, it's inevitable that you start meandering around. Within two minutes of standing, one activates a series of metabolic processes that are beneficial. Once you sit, all of those things get switched off."

--ASSOCIATED PRESS