Apple sold a record breaking 15.43 million iPads in the last three months of 2011, which means a lot of people are starting to use tablet computers. And with last week's news that Apple is planning to bring textbooks to the iPad -- well, that's a lot more people who may start to use tablets, too.

But, do they know how to use them safely?

A new study published by researchers from the Harvard School of Public Health, in conjunction with researchers at Microsoft (a long-time Apple rival), is the first of its kind to examine the physical effects on the head, neck and shoulders of spending time staring at a tablet.

The good news is that it is not all bad news. The researchers found that people are more inclined to move around and shift positions when they use a tablet compared with people who are sitting at a desktop computer. That's definitely good. However, tablet users that hold the device almost at their lap, or rest the tablet in a case on their lap, are putting a lot of strain on the neck muscles -- much more than someone using a laptop or desktop computer.

"If you think about your position when you are hunched over looking down, your head is hanging out over space, so you are using your neck muscles to support the weight," said Jack Dennerlein, director of the Harvard Occupational Biomechanics Laboratory, and lead author of the paper.

Definitely not good.

In the paper, published earlier this month in the peer reviewed "Work: A Journal of Prevention, Assessment, and Rehabilitation," the researchers identified four ways that people use tablets — the lap-hand (holding the tablet down at your lap), lap-case (resting the tablet in a case on your lap), table-case (resting the tablet in its case on a shallow angle on a table) and table-movie (resting the tablet at a steep angle on a table).

They concluded that the best position is the table-movie position because it is the only position in which the user's posture approached neutral. All the other positions put a lot of strain on the user's neck muscles.

Dennerlein said those who use tablets should make sure to move around as much as possible -- "Don't get stuck in one position!" he said. The next most important thing is finding a good case that allows you to prop up your tablet at the most comfortable angle. He added that companies that distribute tablet computers to their employees should make sure to give out cases as well, in order to prevent injuries.

Next up, Dennerlein and his team plan to tackle the effect of tablet computing on the arms and wrist.

One additional note: When we reached out to Apple to see if they had any comment on the ergonomics on using the iPad, a spokesperson pointed us to a large section on ergonomics on Apple's website. The section is impressive, but the suggestions and diagrams are all related to desktop computers, and the site did not have any recommendations on how to most safely use a tablet. We called the rep to see if we had missed anything, but we didn't hear back by press time.

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--Deborah Netburn

Photo: A model looking at her iPad in a position that the Harvard study says is bad for your neck. Credit: Markus Schreiber / Associated Press