Love your iPad? Take care how you use it. As convenient as iPads and other tablet computers are, they can cause physical problems because people often peer down at them in their laps. Viewing a computer screen is less stressful on the body when users are seated looking straight at the screen, not from above or below.

Desktop monitors and laptops can also cause problems for their users when placed too low, forcing their user to hunch over to see the screen. This places a strain on the neck muscles and can lead to pain and discomfort. Neck pain isn't just an annoyance; it can have serious consequences. A 2007 study found that chronic neck or back pain was the leading national cause of missed work days.

The researchers caution that while using the tablet on a table at a steep angle appears best for the neck and shoulders, this may not be the most comfortable for the hands and wrist.

Desktops and laptops have been around long enough for their ergonomics to be investigated and usage guidelines to be established. Tablets are new enough that there's little information available. Researchers from the Harvard School of Public Health recently conducted a study on how people use tablet computers and hope that their findings will help users place less stress on their bodies while using these devices.

Laptops have an adjustable screen that allows users to change the viewing angle. Tablets don't. What they do have is a case that the tablet can be propped against. Most cases can only be set at two different angles. This doesn't give the full range of viewing positions that a laptop screen does, but it's still helpful.

The researchers monitored 15 veteran tablet-owners using two popular brands of tablet computers. The subjects engaged in typical computer tasks: Internet browsing, reading, game playing, email sending and responding and movie watching. While they did this, their head and neck postures, gaze angle and distance were recorded by a 3D infrared motion analysis system.

Subjects used the tablets in four different positions: in the lap with the tablet held with their hand; in the lap with the tablet resting against its case at the low angle position; on a table against the case at the low angle position and on a table with the case at its high angle position.

Only when used on a table at the high angle position did users' posture approach neutrality--a straight-on viewing position. From the other three positions, users were looking downward at their tablets, at a steeper angle than has generally been reported for laptop or desktop computer users. Over an extended period of time, this is likely to cause discomfort.

There were significant differences between the two tablet models. One model's case allowed for tilt angles of 15 and 63 degrees while the other allowed tilting and at 45 and 73 degrees. That's a pretty big difference, and other models on the market will probably show differences just as large.

The researchers caution that while using the tablet on a table at a steep angle appears best for the neck and shoulders, this may not be the most comfortable for the hands and wrist. So there's likely to be some tradeoff between what's most comfortable overall. The study did not address that issue.

An article on the study was published online by Work and offers pictures of people using their tablet in all four positions, so you can actually see what these positions are doing to your posture.