How to properly position the touchscreen. Unlike laptops, tablets like Apple's iPad and e-readers like Amazon.com's Kindle function vertically, horizontally, and anywhere in between. Horizontal use is typically less stressful, especially when the tablet is in a comfortable position for your arms and hands (similar to how you should use a keyboard on a laptop or desktop PC) -- though the fact the screen is positioned at or near lap level means you're likely to bend your neck, which is problematic for your posture.
Touchscreens positioned upright are ergonomically inferior. Like the futuristic computer screen that Tom Cruise's character used in the 2002 movie "Minority Report," vertical touchscreens such as in the new breed of Windows 8 PCs expected later this year (and in some current PCs) force you to use the large muscles in your shoulder and arms in ways that promote fatigue. Then-Apple CEO Steve Jobs put it aptly at a press conference in October 2010: "Touch surfaces don't want to be vertical." The more perpendicular the screen, the more you have to bend your wrist to type, a posture that anatomists call "dorsiflexion." That puts more pressure on the median nerve and the other structures in the carpal tunnel in the wrist.
Vertically oriented touchscreen monitors require you to reach forward and lift your arm against gravity, which tires your muscles rapidly. That also happens to some extent when you use a mouse or trackpad while sitting too far away from your desk, but the fix is easy: Move closer.
If both horizontal and vertical positions are problematic, what angle is acceptable? Unlike desktop computer setups, where there are well-established guidelines based on scientific research, recommendations for people who use touchscreens are scarce and sometimes contradictory because they depend on the task you're doing. For reading, it's best to place the device so that you can see the entire screen clearly. Generally, that means a steep angle close to perpendicular to your line of sight -- in other words, like that of a standard monitor. But for typing and tapping, shallow angles (about 30 degrees) are best.
Avoiding injuries from typing and tapping. The position of your wrist also affects the likelihood of injury from performing multitouch gestures on touchscreens. According to Alan Hedge, director of the Human Factors and Ergonomics Laboratory at Cornell University, the more you dorsiflex your wrist, the greater the chance of injury. But, he adds, most gestures don't require too much force, so you're usually safe as long as you don't bend your wrist excessively or repeat gestures too rapidly.
In theory, the onscreen keyboards on tablets and smartphones pose the same risks of RSIs and related injuries as physical keyboards. Currently, the main unique problem with touchscreen keyboards is their lack of tactile feedback. Unlike mechanical keys, which move and offer resistance, virtual keys don't react when they're pressed. As a work-around, manufacturers typically let you turn on audible key clicks, but that's not always effective, particularly in noisy surroundings. As a result, says Hedge, users strike virtual keys with as much as eight times the force as they tap real ones -- and all that force puts strain on your fingers, wrist, and forearm. If you have to type more than a few sentences at a time on a tablet or smartphone, consider using a Bluetooth or other external keyboard.
At the same time, onscreen keyboards confer unique advantages, not just risks, such as the ability to provide alternative layouts that place keys in less stress-inducing positions. Unfortunately, that's a benefit that vendors haven't embraced much yet.