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.
Unlike traditional computers with which we spend hour upon hour on, smartphones and tablets are used most often for doing stuff that doesn’t require as long. The big exception I can think of is watching full length movies, exactly when we’re not thinking of posture and want simply to relax.