Apple’s new MacBook Air borrows a lot of things from the iPad, including hyperportability and instant-on flash storage. But the Air won’t use an iPad-like touchscreen. Neither will any of Apple’s laptops. That’s because of what designers call “gorilla arm.”
And while Apple points to its own research on this problem, it’s a widely recognized issue that touchscreen researchers have known about for decades.
“We’ve done tons of user testing on this,” Steve Jobs said in Wednesday’s press conference, “and it turns out it doesn’t work. Touch surfaces don’t want to be vertical. It gives great demo, but after a short period of time you start to fatigue, and after an extended period of time, your arm wants to fall off.”
This why Jobs says Apple’s invested heavily in developing multitouch recognition for its trackpads, both for its laptops, on its current-generation Mighty Mouse and on its new standalone Magic Trackpad.
Avi Greengart of Current Analysis agrees it’s a smart move, borne out of wisdom gathered from watching mobile and desktop users at work.
“Touchscreen on the display is ergonomically terrible for longer interactions,” he says. “So, while touchscreens are popular, Apple clearly took what works and is being judicious on how they are taking ideas from the mobile space to the desktop.”
But Apple didn’t have to do its own user testing. They didn’t even have to look at the success or failure of existing touchscreens in the PC marketplace. Researchers have documented usability problems with vertical touch surfaces for decades.
“Gorilla arm” is a term engineers coined about 30 years ago to describe what happens when people try to use these interfaces for an extended period of time. It’s the touchscreen equivalent of carpal-tunnel syndrome. According to the New Hacker’s Dictionary, “the arm begins to feel sore, cramped and oversized — the operator looks like a gorilla while using the touchscreen and feels like one afterwards.”
According to the NHD, the phenomenon is so well-known that it’s become a stock phrase and cautionary tale well beyond touchscreens: “‘Remember the gorilla arm!’ is shorthand for ‘How is this going to fly in real use?’.” You find references to the “gorilla-arm effect” or “gorilla-arm syndrome” again and again in the scholarly literature on UI research and ergonomics, too.
There are other problems with incorporating touch gestures on laptops, regardless of their orientation. Particularly for a laptop as light as the MacBook Air, continually touching and pressing the screen could tip it over, or at least make it wobble. This is one reason I dislike using touchscreen buttons on cameras and camera phones — without a firm grip, you introduce just the right amount of shake to ruin a photo.
Touchscreens work for extended use on tablets, smartphones and some e-readers because you can grip the screen firmly with both hands, and you have the freedom to shift between horizontal, vertical and diagonal orientations as needed.
On a tablet or smartphone, too, the typing surface and touch surface are almost always on the same plane. Moving back and forth between horizontal typing and vertical multitouch could be as awkward as doing everything on a vertical screen.
This doesn’t mean that anything other than a multitouch trackpad won’t work. As Microsoft Principal Researcher (and multitouch innovator) Bill Buxton says, “Everything is best for something and worst for something else.”
We’ve already seen vertical touchscreens and other interfaces working well when used in short bursts: retail or banking kiosks, digital whiteboards and some technical interfaces. And touchscreen computing is already well-implemented in non-mobile horizontal interfaces, like Microsoft’s Surface. Diagonalized touchscreen surfaces modeled on an architect’s drafting table like Microsoft’s DigiDesk concept are also very promising.
In the near future, we’ll see even more robust implementations of touch and gestural interfaces. But it’s much more complex than just slapping a capacitative touchscreen, however popular they’ve become, into a popular device and hoping that they’ll work together like magic.
Design at this scale, with these stakes, requires close and careful attention to the human body — not just arms, but eyes, hands and posture — and to the context in which devices are used in order to find the best solution in each case.
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Even if you’ll never get your hands on one, you’ve got to love super-rugged computers. 
