"Gorilla arm"
Gorilla arm was a side-effect that destroyed vertically-oriented touch-screens as a mainstream input technology despite a promising start in the early 1980s.[2]
Designers of touch-menu systems failed to notice that humans aren't designed to hold their arms in front of their faces making small motions. After more than a very few selections, the arm begins to feel sore, cramped, and oversized -- the operator looks like a gorilla while using the touch screen and feels like one afterwards. This is now considered a classic cautionary tale to human-factors designers; "Remember the gorilla arm!" is shorthand for "How is this going to fly in real use?".
Gorilla arm is not a problem for specialist short-term-use devices such as ATMs, since they only involve brief interactions which are not long enough to cause gorilla arm.
Gorilla arm also can be mitigated by the use of horizontally-mounted screens such as those used in Tablet PCs, but these then have the problem that the user's need to rest their hands on the device increases the amount of dirt deposited on the screen, and occludes the user's view of the screen.
