gorilla arm

n. The side-effect that destroyed touch-screens as a mainstream input
technology despite a promising start in the early 1980s. It seems the
designers of all those spiffy 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?.