When people interact with computers, some interfaces require them to hold their arms up. They range from the light pens used to operate radar screens from the 1950s, to the futuristic displays used in the film Minority Report, to modern VR peripherals.

Light-gun–operated radar screens used by NORAD

But holding your arms out like that makes them tired. Human-computer interaction researchers have a wonderful term for this fatigue: they call it “gorilla arms”. As Steve Jobs said when he explained why he thought touchscreens on laptops were a bad idea:

“We’ve done tons of user testing on this 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.”

The term was originally coined in the 1980s, when touchscreens were first taking off. As the New Hacker’s Dictionary puts it:

“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.”