What is the fastest muscle movement in the human body?

Blinking, at around a tenth of a second, driven by the orbicularis oculi - and the muscle behind the eye that moves it is even faster. You blink perhaps 15 to 20 times a minute, several times more often than tear film maintenance actually requires, which means most blinking is doing something else.

Part of it appears to be mental punctuation. People blink at predictable moments - at the ends of sentences while listening, at scene changes while watching film, at natural breaks in a task - and audiences watching the same film blink in near-unison at the same points. It looks like the brain taking micro-breaks at moments when it judges nothing important will be missed.

What is odd is that you never see the darkness. The visual system suppresses input during a blink and appears to bridge the gap, so a total interruption several hundred times an hour goes unnoticed. The same suppression happens during rapid eye movements, which is why you cannot watch your own eyes move in a mirror - look from one eye to the other and the motion is simply absent, though someone standing beside you can see it happen.