The fastest muscle in the human body is in the eyelid.
Why it is true
The orbicularis oculi muscle, responsible for blinking, is the fastest in the human body. Each blink lasts only about 100 milliseconds, equivalent to 10 blinks per second if there were no pauses.
Source: Journal of Anatomy
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.