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The limit to how fast a human can run is 9.48 seconds for the 100-meter race, 0.10 seconds faster than Usain Bolt's current world record, according to Stanford biologist Mark Denny. That is, if you are talking about natural human beings.
In a 2008 study published in the Journal of Experimental Biology, Denny modeled the fastest human running speed using records of men's 100-meter race results going back to the 1900s. He plotted the annual best times in the race into a graph and used computer programs to come up with an equation whose curve best models the behavior of the actual graph he obtained.