Scientific and technological advances are making that an awfully complicated question.
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. Denny 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. The curve showed humanly achievable time for the 100-meter race would level off at 9.48 seconds. “They haven't plateaued yet, but you can definitely see the data are bending a little towards that plateau,” Denny says.
Denny, who also modeled the best times for racing thoroughbreds and greyhounds in the same study, found there’s a speed limit for these races as well, with little improvement in the Kentucky Derby since the 1950s and dogs’ performances leveling off in the 1970s.
For the rest of the story: http://www.popsci.com/science/article/2013-05/fyi-there-limit-how-fast-human-can-run?src=SOC&dom=tw