RIO DE JANEIRO, BRAZIL - AUGUST 16: A view of competitors during the Women's 200m Semifinals on Day 11 of the Rio 2016 Olympic Games at the Olympic Stadium on August 16, 2016 in Rio de Janeiro, Brazil. (Photo by Quinn Rooney/Getty Images)
It's not how quickly they put one leg in front of the other. Research shows runners of all abilities — you, me and Usain Bolt — move their legs back and forth at about the same frequency when we're running flat out.
But this contact with the ground creates a sort of speed limit. Since it's so brief, runners don't have enough time during each stride to push as hard as they could. Their muscles just don't contract fast enough.
Researchers in Texas and Wyoming once tested how fast people could theoretically contract their leg muscles by having them hop on one leg on a treadmill.
They found if we could get every bit of power from our leg muscles while running, humans would be able to run as fast as 42 mph. Compare that to Bolt's top speed of about 27 mph.
That's almost but not quite enough to keep up with ostriches. Long, springy legs can propel the birds forward at 43 mph, making them the fastest runners on two legs.