Humans could perhaps run as fast 40 mph, a new study suggests. Such a feat would leave in the dust the world's fastest runner, Usain Bolt, who has clocked nearly 28 mph in the 100-meter sprint.

The new findings come after researchers took a new look at the factors that limit human speed. Their conclusions? The top speed humans could reach may come down to how quickly muscles in the body can move.

Previous studies have suggested the main hindrance to speed is that our limbs can only take a certain amount of force when they strike the ground. This may not be the whole story, however.

"If one considers that elite sprinters can apply peak forces of 800 to 1,000 pounds with a single limb during each sprinting step, it's easy to believe that runners are probably operating at or near the force limits of their muscles and limbs," said Peter Weyand of Southern Methodist University, one of the study's authors.

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