Scientists think they've worked out the top speed that humans might be capable of reaching in the future.
They reckon we might be able to run at 40mph - that's 12mph quicker than the fastest man on the planet, Usain Bolt.
They used to think this was impossible, as human limbs couldn't stand the force of hitting the ground at that speed.
But now researchers in America have found human legs are much stronger than they thought and that athletes could cope with speeds of up to 40mph.
The scientists used a high speed treadmill to measure the force on athletes' legs as they ran and hopped.
They found that when athletes hopped at top speed, their legs had to cope with pressure that was sometimes twice as great as running.
They said this showed that the only thing slowing us down was how quickly our muscles could contract to cope with this increased force.
Usain Bolt ran the 100m - from a standing start - at 23.35mph in August 2009. But he dashed from the 60m to the 80m mark in 1.61 seconds, meaning his top speed was 27.79 mph.