Humans could run at 40 miles per hour, according to scientists, who reckon they have established the theoretical limits of running speed.
The researchers reckon that it's possible to improve significantly on the 28mph achieved by sprinter Usain Bolt.
It had previously been thought that speed was limited by the force with which the limbs can strike the running surface.
"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.
"However, our new data clearly show that this is not the case. Despite how large the running forces can be, we found that the limbs are capable of applying much greater ground forces than those present during top-speed forward running."
Rather than force, say the researchers, the critical factor is time – specifically, the very brief periods available to apply force to the ground.
In elite sprinters, ground contact times are less than one-tenth of one second, and peak ground forces occur within less than one-twentieth of one second of the first instant of foot-ground contact.
The researchers used a high-speed treadmill and had subjects perform at high speeds in different gaits.
They found that the ground forces applied while hopping on one leg at top speed exceeded those applied during top-speed forward running by 30 percent or more, and that the forces generated by the active muscles within the limb were up to double those of the hopping gait.
According to Matthew Bundle, an assistant professor of biomechanics at the University of Wyoming, "The very close agreement in the briefest periods of foot-ground contact at top speed in these two very different gaits points to a biological limit on how quickly the active muscle fibers can generate the forces necessary to get the runner back up off the ground during each step."
The researchers said the new work shows that running speed limits are set by the contractile speed limits of the muscle fibers themselves.
"Our simple projections indicate that muscle contractile speeds that would allow for maximal or near-maximal forces would permit running speeds of 35 to 40 miles per hour and conceivably faster," Bundle said.