Future running robots are likely to resemble ostriches or dinosaurs, new research suggests.
Scientists believe the efficient way flightless birds - and their dinosaur ancestors - get around on two legs could usefully be employed in robot design.
A study has demonstrated how an ostrich's speed, energy conservation and ability to stay upright is unmatched in other animals, including humans.
"Birds appear to be the best of bipedal terrestrial runners, with a speed and agility that may trace back 230 million years to their dinosaur ancestors," said US lead researcher Dr Jonathan Hurst, from Oregon State University.
Running birds such as ostriches and emus are not necessarily the most graceful movers, but they save energy and avoid falling, Dr Hurst pointed out.
"These animals don't care that they appear a little unstable or have a waver in their gait," he said.
"Their real goal is to limit peak forces, avoid falling, be safe and be as efficient as possible. If their upper body seems to lurch around a little as a result, that's okay. What they are accomplishing is really quite elegant."
The research showed that a wide variety of birds with very different body sizes all employ essentially the same running technique. They ranged from the tiny quail to the ostrich, whose body mass is 500 times greater.
To hop over objects on uneven ground, they all used a largely "vaulting" motion followed by a more crouched posture when on top of the obstacle.
Unexpectedly, the scientists found that running birds do not prioritise stability in a rigid "robotic" way. Instead, they allow their bodies to bounce around to some extent, while ensuring that they never fall.
In order to stay upright, their leg movements may speed up or slow down, much like those of a scrambling rugby player.
Oregon State University doctoral student Christian Hubicki, who co-authored the study, published in the Journal of Experimental Biology, said: "We should ultimately be able to encode this understanding into legged robots so the robots can run with more speed and agility in rugged terrain.
"These insights may also help us understand the walking and running behaviours of all the common ancestors involved, including theropod dinosaurs such as the velociraptor."
Working with colleagues from the Royal Veterinary College in London, the scientists looked at five species of birds, and developed a computer model that simulated their behaviour.
They concluded that robot design "must embrace a more relaxed notion of stability" rather than focusing on a rock-steady gait.
"The running robots of the future are going to look a lot less robotic," said Dr Hurst. "They will be more fluid, like the biological systems in nature. We're not necessarily trying to copy animals, but we do want to match their capabilities."
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