MABEL, a two-legged robot that runs like a human being, is believed to be the world's fastest bipedal robot with knees, hitting a peak pace of 6.8 miles per hour.

Two-legged robots with good running form have the advantage of being able to travel over rough terrain and inside places built for humans. They could one day serve as robotic soldiers or rescuers, the engineers say.

The robot, developed in a University of Michigan lab, has been programmed to run at varying speeds in the same way a human moves its limbs. When running, it spends about 40 percent of its time in the air, just like humans.

"It's stunning," said Jessy Grizzle, a professor in the Department of Electrical Engineering and Computer Science. "I have never seen a machine doing a motion like this."

Few robots can run, and the researchers say no machine but MABEL can do it with such a human-like gait. Its weight is distributed like a person's. It has a heavier torso and light, flexible legs with springs that act like tendons.

MABEL is in the air for 40 percent of each stride, "like a real runner," Grizzle said. Other running robots are almost speed-walking. Their so-called flight phase when both feet are off the ground lasts for less than 10 percent of each step.

MABEL was built in 2008 in collaboration with Jonathan Hurst. Grizzle and U-M doctoral students Koushil Sreenath and Hae-Won Park have been progressively improving the feedback algorithms that enable the robot to keep its balance while reacting to its environment in real time.

MABEL started off walking smoothly and quickly over flat surfaces. Then it moved on to uneven ground. It took its first real jog in late July, and with that, Sreenath met the ultimate goal of his research just days before he was scheduled to defend his thesis.

"The robotics community has been trying to come up with machines that can go places where humans can go, so a human morphology is important," Grizzle said. "If you would like to send in robots to search for people when a house is on fire, it probably needs to be able to go up and down stairs, step over the baby's toys on the floor and maneuver in an environment where wheels and tracks may not be appropriate."

"Imagine a future where you don't have to first clear a path and build roads before a vehicle could move around ... but rather, we have a class of running machines like animals that could transport you around with no roads, but with a smooth and efficient ride," said Sreenath.

MABEL is funded by the National Science Foundation and the Defense Advanced Research Projects Agency.

Watch the below video to see MABEL running like humans: