Typically, bipedal robots like Agile Technologies' Cassie require plenty of processing power to run the balancing algorithms, gyroscopes and sensors needed to keep it on its feet. However, the PER's elegant design, with a single motor that drives the legs in an elliptical motion, means all that weight and tech simply isn't needed. The body's shape adds to its stability and the robot's legs have "reactive resilience": when the legs feel resistance, it adds more power to push through, as well as mechanically adjusting the rear leg to stabilise its sprint. If the robot was scaled up to human size, its speed could apparently reach between 20 or 30 mph.

Biped robots are typically harder to balance, more power-hungry and more expensive, but as they can dynamically balance, like Boston Dynamics humanoid offerings, they can get to places other robots can't. "Robots with legs will be particularly useful in places where you want a human presence, but it's too dangerous, expensive, or remote to send a real human," Pratt told Technology Review.