The first pair of biologically accurate robot legs can walk like clumsy human babies placed on a treadmill - placing one foot in front of the other with surprising confidence.

The robot legs work despite having just a simple nervous system to control the hips. That is because the lower legs' reflex responses take over to complete the walking motion - a possible explanation for how human babies can walk on treadmills without an adult's complete sense of coordination and balance.

"Interestingly, we were able to produce a walking gait, without balance, which mimicked human walking with only a simple half-centre controlling the hips and a set of reflex responses controlling the lower limb," said Theresa Klein, a Ph.D. student at the University of Arizona.

That "half-centre" represents a neural network in the human spinal cord that can send out muscle signals. The simplest neural network, the half-centre, fires off alternating signals to create the rhythm of walking.