The Nagoya Institute of Technology's Sano Lab in Japan has developed a pair of robotic legs that are actually propelled by their own weight. The gait is more normal than most and no external control or motors are needed. Made of aluminum, they were adjusted to the same thigh and leg lengths as a human and the same weight. They work in the same manner of "falling" in that they just need a small push and downwards slope.
When tested last year, the legs went non-stop for 13 hours. Their hope is to create a commercial version in a year or two for testing sports equipment or assisting those who have trouble walking but we still want to know, what happens when it needs to go uphill?