d) Researchers need to understand why different forms of locomotion evolved.
b) Long-held assumptions, such as the need for energy efficiency, have already been overturned.
a) For example, a mechanical ankle brace can improve the metabolic efficiency of human walking, implying that walking is inefficient.
e) But variation of movement is important, too: such an ankle brace holds you back if you try to skip, gallop or skitter.
c) Similarly, legged robots struggle to deploy different gaits, just as roboticists struggle to enumerate them.