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When thinking about robots that emulate animals, I would think roboticists would try to emulate humans which have distinctly forward jointed legs. Yet, I see in many instances that robots have more bird-like legs with shorter femurs.

Why are robots designed with these legs? What advantage do they provide over more human-like legs?

enter image description here

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    $\begingroup$ who runs faster, a human or an ostrich? ..... that picture may be similar to a bird, but it is also similar to a human on tiptoes .... the gait is more complicated and less springy if you use the heel-toe gait of humans .... on the model in the picture, the heel is the backward facing joint $\endgroup$
    – jsotola
    Apr 25, 2018 at 23:17
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    $\begingroup$ Your claim does not seem to consider other relevant factors. Birds are also also be comparatively lighter weight for their size and have their lumbar vertebrae aligned horizontally, not vertically, so that shape alone may not account for maximum speed, let alone agility and acceleration. $\endgroup$
    – John Joe
    Apr 25, 2018 at 23:19
  • $\begingroup$ i was not thinking about the skeletal structure of birds at all, i was just stating the first thing that popped into my head ... i have no expertise in the area .... almost all animals that i can think of have their hind legs like the example in the picture (excluding marine animals and insects) ... so there must be an advantage to that form $\endgroup$
    – jsotola
    Apr 26, 2018 at 2:33

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One chooses different leg designs for different reasons. Sometimes there are specific design goals, involving the workspace, manipulability, and dynamics, but sometimes the designs are less intentional, with choices made for other reasons.

Petman, the predecessor to the Atlas series was designed to mimic human kinematics for testing protective clothing. The iCub series were also specifically mimicking humans rather than designed to a performance spec, and the first generation of those could not walk.

Hurst (the lead person behind Cassie which is the robot you pictured) switched from his previous approach (Atrius) to the bird approach after collaborating with people that studied bird walking.

http://jeb.biologists.org/content/217/21/3786

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  • $\begingroup$ Do you have any sources for "mostly aesthetic"? $\endgroup$
    – FooTheBar
    Apr 26, 2018 at 11:07
  • $\begingroup$ Aesthetic was a poor choice of word. Added a couple examples of projects where the goals were not performance driven. $\endgroup$
    – hauptmech
    Apr 26, 2018 at 13:52
  • $\begingroup$ You keep mentioning these "reasons" but you never actually explain what they are. Why does a roboticist choose to shorten a femur and lengthen a metatarsal? What kind of mechanics has been solved that shows this structure is more efficient for which action? $\endgroup$
    – John Joe
    Apr 26, 2018 at 16:07
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    $\begingroup$ The point is that robot designs are not always optimized for the same goals, and sometimes the goals are not performance ones. For the specifics of the choices made for the robot pictured, you can read the paper I linked or one of the other papers that group wrote, or you could ask them directly. $\endgroup$
    – hauptmech
    Apr 26, 2018 at 19:15

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