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Asking about the possibility of a humanoid having its feet be inverted, i.e. knuckle walking with its lower limbs. Basically that the foot looks like acute angle, going forward from an ankle/wrist, then back under itself towards the rear. We see this thing in quadrupeds fore limbs occasionally, but i cant think of an example on the back legs.

I could see it functioning as some sort of spring to aid in walking. Plenty of other animals have bent legs that act like springs, would it be to uncoordinated to have a straight section of leg then at the bottom a zigzag section. Or is there a lack of leverage and the being would have difficulty stepping off? Something like this, enter image description here

or this, but with a a more sensible number of joints in the leg.

enter image description here

And apparently the occasional fashion designer has done something similar. Branded as variously cantilever/floating heels. enter image description here enter image description here

I actually just found a model with some interesting Aimee Mullins. https://images.app.goo.gl/LHuZJLdRdcyMEanZ9

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  • $\begingroup$ Does it matter if this can exist on Earth within the limited understanding of evolution that we have? What does a "yes" or "no" answer mean to you? Not only has SciFi embraced this foot design before, I think it's cool. Are you worried about believability? Can you explain what problem you're trying to resolve with the question? $\endgroup$
    – JBH
    Commented Jul 17, 2023 at 0:24
  • $\begingroup$ Its more of a general question i guess. Seeing this foot design in several scifi shows makes me really curious about the ergonomics of it. Would the foot be like the Vagus nerve in that its not ideal but still works perfectly fine, or that its just the concept artists being fancy and isn't viable? All animals I can think of have a 'spring' in the top part of the leg, Having the 'spring' in the lower part seems similar at first but i have a gut feeling that there is some fundamental forces being to much and i just can't see it. $\endgroup$
    – tom Bebop
    Commented Jul 17, 2023 at 7:15
  • $\begingroup$ For future reference, general "is this possible?" questions don't work well here. Per the help center, our goal is to help you build an imaginary world of your own creation and Stack Exchange's policy is for people to ask specific questions for help overcoming specific problems. Questions of curiosity aren't what they want. From our perspective, if that type of foot evolved on your world, it would be possible and work fine. $\endgroup$
    – JBH
    Commented Jul 17, 2023 at 15:12

3 Answers 3

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An inverted foot design like you describe could work for a humanoid, but would require some anatomical adaptations:

  1. The ankle joint would need to be very flexible to allow the foot to bend back under itself. An additional joint may be needed in the "arch" area.
  2. The foot bones and connecting tendons/ligaments would need to be structured to support walking on the knuckles. They would need to be reinforced compared to a normal human foot.
  3. Additional muscle mass would likely develop in the calves to control the more complex foot movements.
  4. At the tips of the toes there may be hardened nodules or small hoof-like structures to aid in grip and pushing off.
  5. Balance would be a challenge at first as the center of mass shifts backwards. The being may need a counterbalancing tail.
  6. Stepping motion would involve carefully rolling the foot forward as body weight shifts ahead before lifting the foot. It can't just be picked straight up.

So in summary - it could plausibly work with the right adaptations, but the gait would be slower and more deliberate than a typical bipedal humanoid. The advantages could include shock absorption and ability to grip irregular surfaces.

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  • $\begingroup$ 1- interesting depends on how big the foot part is though doesnt it? 2- Human feet seem to have an unnecessary number of bones compared to other animals. I was thinking fewer bigger 'fingers'. 3- Good point, and more muscle on the lower leg would further inhibit running. 4- psuedo-hooves aren't that much of a leap. sounds reasonable as lots of animals have some for of foot pads. 5-if the mass shift backwards wouldnt we want weight out front? 6-Thats what interest me most, how does it step, i magine it would be quiet unnerving to watch, human-like but not. $\endgroup$
    – tom Bebop
    Commented Jul 21, 2023 at 13:05
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  1. I think you will find there is already a class of humans who go up on their toe knuckles. They are called ballerinas.

  2. I suspect that a creature knuckle walking full time will be doing it to defend the inside of the hand/foot. Given that the stress on the joints is very bad, this would imply a critical need to protect the inside. Possibly as manipulative members (hands).

  3. I suspect that to get this in a biped, it would need to start as a quadruped, develop the knuckle walking, and then develop into a biped.

  4. I think I saw a creature in a sci-fi story once that went both quadrupedal and bipedal, and did knuckle walking on its forelimbs. This was to protect the hands. (I'm afraid I can't give you a reference.)

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  • $\begingroup$ Comparing it to ballerinas says a lot about this concept. I have seen photo's of a ballerinas feet after a show, it is not pretty. $\endgroup$
    – tom Bebop
    Commented Jul 17, 2023 at 7:25
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    $\begingroup$ True, and why girls not full grown are not (these days) allowed to work en pointe. But you should also remember that, while they do it, they aren't really evolved to do it. $\endgroup$
    – David G.
    Commented Jul 17, 2023 at 7:29
  • $\begingroup$ I think the Demons (which are actually hostile aliens) from A Plague of Demons worked like that. $\endgroup$ Commented Jul 17, 2023 at 15:16
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I don't think this would evolutionary make sense, for 2 key reasons.

1: Centre of Gravity. We have our CoG slightly forward and so having a foot that extends forward is a stable platform and makes sense, the inverse, however not so much.

2: Running. Now, even if we assume that your creature has long since evolved from chasing their prey down on the plains or running away from a predator - such a design would make Running really difficult, especially at full sprint speeds.

That said...

If they evolved as an apex herbivore, with no natural predators (so limited requirement to run) and their mating ritual involved a pushing contest. Such a foot arrangement (digging one's heels in... literally) could be beneficial and explainable that way.

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  • $\begingroup$ That is kind of the conclusion i kept coming to, walking is probably fine but running gets the forces being applied up above tolerable levels to quickly. $\endgroup$
    – tom Bebop
    Commented Jul 17, 2023 at 7:22

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