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Allow me to apologize in advance for the layman's terminology.

I'm wondering what the anatomical term for a cat- or a goat-style hind leg is.

Cats, goats, t-rexes, and many many other animals don't have human hind legs (i.e., with one knee / one pivot point). They have two pivot points, with one bone going down, then another going back-ish, then another going down again.

I've googled and googled for the answer, but I'm completely at a loss as to what this is called! Is there any name for that style of leg? I'm sure there's a lot of variation inside the category, but there is a category, correct?

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Welcome to Biology.SE!

I think you are talking about plantigrade, digitigrade and unguligrade.

Please note that the number of joints in mammals does NOT vary, but only the relative length (and shape) of the different parts of the leg.

enter image description here

  • A Plantigrade walks on the sole of the foot. 'Sole' translates to 'planta' in Latin hence the name.
    • Examples: Human, squirrel, raccoons, ...
    • Below is a chimpanzee skeleton

enter image description here

  • A Digitigrade walks on the digits (the toes).
    • Examples: dog, cat, hyena, ...
    • Below is a cat skeleton

enter image description here

  • An Unguligrade walks on the nails. 'nail' translates to 'ungula' in Latin hence the name.
    • Examples: cow, reindeer, goat, ...
    • Below is a cow skeleton

enter image description here

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    $\begingroup$ I think the final point you make (about the actual structure being invariant) is a major one and should be moved to the top so it isn't missed. The rest works well as an explanation of how that can be in spite their apparent dissimilarities. $\endgroup$ – Harris Sep 2 '16 at 18:25
  • $\begingroup$ I agree with you. I moved it on top. I thought the same when writing the answer but did not really know how to format. Hope that looks good enough. Feel free to edit to improve the format if you want. Thanks $\endgroup$ – Remi.b Sep 2 '16 at 18:41
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    $\begingroup$ I'd say that sentence placement, being immediately followed up with a side-by-side comparison chart, is perfect. $\endgroup$ – Harris Sep 2 '16 at 18:46
  • $\begingroup$ WOW I never noticed that they're all technically the same thing! That was more than helpful and extremely interesting! Thanks so much! $\endgroup$ – mkrell Sep 3 '16 at 21:16
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Birds are also confusing for a lot of people.

  • Specifically, they assume that birds only have a two-jointed leg and therefore their knees "bend backwards."

enter image description here

However, like all tetrapods, the leg structure and number of joints doesn't really change a whole lot:

http://sunny.moorparkcollege.edu/~econnolly/amphibiantetras_files/image010.gif

enter image description here

Copyright Scott Hartman 2011

The confusion is a result of the fact that -- similar to what @Remi.b points out in his answer -- birds are generally digitigrade animals. [see here].

  • In the case of the "backwards bending" bird knee: that's really the ankle joint.

    • The bird knee joint between the femur and tibiotarsus points forwards, but is hidden in feathers.

    • The bird ankle joint between the tibiotarsus and tarsometatarsus points backwards.

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