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I've been thinking of creating a setting similar to that of BattleTech, but set some time around when the industrial revolution is in full swing. This setting would also include bipedal mechs of various sizes, shapes, and weights. When I say mechs, I mean "walking weapons platforms", and not giant human/gundam mechs.

Advantages/disadvantages of such mechs aside, I wonder if there is a valid reason for a bipedal mech to have reverse jointed legs. As an example, think of an Atlas from BattleTech, with its regular jointed legs, versus a Daishi/Dire Wolf with "inverted" knees. Both are 100 ton mechs.

Is there any practical reason other than flavor as to why such a mech would have inverted knees? My wholly uneducated pure speculation would be that a regular jointed mech would have an easier time scaling uneven terrain or steep cliffs and would have overall better balance, whereas on a flat surface a reverse jointed mech could theoretically reach a higher maximum speed. Does this idea have any merit or would it be purely for design variation and fluff?

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    $\begingroup$ Do you actually have a detailed mechanical design for your machine? Because if you don't, there is no way to have a meaningul discussion about such a nebulous concept as "reverse jointed legs". Reversed with respect to what? The legs of your machine will very obviously need a minimum of three joints (one with the body, one between leg segments, and one with the feet). For evolutionary reasons tetrapod legs have six joints, with different degrees of freedom and preferred direction of motion. (In tetrapod legs, counting from the body, the first four joints have alternating preferred directions.) $\endgroup$
    – AlexP
    May 4, 2022 at 17:07
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    $\begingroup$ I think the terms you are looking for is: digitigrade vs plantigrade [Pros and cons of having digitigrade and plantigrade legs?][1] [1]: worldbuilding.stackexchange.com/questions/153072/… $\endgroup$
    – Gillgamesh
    May 4, 2022 at 18:09
  • $\begingroup$ So, ED-209? The guns would get in the way of the 'calves' when couching. Cassie ("the most evolved bipedal robot on the planet") looks like it's walking on its tip toes, which the link above talks about a lot, and it can crouch putting its groin almost all the way to the ground. futurix.it/2018/03/… ... Toy AT-STs would fall over if they weren't (permanently bent into a crouch). I feel like they'd be easier for a computer to balance (?), but as soon as we could we went straight into trying to make them all walk like humans. $\endgroup$
    – Mazura
    May 5, 2022 at 1:36
  • $\begingroup$ Atlas from BattleTech, with its regular jointed legs, versus a Daishi/Dire Wolf with "inverted" knees ... vs ED-209 which has thighs on gantries not knees, vs AT-STs which have five joints in their legs. If it doesn't have ball jointed hips that can swing, the legs must extend and contract. OCP and SW win vs Batteltech. $\endgroup$
    – Mazura
    May 5, 2022 at 1:49

4 Answers 4

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Joints

Let's say these fine engineers have taken clues from biology and are trying to select some joint for their walkers. They would likely be looking at various "knee joints" in the animal kingdom.

The big news here is that many of these "knees" are not knees at all. Frequently, these other animals are walking on wrists or even fingers. They all serve a similar purpose: to change the distance between base and top of the limb. In this, there really isn't anything stopping one or the other from performing just fine.

A Fine Point: Docking!

If your walker needs to raise/lower next to a wall, tower, or building, which I will call performing a "docking" maneuver, reversed knees come in handy here. Reversed knees allow you to walk up to the structure and raise/lower without repositioning the torso.

You can try this with your own legs and a wall: attempt to keep your torso as close to the wall as you can while squatting down. If you face the wall and try to lower your torso, note how your legs (knees) must go forward and push your torso away from the wall. If you face away from the wall and attempt this same exercise, you can raise and lower yourself and stay much closer to the wall. When you are facing away from the wall, you are like a digitigrade (reversed knee) walker.

Since mobile things tend to encounter obstacles and "docks" in front of them instead of behind, there is more likely to be space behind for the joint to move. It's a small difference but may help in certain situations.

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The difference between a knee and an ankle

Forward bending joints and rear bending joints on rear and bipedal legs have separate purposes in the animal kingdom. The knee helps us adjust how far our foot is from the ground. The ankle absorbs impact elastically and redirects the energy into the next push. This is why horses and kangaroos look like they have backwards-bent knees. Those are actually their ankles.

So, for something as big as a mech, a longer "foot" -- a backwards-bent knee -- would be essential for absorbing the weight of any two-legged mech that was capable of actually "running," with running defined as a gait where both feet are off the ground at the same time.

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    $\begingroup$ I like this explanation. Where are the "shock absorbers" in a mech? I'm sure we can think of a few possibilities but building the shock absorption into a lengthy "backwards knee" configuration seems quite reasonable and surely allows for a greater depth of absorption without compromising forward movement. $\endgroup$
    – JamieB
    May 4, 2022 at 20:21
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    $\begingroup$ A million upvotes, if I could. Digitigrade legs are not "reverse articulated" or anything like that. They walk on their "digits" (i.e. toes), and that "reverse" bend is the ankle. $\endgroup$
    – T.J.L.
    May 5, 2022 at 12:24
  • $\begingroup$ A million upvotes, if I could. - well, here's one more. $\endgroup$ May 5, 2022 at 14:13
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    $\begingroup$ Thank you! Look up "Jumping stilts" for some fun videos that take advantage of this principal. $\endgroup$ May 5, 2022 at 16:10
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    $\begingroup$ Typo: gate -> gait? $\endgroup$
    – void_ptr
    May 5, 2022 at 18:12
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Your knees are always in the way when you're squatting down and trying to see and work in the area on the ground in front of your feet.

It would be much more convenient in those situations if they were out of the way behind you. The best alternative is to kneel, and nobody wants their mech to do that!

To a 100 ton mech, people and many other people-sized things are small, and will often need to be manipulated in this position.

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Let's look at the anatomy of a pair of Mechs in popular fiction:

MadCat Mk2 & VF-1 Valkyie

The savvy one might recognize the iconic MadCat Mk2 from Battletech/Mechwarrior and VF-1 Valkyrie from SDF Macross in GERWALK mode. Both seem to have reversed Knees... but do they? Red appears as the upper leg, yellow the lower, green the foot - but they are not having a knee in the traditional sense.

The "legs" of the Valkyrie are not legs at all, they are Thrusters, and the joint between the red upper leg and the yellow lower leg is actually at its furthest extent pushed to the front at the moment in the picture. The Valkyrie doesn't "walk" they skate on the exhaust on this configuration, and the next configuration to humanoid does prevent this "under sweeping" of the legs under the torso.

The MadCat Mk2 baffles with its anatomy, the legs being apparently reversed, but there is genuine engineering thought in the madness here, and on a really technical basis, this mech behaves completely differently from normal anatomy: The red upper limb rotates around the hip joint to go forward and under the body, while the lower limb rotates also forward but faster to get off the ground, and a little sideways to get direction done. The result is, that the green foot is pushed forward and lifted. Now, yellow rotates backward, presses the foot onto the ground, until the weight is on it. As the other foot does the lifting cycle, the whole Mech's body moves down as the red-Yellow and red-hip joints rotate simultaneously reverse to the first movement. That's not how a leg or knee works, but it's how your elbow does move your arms - with some twisting coming from that flexible joint. Yes, technically the MadCat Mk2 walks on arms when it comes to anatomical comparisons. And you totally can be done as a controlled fall forward which is exactly how the MadCat Mk2 bobs down during each step.

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