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Humanoids with more than 2 pairs of arms weren't an unknown concept in human culture. From ancient mythology, there are the hundred-armed Hekatoncheires (who have their own thread here) and six-armed Gegenees from Greek myth, the many Hindu deities that were depicted with as many as a thousand arms (such as Avalokiteśvara).

And from modern times, there's the six-armed Spiral from the X-Men comics... Spiral from *X-Men*

... the few times Spider-Man) ended up growing four extra arms (as one step among many in a gradual mutation into a "Man-Spider"... Peter Parker as a six-armed Spider-Man (right), and as the Man-Spider (left)

... and the Shokan race from Mortal Kombat.enter image description here

Science considers any real-life instances of this as falling under the umbrella of a medical disorder named "polymelia", which applies to all limbs rather than just arms. Personally, I'll draw upon both that term and the name of the aforementioned Hekatoncheires to coin the terms "polycheires" and "polycheirid" (lit. "many-handed one") for all more-than-two-arms humanoids, with "poly-" being replaced with the appropiate numerical prefix when a particular number of arms is specified (e.g. "tetracheirid" for four arms, "hexacheirid" for six arms, and so on so forth).

Now, with that introduction done, we come to the big question: How does one design an anatomically correct musculoskeletal system for a polycheirid? What I've managed to gather on the subject makes it seem as if even adding a single extra pair of arms would require significant modification to the human(oid) torso's muscles and bones to be possible. Most importantly, the idea of having two or more arms sprout from the same spot is apparently impossible from an anatomical standpoint, despite it being a somewhat common design approach in modern fiction.

Supposing we do figure out the anatomy, what can one expect from the polycheirid in terms of the arms' movement range, ability to apply force, and so on so forth?

PS: For the record, I would've added this to the Anatomically Correct Series, but the page (and the entire Meta Worldbuilding section) seem unaccessible to me at the moment.

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  • $\begingroup$ Not directly related, but it's odd that the Na'Vi from Avatar did not also have six limbs, as quite a few of the other creatures there did. They probably should've.However, it seems like whatever the evolutionary pathway was there, it must've split off earlier than on Earth (here all mammals, bird, and reptiles still have four limbs, as you've no doubt noticed, but 'bugs' of all sorts frequently have more). $\endgroup$ – N Choe May 12 '18 at 2:38
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This is quite a difficult question to answer, for the sake of the bigger picture. You see, the arms require skeletal support (primarily the clavicle and scapula).

human skeleton

As well as muscular support (all the muscles in the arm and the pectorals on the chest).

Muscular system

So, what you would need, is a skeleton with seemingly redundant bones to support the limbs spread out to ensure it can support this - and you'd have to add more shoulder breadth to allow for the added arms to rest comfortably, unless you plan on making this 'human' walk on all six (or more) limbs.

Once you have the bone space, you'll need the Agonist and Antagonist muscle pairs to allow for movement. As with the pectorals, I would advise for this to be placed over the bones to allow for muscle growth as needed.

Problems this will create? Not exactly something you'll want to ignore, after all. Well, the ones I can come up with (and I'm no biology major), are fat distribution and the health risks this will pose. You see, in humans, we allow for subcutaneous (under the skin) fat layers, but only in certain places, dependent on the dominant hormone of the specimen - testosterone allows for more fat on the belly, oestrogen allows for mammaries, hips, butt, and thighs. Given these muscles will create an issue with at least the testosterone level of the species, excess fat on the tummy is going to be an issue.

So how to distribute the fat? I'm not sure. Wolves, for example, keep fat around the vital organs:

A well-fed wolf stores fat under the skin, around the heart, intestines, kidneys, and bone marrow, particularly during the autumn and winter.

As quoted from this source. But fat, while healthy and needed, is also bad for the specimen in the long-term (heart disease, high cholesterol, etc.), so you want to keep it away from vital organs. How you solve that... well, I guess I'll leave that to you.

As an aside, there is an artist on DeviantArt that delves into stuff like this (though more commonly anthropomorphic animals and making them anatomically correct). It might be worth checking his art, see if you can get some inspiration.

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  • $\begingroup$ When you say "redundant bones", do you mean "extend the torso vertically"? Because I've been contemplating that approach for a while. And your advice to place the extra muscles on the bones implies there are somehow other places to put them than on bones. Also, why would excess belly fat be an issue for the extra arms when they project from the chest's sides? (Unless you're referring to the cases where they emerge from the abdominal region, as with Spider-Man... which is rather implausible anatomically, IINM.) $\endgroup$ – MarqFJA87 Mar 17 '17 at 21:31
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    $\begingroup$ Redundant bones--bones that would otherwise have no use. So I'm suggesting closing the ribcage and adding the two bones I mentioned to stabilize the limbs. The muscles over the bones, because if you intertwine them any other way, you add pressure on the vital organs or digestive tract. Belly fat is an issue, because you are sacrificing space for muscles to work the limbs--even if you only add one pair, consider how much more powerful the pecs will have to be, then add more muscles for the added limbs. There isn't space left for fat in the 'males sex'. $\endgroup$ – Fayth85 Mar 17 '17 at 22:30
  • $\begingroup$ I see. What about vertical extension of the torso, particularly the ribcage? It would provide space for the extra bones and muscles without those problems you pointed out, wouldn't it? $\endgroup$ – MarqFJA87 Mar 18 '17 at 11:14
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    $\begingroup$ @MarqFJA87 It might, but nothing is without consequences. Adding height adds weight, which requires stronger muscles to keep it all moving, which requires more energy to keep in motion, which cuts into the fat reserves because there would be no 'excess energy intake'. You need to see this as a 3D puzzle--everything must stay in balance--adding something takes something else away. There are evolutionary reasons why two arms makes more sense (energy requirements to benefits ratios, and the like). But if this is something you want done, then yes, extending the torso would do the trick. $\endgroup$ – Fayth85 Mar 18 '17 at 13:40
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    $\begingroup$ Just remember what I said. Add shoulder breadth for the upper pair, so that the lower pair can sit comfortably beneath them. Otherwise you'll have a very uncomfortable 'idle stance', which costs more energy that is needed. And try to imagine what a being--having said extra limbs from birth--would do with them when they're not being used. Are they folded across the chest? Do they swing with their gait? Are they hyper-aware of them being there, or are they kind of just... there? And do not forget the higher learning curve to properly use these limbs--can they learn to write with them? $\endgroup$ – Fayth85 Mar 18 '17 at 13:44
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I think Burrough's ‘green martians’ were probably one of the better designs for this. E.g. Tars Tarkus.

Here are some images from the 2012 cinematic version:
Tars Tarkus — anterior — arms outstretched Tars Tarkus — anterior — arms down

Here is Tars on the cover of ‘Gods Of Mars’:
Tars Tarkus and John Carter back–to–back

I did not remark the source for this one.
one green–martian — rightly anterior — with spear and shield held by both intermedial arms

Here's a version which doesn't resemble the original depictions so much as do those from the 2012 movie, but which does show the characteristic structure as I wish to proffer — albeit exaggerated and rather bulky:
green–martian –esque — bulky and ruddy — anterior — full body

Here is one more for sake of variety:
Tars Tarkus with lavish spear
Source: http://the-first-magelord.deviantart.com/art/Tars-Tarkas-WIP-255092129

My idea has always been this: If you wish to add a pair of extra arms, then simply extend the height of the thorax by addition of clavical units.
To reduce the total height, you redistribute and reshape the internal organs to accomodate the new bones — all as compared to the human structure.
Of course — and similarly to what was stated in this answer, — you will need to provide additional scapulae and sternum on which to anchor the muscles for the arms.

Flexibility and strength of any limbs depends, of course, on the mechanical advantages of their leverage in relation to the centers of mass.
Well, anyway, it seems that the Green Martian design overcomes the problems with crowding extra arms on a humanoid frame by simply adding height: the original descriptions put them at 10 or so feet (3 m) tall.
The curvature of their vertebra would need to be somewhat different from ours — and not simply stretched vertically, nor like two torso stacked one atop the other. The added load atop the bottom vertebra would probably require that they be broader so as to distribute the forces over a larger area, for one thing. Too much curvature at the bottom would be difficult when maintaining an upright posture, so you'd probably sacrifice some flexibility there for better stability and support.

Unless you want to go for some vertebrate polypede thing, such an approach does limit you to one additional pair for concern of height.

It should be noted that the original description of these Green Martians also allowed them to use their intermediary limbs as legs, if they so need. Thus, their bones were probably not quite like additional shoulders so much as a hybrid between shoulders and hips.

Furthermore, note that not all depictions of Burrough's Green Martians have been portrayed so novelly. This one looks like a humanoid, and the lithographer obviously was skilled, but the extra arms are carelessly applied:
green–martian riding thoat — frontispiece for 1920 Chicago edition of ‘Thuvia, Maid Of Mars’

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  • $\begingroup$ Note in many cases the upper set of arms is also wider set to allow them a full range of motion, this however does limit how far you can raise the lower ones which will be true no matter what you do. you will never be able to hunch the lower set of arms. $\endgroup$ – John Apr 11 '18 at 5:27
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I think your guy will look something like this, skeletally and muscularly. This is crude, but you can get the picture.

The skeletal and muscular changes are necessary to accommodate the extra arms. You CANT just stick them on the side and hope they work. What happens inside the body is what makes the arms work.

EDITs: Edit 1 had no Abs, no mid section. This change is more like it. However, as you can see it means he has a disproportionate upper body mass. His legs MIGHT be able to take the weight stress IF they are much more powerful/muscular, but I'm more concerned about his heart, hips and knees than I was before.

Initially I was thinking one ribcage that covers the whole area (even though the art makes it look like its two), but now I'm thinking that maybe it does need two separate ribcages to buy back some torso mid-section flexibility. So the art might be more accurate than I thought at first.

EDIT 3 - Final art:

The artwork was bothering me so I decided to put this together, which is the best visual description yet, I think. Just imagine a more powerful lower body than upper body, to carry the weight, and this makes some kinda sense. The greater leg strength might also make him faster.

enter image description here

Enjoy!

Edit 4: I know I said I was done, but then I had some additional ideas for alternate ways a multi-armed humanoid might work:

More arms from the same area: Imagine the extra set of arms from the same region of the upper body, but from behind instead of from the front. So you'd have arms from the chest and from the back. The back arms might even be able to reach backward!

enter image description here

enter image description here

And also - Arms at the hips: This came about because I thought maybe too many arms at the shoulders was making the upper body too heavy, and the arms would kind've bump into each other, so... arms at the hips. I don't know how the pelvis inner skeleton would accommodate that, but there is enough space in the area for some changes, so I'm gonna say not IM-possible.

If the arms at the hips were smaller than the regular ones, they might not be quite as strong, but they might still be very useful and they wouldn't get in the way of the upper arms.

Enjoy!

enter image description here

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    $\begingroup$ Thanks for the effort and the multiples pespectives, I for one really appreciate this $\endgroup$ – GlorfSf Apr 9 '18 at 14:44
  • $\begingroup$ You're welcome, and thanks. I've given this sort of thing lots of though in the past because whenever I think of multi-armed humanoids or aliens I always think of what would be happening "inside" the body too. It complicates things, but it also makes it more real and more fun. $\endgroup$ – Len Apr 9 '18 at 14:50
  • $\begingroup$ For fun you guys might also like worldbuilding.stackexchange.com/questions/107781/… $\endgroup$ – Len Apr 10 '18 at 19:23
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Unquestionably the skeletal and musculature foundation for functioning arms would have to be duplicated in some fashion, but there are a lot of variables to consider beyond the human-like arrangement offered in other answers.

It's worth noting that 'humanoid' does not require an endoskeleton. Take, for example, the Prawns (Poleepkwa) of District 9:

Concept art of Prawn/Poleepkwa from 'District 9'

Note the secondary anterior arms tucked into the lower thorax. The range of motion and utility of these limbs is certainly less than the primary arms, but they are positioned such that there's little interference between the two sets.

A similar arrangement of limbs on a mammalian/endoskeletal frame would likely entail the sternum bifurcating and descending to replace the costal margin (and secondary scapulae over the false ribs). This would require significant changes to the respiratory system as the lower ribcage and upper abdomen wouldn't be as flexible; diaphragm-based breathing might not even be possible without further changes to the ribcage.

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  • $\begingroup$ Your ribcage concerns are well taken. In my answers crude artwork for it the ribcage would either be A) longer from top to bottom (which would create some inflexibility in the a least the upper half of the torso), or B) be bifurcated as you say somewhere in the middle to give back some flexibility. However, one could argue that the humanoid might need that flexibility in the middle of his longer ribcage. After all, we don't. Maybe that's just the way he is. Maybe it just creates more room for internal organs. His heart will have to be stronger, but its not like it would have to be doubled. $\endgroup$ – Len Apr 5 '18 at 14:16
  • $\begingroup$ @Len The problem is introducing flexibility at or near a point that should be the anchor for pectoral (or pectoral-like) muscles. Too flexible and you risk breaking ribs or collapsing the diaphragm by picking up something too heavy. $\endgroup$ – rek Apr 6 '18 at 4:24
  • $\begingroup$ And It gets worse! It would need to twist too, not just bend a little! Its a constant second guessing game with this anatomy! $\endgroup$ – Len Apr 6 '18 at 19:12
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Four Arm Mutation

I was thinking of something along of an ant where there is a separate abdominal structure between each set of arms. This would would enable flexibility and independent movement whilst giving two fully functional sets of appendages.

This would also give sufficient space for pectoral muscles, shoulder blades etc enabling a full range of motion when compared to a stacked arm option.

As you can see I have assumed that the abdominal section between each set of arms is reduced when compared to a normal human and I would have thought the four armed variant would be thinner as it would have similar organs to distribute through a larger frame.

Four Arm Mutant

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Modify the lower torso

Use modified legs instead of trying to add extra pairs of arms.

Crabs' claws are the analogue of hands. Then they have 8 legs in addition.

Rather than trying to modify the upper body with all its vital organs, it is comparatively 'easy' to add extra pairs of legs at the lower end of an elongated spine. The upper pairs of legs stick out to the side and of course the 'toes' act as fingers. The muscular support is derived from extra hips.

Note:

Others have provided similar ideas but I believe that having extra hips instead of extra shoulders is sufficiently different for me to suggest it.

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