How the limbs should be correctly positioned in the body of a "quadrupedal hexapod" humanoid?

Question

This question basically is about problems with combine the question Anatomically correct multiarmed humanoids but turning the position to quadrupedal and specializing one pair of the two pairs fo arms to other work.

This creature could arise thinking if is completly necesary is the upper part of a centaur should be erected over the horse body. So I thougth maybe a more floor leveled position might still useful for other kind of fictional races.

That's why I made a set of schemes about how should be the arrangement between the two pairs of forelimbs, one pair as arms and the other as legs.

The implied problems for this, are the viability of this kind of arrangements, if they really are able to work as is required or if is needed to change the arms' positions to permit it working and other body's proportion changes.

This practically can be like a centaur but removing the problems of second erected torso but getting the problem of have more limbs in a littler space and each with an specific specialization.

The question implies, how to correctly place these extra arms in a quadruped position without interfering with each other and that each pair is useful in a different way?

This also might require to think about how long should be the arms, and optionally to think how turn bipedal keeping the arms useful.

This question might be related too, How to design a humanoid with facultative quadrupedism?

Answer 1

Bird + Lizard = Solution

A simple solution to your problem could exist by combining and altering two different shoulder-types into a single girdle with 4 joints

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Specifically, you would first take the coracoid and scapula of a bird and a lizard and put them together on the same ribcage. The arms would go on the bird shoulders, and the legs on the lizard shoulders

The musculature should be easy to rearrange: Forwards and backwards muscles for the limbs shouldn't be affected, and so you wouldn't need to do much with them. As for the vertical muscles:

The ribcage would have to be extended vertically, and the muscles to move the limbs inwards would be attached to the bird-coracoid in the middle, perhaps with some backwards extension to increase the muscle size. It also seems plausible that the arms' pectoral muscles could slip under the leg muscles, for a more normal arrangement

The arm-raising muscles can be kept bird-like, or could use a more typical system with the muscles on the back as in humans and other mammals. The leg-lowering muscles can just be connected to the sternum