How do you fix a shoulder caving in on itself?

Howdy! I’m at the tail end of working on a model of mine, but there’s one last thing I’m not sure how to fix.

When I bend the arm upwards like so, a cavity begins to form. This is probably because I built this model in an A pose instead of a T pose. My problem is that no amount of weight painting seems to make this better. I’ve been able to get the cavity to be smoother, but not make it better. Rotating the shoulder bone does help, but the cavity is still very noticeable.

Here’s what I’ve tried so far:
Shape Keys - Seemed like the right answer at first. Just set up some drivers and fix the shoulder manually and we’re all good. However, edit mode reverts the model to its position before it was posed, thus leaving me blind if I was to try this. Skulpt mode seemed like the fix from here, but I found my ability to manipulate the model in this mode is very limited for whatever reason (Probably just my own unfamiliarity if I’m being honest)
Royal Skies - I watched his tutorial on how to fix issues like this on a shoulder and they were very good, the problem is that his method only works when a character is in a T pose due to the values present in his formulas. While his method could work, I’m not sure what kind of math I’d have to add to his drivers so that I have the proper offset.

I figured instead of bashing my head on another wall, I’d simply ask for help. I imagine there are probably dozens of ways to do this so I’d love some advice! Thanks in advance!

There’s an excellent add-on called RBF drivers that makes the shape key solution you’ve found much easier to work with and visualize. I’d highly recommend it. If you’d rather not buy that, shape keys really are the only way to get good shoulder deformation, so you can just use your method blind. Shoulders are the worst part of the body to animate by far, I wish there was a better option than shape keys, but because of the way the human shoulder works, there’s no really a way to build it in an armature without like 8 bones per shoulder

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This kind of issue typically occurs when the Armature of your character has Unchecked: Preserve Volume feature.

Select the character’s mesh Object, then check for its Armature properties at the Modifiers Properties Tab in the Properties Editor; you’ll be able to find that box to check. In certain cases, this single feature may solve up to 100% of the issue.

If this doesn’t fix the issue, then the solution might be a bit more tricky. And this could be a huge topic.
Personally, what I’m doing to solve those Organic Rigging issues is to work with additional, well-place Bones, that share & concur strategic Weight Paints with the “main Bones” (of the limbs) on those complex region, but those extra Bones have special Bone Constraints —instead of Drivers; examples of Bone Constraints would be “Copy Rotation”, “Strecth To”, so that you can apply “Corrective Bones” to your model on complicated Poses. Think, like, as if your common Armature is more like a “stick figure” Armature, but in order to have proper Organic Rigging, you need, besides the stick figure, some degree of a “volume” Armature; because not naturally the stick figure Bones (with their “one-dimensional” Weight Paint) will handle all the complex shapes required.
Currently, I have had good results with this for the hips and thighs regions; for the shoulders region, somehow I didn’t even require extra solutions; but I have that structure spine + shoulder.L + arm.L Bones… and the mesh Topology and Weight Paint of those main Bones is delicate. I could probably benefit from using the method there too… but I’ll require a specific research in order to understand what is to be improved.

Feel free to ask anything, I might bring up some examples, but it’s a complex topic. So, first, you should check that Preserve Volume thing, because this mostly fixed my problem and it is something that we can easily forget about or don’t even know about.


Preserve volume did the trick. The shoulders are just fine now. This did end up messing with other parts of the mesh to an extent, but now it’s nothing that a little more weight painting can’t fix. Thanks a bunch!

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I have a hard time seeing in your image: does your rig have an actual shoulder bone, like this?

The bone of the arm has a limited range (in real life): it doesn’t go higher than 90 degrees, anything above that is the shoulder bones rotating. So basically, it’s possible to to rig a character pretty convincingly even without preserve volume, but you need the proper shoulder bone placed in the right spot and knowledge of what range of motion the arm can do before the shoulder starts rotating.