https://theslyd.tumblr.com/post/159067861786/tutorial-how-to-fix-under-arm-glitches-under-arm

Tutorial: How to fix under arm glitches

Under arm glitches are often caused by incorrect weight paint. Read **Weight transfer in Blender** and Tutorial: How to do a SMOOTH weight transfer first to understand the process of weight transfer.

Your new mesh and the base mesh you’re transferring weights from are not identical. Weight value on some vertices that works on the base mesh don’t necessary work on your new mesh when the sim moves.

Open your mesh in blender. Make sure you’ve done weight transfer.

1. In the Properties panel, click the wrench icon to open Modifiers.

2-3. In the Armature Modifier named ‘geo_skin’ (your mesh should have one if it’s exported from sims4studio), make sure value of Object is ‘rig’.

4-5. Select the rig object and turn on the eye and arrow icon to allow you see and select the bones in Pose Mode.

6. Go into Pose Mode.

7. Select the bone on the shoulder.

8. Press R key and move the mouse to put down the arms.

9-10. Now you see the glitches under the arms.

11. Select your mesh. In the Properties panel, click the triangle icon to open Data panel. Expand the Vertex Groups section. Select b_R_ShoulderTwist group. It’s b_L_ShoulderTwist if the glitch is under the left arm. (Update: I made a mistake. This step should be after #13. Otherwise when you go into Weight Paint mode, a different vertext group will be automatically selected.)

12. Go into Weight Paint Mode.