Arms scrunch and lose volume when armature moves I have a simple humanoid armature: no IK rigging yet and it worked fine until the arms started messing up when I try to move them.  As far as I can tell there is nothing wrong with the weight painting, I even cleared all the weights off of all the vertex groups in the arms but when I repainted them the same thing happened.  The mesh doesn't follow the bones at the same speed and deforms wrong and I have no idea why or how to fix it.


The only modifier I have active is the armature so its not a stack order problem, and none of the other bones have any weight on any part of the arm so I really just have no clue what is wrong here and any help would be greatly appreciated.
EDIT: Here is the .blend file
Your arm is affected by two extra bones:
Spine.2

Spine.1

That's why your mesh only follows the arm at &quot;one-third speed&quot;; the other two-thirds of the influence comes from these two Spine bones.
If I quickly clean up the Spine bone's influence, the arm moves correctly.

Btw, one quick way to check what bones influence a vert is to select the vert in Edit mode and look in the sidebar under the &quot;Item&quot; tab. You can see a list of all groups the vert is assigned to and their weights. See the extra Spine.1 and Spine.2?