Mesh distorts and stretches when posing with vertex groups

I am trying to pose my model, but whenever I rotate the bone, it stretches the mesh, and distorts it. I used vertex groups in order to assign the weights.

These are pictures of my character’s arm, displaying the problem.

Resting position in Object Mode:

Bent at roughly a 90° angle using Pose Mode:

My vertex groups in Edit Mode with weights displayed:

As you can see, in the second picture the arm stretches, and there are odd bumps/ripples when rotated.

Can anyone please help me understand why this is happening, and how I can fix it? Thank you.


you need to weightpaint the mesh for every bone. At the moment it looks like you did it only for the forearm and not for thre upper arm. Autoweights can give you a good start.
Try also to blend both weights better… red>yellow>green>cyan>blue to smooth everthing.
This question would have been better placed in forum animation an rigging i believe ;).
(edit) … your mesh is realy ugly for weightpaint and animation. model it clean with nice and even edge loops and edge rings.