Arm bending physics

I don't know, but some ideas if you want to experiment. (Fundamentally CH is 2d animation, so does not have much support for 3d effects like foreshortening.)

- I wonder if you can restrict movement near the elbow with sticks - e.g. put a very short stick from elbow a short way up the arm so the rest of the arm shrinks etc, not the bit near the joint.

- I was wondering if a small stick across the arm (like a bandage wrapped around the arm) near the elbow would help stop shrinkage of the rest of the arm reach the joint - but I suspect it would just move the stick.

- Try some short sticks along both edges of the arm near the elbow, again to try and discourage CH from stretching that area in preference to the rest of the arm

- The final fallback is to create a second arm and put it in another swapset - you draw it with shorter arms! It is sort of "lets bypass the whole problem by replacing the artwork with custom drawn artwork for the effect I want!" - this is more effort, but almost guaranteed to work. The puppet can slow down too if done too frequently.

I understand what you are saying.

I don't know any more. I'm sorry.

We need the support of someone with more expertise.

I'm not good at English. So it may not be the answer you want.

1）I added the arm movements based on your PSD file.
2）I used the stick tool to add the bones.
3）I added Limb IK behaviors .
4)  I set the elasticity to 0% in the IK parameters of the limbs.

I think the arm no longer stretches and contracts.

I uploaded the puppet below.

https://www.dropbox.com/s/l21shib63pv20b4/edit_%D0%A1%D0%BD%D0%B8%D0%BC%D0%BE%D0%BA.puppet?dl=0

An explanation video has also been uploaded to YouTube.
https://youtu.be/9nFho3G7DQI
Thank You.

One trick that sometimes works is the cut the artwork into two groups at the elbow (upper arm and lower arm) then arrange one layer above the other. No independent group needed, just organize the layers so one is above the other. Might need some fiddling and experimentation, but it gives you a bit more control. (It is not a perfect solution.)