I'm sorry to ask many questions.

I want to make a model in which the chest fluctuates due to breathing while walking.

I want to give the model a walking animation and edit the chest frame by frame.

I'm trying to automate this task with a script.

I need to select the chest to deform the chest from the script.

There is no problem if the body parts are separated, but I am thinking of importing a model that does not have separate body parts.

In that case, I'm thinking of creating a chest as a prop in the "setup room" and selecting it from a script for editing.

But,  when animation is added, the position of the chest props shifts.

I want a model that looks natural.

I want to set the chest-shaped props to match the movement of the body.

I also failed using the parent feature.

How can I adapt the chest-shaped props to the movement of the body?
Or is there any other way?

Thank you.

Looking at several of your threads and the theme is using Python to move bits of the body around and then exporting the resulting mesh.  If it was me, I would take the model use the morph brush to move parts of the chest outwards and then use the resulting dial to move the chest in and out along the timeline.  The advantage is the mesh stays together with the rest of the body and you can make bits expand more than other areas.  Then use your python to change the value of the morph channel so that it moves i and out.

Showing Morph Brush Breathing by Richard60 (renderosity.com)

I am pretty sure this the effect you are trying to achieve.  In the video link above I made a very quick morph and then applied it along the timeline about 1/3 strength otherwise it would show spikes etc. as this goes across several body parts. 
And it does not tear as in your pictures, since it leaves the body parts attached to the rest of the body.

And you can move to each frame and export the model as an obj for use where every else you seem to need to, and it will have the movement baked in.

I agree with Richard60, morphs are the way to go for these kinds of things.

Looks like you are using the Poser 4 Nude Male High res (Dork)

While it's probably best to learn how to create morphs, here's a simple Breath morph injection file as an example. Just put the included 3 files together in any pose library of choice and apply the pose to Dork. The pz2 calls on the pmd but the file path is absolute. This will cause a morph dial to appear in the Dork's Body that causes a breathing deformation. There's limits set but you can change those in that dials settings if you so choose...

Dork Breath DL

This looks perfectly fine, however, a normal person does not chest breathe alone; the diaphragm pushes down on the abdomen as a breath is taken causing the belly to distend as well.  You can get away with it if the model is completely dressed with loose clothing but shirtless the action looks wrong.

This looks perfectly fine, however, a normal person does not chest breathe alone; the diaphragm pushes down on the abdomen as a breath is taken causing the belly to distend as well.  You can get away with it if the model is completely dressed with loose clothing but shirtless the action looks wrong.

The change you suggested.

Update Dork_Breath

The Belly_Distend is a separate dial rather than baked into the Breath_Chest. There's a controller master that drives both simultaneously or you can dial to taste separately. As before limits can be changed. The dials (including the controller master) work in both positive and mild negative at default.

If you want Adams Apple motions and neck artery pulse that'll be 2 bucks :)...

Sorry for replying late.

Thank you very much for your response.

I am very grateful to you for making a morph breathing model sample.

I think this expresses realistic movement.

The methods you have taught me get me closer to what I want to do!

thank you very much.