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.