At this point, you’re just trying to ridicule me over my choice of words and not actually trying to interpret them in the context that you yourself set:
they have a sack of muscles somewhere inside their body
Why mention “inside their body” if you didn’t mean “deep” inside? All organs are “inside” the body. Therefore, I interpreted your words meaning truly “internal” organs, that that don’t manifest themselves on visual inspection, like heart or bladder. Lungs, while technically inside, are peripheral and visibly expand - a critical distinction in this context.
So you specify “inside” and then mock my adherence to that framing, instead of addressing the core biomechanical issues being discussed.