Where there is no movement, breath is movement.
A brief anatomical lesson:
Your lungs are organs, and to that effect, they do not " breathe themselves".
Your lungs' resting state is that of a vacuum, and it takes a muscular effort from the "auxiliary muscles" ( so to say, the muscles that surround them) to pull them apart in order to bring oxygen in to your body. Let's think about this for a second... what "surrounds your breath?"
Well, first of all, it comes in through your nose or your mouth, and travels down through your throat canal, then lodges in your lungs, where it is converted in food for your blood, and is then released through your mouth or your nose, as the muscles that were holding you open relax back in to their resting state.
So we're talking about, oh, Face muscles, head muscles, neck muscles, chest and back muscles and ... does it go deeper? Let's see. Purse your lips and blow out like you're blowing out a birthday candle. Did you feel that? Ok, so let's include the pelvic floor.
How are these muscles involved? In different ways:
Up at the top, your nose/mouth/throat could be seen as your apertures, or : different openings to allow air in at different speeds. This is an area somewhat out of control, in that many of the articulations are created by nonspecific desires, such as " that smells great, let me open my sinuses and breathe deeply", or " I'm gonna sing this bass note for 4 bars now" a blending of both conscious and unconscious actions in the body, Both having very specific results>
Traveling down through the neck, your chest/upper back muscles are the primary ones involved in actively opening the body to receive breath. Think about how you spontaneously lift your arms when you yawn - a perfect example of unconscious movement for a specific and desired result.
Below the ribs, the abdominals and pelvic floor muscles come in to play, mainly because of their interaction with the movement the upward movement of the diaphragm, of the peritoneum ( the sac that surrounds your digestive system), and the resulting vacuum that pulls the lowest ribs down, and engages the transverse abdominus up towards the lower back.
To break down the organ movement, imagine two accordions on top of a water balloon. The accordions fill with air, through an active " pulling apart". The water balloon responds to a liquid or solid ( as opposed to air) volume, but can be easily displaced according to the pressure of the above accordions. These two organ systems are separated by a muscle called the diaphragm, which deserves a lot more attention than I am going to give it here.
As you ask your body to intake air, the muscles of your chest flex, responding to the allotment of air decided by your upper apertures, given a range of circumstances such as temperature, stress conditions, sound issues, or ease of bodily movement. They cause the lungs to inflate, pulling in air and expanding the space inside the ribcage. That pushes down on on the diaphragm, which is forced to inflate, and at the same time, depress the digestive organs, pushing them out below the ribcage.
After a pause, the body exhales, a controlled release of air. This is again achieved by conscious/unconscious actions of the skull muscles. The lungs deflate, they pull up on the diaphragm, which ( if left uninterrupted) pulls the peritonium up with it, and initiates the upwards and backwards movement of both the pevic floor and transverse abdominus as the muscular body is moved towards the spine.
That happens every time you breathe.
So here's where it gets interesting.
There is a term called " tidal volume"
It refers to the amount of breath that you need to keep yourself " not dead". In essence, at a state of basic human body function expectations.
It is demarcated by " apneas, as in, a pause in the breath. Ideally there is one between both the inhale and the exhale. It's usually the pace at which you breathe when your are sleeping.
Let's just think about that... if the basic amount of oxygen exchange necessary for human survival is the range we have when we're sleeping, then our stressed out "awake" breathing patterns are not even keeping us functioning at a level that supports survival.
Tidal volume is about 35% of your lung capacity, located in the middle of your range. Above that is called "Inhalatory Reserve Volume" and below is called " Exhalatory Reserve Volume".
You can in fact, expand your lungs' capacity to breathe and by doing so, stretch your body out from the inside by working in to the IRV and ERV ranges, and I explore the key to developing that in this 10 minute breathing sequence for upper body tension.