The most important muscle for breathing is the diaphragm.
Actually, the diaphragm is a sheet of tendon with muscle around the edge. It is a kind of dome with two extensions that separate the thoracic cavity from the abdominal cavity.

During inhalation, the diaphragm moves downwards: the dome flattens into a kind of inverted saucer. This stretches the lungs downwards and allows air to flow in. The diaphragm makes space from within. But not just by moving downwards: imagine the dome flattening out. It gets wider too. The diaphragm is encased by the lower ribs, so they move sideways, and also by the sternum. There’s actually not that many places where the diaphragm is restricted, which makes it flexible in shape.

At the same time, there is also a downward movement from the centre of the diaphragm that flattens out. Your organs are gently pushed down, causing your abdomen, flanks and back to expand. This is a natural massage for the organs. The movement also helps your heart function (the centre you see between the two domes is connected to the pericardium via a central tendon) and there are also other benefits to using your diaphragm.

There are many forms of breathing. When we want to learn to use the diaphragm, we often think of the abdomen. The abdomen expands as the organs are gently pushed down by the diaphragm. However, you might only be moving your abdomen in this way, while your back, flanks and chest are not expanding. The best approach is to let your entire torso move in a relaxed way at the same time.

Also have a look at this great video to see the expansion of the lungs while the diaphragm (not seen by itself but is below the lungs) moves downward.

In the image above you can see the extensions of the diaphragm. You could say that these crura are the deepest elements of the diaphragm. When you start a breathing movement from this centre of your body, your entire torso will move too, simply because everything is connected there. This happens without you having to think about all the different parts of your body. So you don’t have to think about inhaling low and then letting the breath come up to your chest.

To change your breathing from belly breathing to a more optimal way you want to learn how to breathe from this centre of your body, or you could also focus on your lower back.
Many people suffer form a stiff lower back so it can take some time to make space for the breath.

The online course Effortless breathing for musicians helps singers and wind players to use the diaphragm in an optimal way, through various Feldenkrais exercises. These exercises help you to become more aware, get rid of tensions and create more space for the breath.

When you start a breathing movement from the centre of your body, your entire torso will move, simply because everything there is connected. You could imagine that centre of your body (where the crura and lumbar vertebrae are connected) as the button of an umbrella. As soon as you keep your focus on that during inhalation, the spokes of the umbrella (ribs) open sideways and upwards.

Het bericht Diaphramatic breathing vs belly breathing verscheen eerst op Floor Wittink.