When we breathe in the chest (like in Standing Deep Breathing), the abdominal muscles are relaxed.
This seems to confuse a lot of people who are used to hearing “suck the stomach in” or something like it in this exercise. When we inhale into the chest, we use the intercostal muscles between the ribs and not the abdomen. (More on that here.) The abdomen does appear to come in, but this is directly a result of the chest lifting. There is no action in the abdominal muscles on the inhale.
Muscles only have the ability to pull. They cannot push.
When a muscle engages, it pulls its two attachment points closer together. So, no action in the abdomen can push the ribs up. (In fact, engaging the rectus abdominis will keep us from affectively breathing in the chest, because it will pull our ribs down.)
In chest breathing, we have a second set of intercostal muscles that exhale. However, it is true that the transverse abdominis can help us exhale, especially if we are exhaling forcefully.
When you practice chest breathing, relax your abdomen. On the inhale, any muscular effort in the abdomen will hinder you.
