The double layers of intercostal muscles are sandwiched in between your ribs and their job is to raise and lower your ribs as you breathe, much like opening and closing an umbrella.
Lung tissue cannot breathe by itself but MUST BE BREATHED INTO by the combined efforts of your intercostals and the powerful diaphragm that is stretched across the floor of your ribcage. Poke around on your ribcage (gently please) and note the flexibility of your lower ribs.
When we shallow-breathe, we invest a LOT OF ENERGY FOR VERY LITTLE OXYGEN IN RETURN. When we deep-breath into the wider and more flexible lower cage, we spend less energy for greater gain. The upper chest serves well as a reserve tank (when you need more oxygen than the lower cage alone can provide). Shallow breathing is lazy breathing or tired breathing or stressed breathing or simply a very bad habit.
Be well. Breathe beautifully.
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