Originally Posted by
LaRoux
There are two types of respiration- cellular respiration and physiological respiration. Physiological respiration is the exchange of carbon dioxide and oxygen between the environment and the body through the lungs. Technically, breathing is separate from each of these, but is required for both of them to occur.
Abdominal breathing is not natural, nor is chest breathing. Natural (and the most efficient breathing) is to use both the diaphram and the intercostal muscles. This is the type of breathing that endurnace athletes use. Also they don't have to having to "train" or "learn" how to do this.
The lower abdominal muscles have nothing to do with breathing.