The process of flowing air in and outwards of the alveoli is described as ventilation. This process has two stages: inspiration or inhalation and expiration or exhalation. To continue this process, the thorax moves and changes its size completely. This happens for the action of two sets of muscles: the intercostal muscles and the diaphragm. The Expand-A-Lung® Breathing Resistance Exerciser is a device which is designed to strengthen these muscles in order to aid people to breathe deeper, simpler and better.