The diaphragm and chest wall muscles act together like a bellows to pump air in and out of the chest.
To breathe in these muscles contract to expand the chest cavity, causing a pressure drop into which the air flows. To breathe out, you simply relax these 'inspiratory' muscles and the chest springs back forcing the air out of your lungs.
During exercise the exhalation is assisted by contraction of the abdominal muscles and the inspiratory muscles undertake most of the work of breathing.
In contrast to observations of inspiratory muscle fatigue, research has not yet identified exercise-induced expiratory muscle fatigue which is why, until there is sufficient evidence to prove otherwise, it would be unnecessary to train anything other than the inspiratory muscles.
Also, while at rest, you breathe around 12 litres of air per minute, but during heavy exercise this can rise to over 150 litres per minute, and in elite athletes, this can be as high as 220 litres.
Scientific studies show that by exercising your inspiratory muscles with POWERbreathe you will increase the strength and stamina of your breathing muscles, reducing inspiratory fatigue and improving your performance.