The muscles of respiration, from the diaphragm and the intercostals to the assisting musculature, need to be trained like any other muscles to increase resistance to fatigue and maximize performance. Training masks allow users to actively work on their respiratory muscle fitness. Originally they were designed to simulate training at altitude.
Training in hypoxic (low oxygen) environments increases red blood cell mass and improves oxygen transport. The use of training masks, however, has no measurable effect on haemoglobin, hematocrit levels and oxygen transport in athletes, as they do not alter the oxygen concentration of the air taken in. However, they appear to add resistance to the respiratory muscles by limiting air supply, thus triggering an adaptive physiological response.