Amann 2012, Pulmonary system limitations to endurance exercise performance

In healthy untrained people the respiratory system is not the limiting factor for maximal exercise, and breathing uses under ~10% of VO2max. In highly trained athletes at high intensity (above ~80 to 85% VO2max) the lungs can become limiting through three mechanisms: exercise-induced arterial oxyhaemoglobin desaturation, the high oxygen cost of the respiratory muscles (up to ~15 to 16% of VO2max), and intrathoracic-pressure effects on cardiac output. Chronic training produces far less adaptation in the respiratory system than in the cardiovascular system and muscle. Moderate-strong.