As the intensity of exercise becomes higher, increases in the oxygen uptake (VO₂) and cardiac output (Q) start to level off, and they will enter a steady state. However, the pulmonary ventilation (V_E) continues to increase in an exponential manner even after the VO₂ and Q level-off. After exercising to extreme fatigue, the oxygen used by the respiratory muscles (VO₂) increases to 10 to 16% of the VO₂max. In this situation, the respiratory muscles use the oxygen that would have been allocated to the active muscles because the oxygen used for the entire body is already in a steady state. Therefore, overexercising must be discontinued due to a shortage of oxygen for the active muscles (in the legs); amount of O₂ debt has reached its limit. Since the total amount of oxygen that can be taken into the body remains constant, the enhancement of the respiratory muscles and their functions (including the respiratory efficiency (V_E/VO₂)) in advance is expected to improve their endurance. Several previous studies suggested that respiratory muscle training (RMT) enhances the endurance of subjects. Furthermore, recent studies suggested that RMT not only enhances endurance, but also improves the physical elements of significantly intense exercise (including speed, muscle strength, and power) performed for a short period of time. It is necessary to conduct further research to clarify the mechanism of RMT in the improvement of physical performance. RMT is not expected to improve the VO₂max, and the following are possible causes of the improvement of performance: improvements in the respiratory muscles and efficiency (V_E/VO₂), a subsequent decrease in the blood lactate concentration, and subjective respiratory distress and sense of fatigue in the active muscles.