When exercise is being performed, if the exhale-hold technique is properly applied, a decrease in O2 concentrations and an increase in CO2 concentrations occur in the lungs, the blood and the muscles. The combined effect of hypoxia and hypercapnia act as a strong stimulus whose main consequence is to increase lactic acid and hydrogen ions production, and therefore to provoke a strong acidosis in the body. Thus, during exercise with hypoventilation, the blood and muscle acid–base homeostasis is highly disturbed. The studies have also reported an increase in all heart activity when hypoventilation is carried out in terrestrial sports. Cardiac output, heart rate, stroke volume and sympathetic modulation to the heart are greater when exercise with hypoventilation is performed in running or cycling. A slightly higher blood pressure has also been recorded. In swimming on the other hand, no significant change in the heart activity has been found.After several weeks of hypoventilation training, physiological adaptations occur that delay the onset of acidosis during a maximal exertion test. The studies have shown that at a given workload, pH and blood bicarbonate concentrations were higher, whereas lactate concentrations had a tendency to decrease. The reduction in acidosis would be due to an improvement in buffer capacity at the muscle level. However, no change advantageous to aerobic metabolism has been found. Maximal oxygen uptake (VO2max), the number of red blood cells and the anaerobic threshold were not modified after hypoventilation training.