Studies at moderate altitude are not conclusive in demonstrating a beneficial training effect for talented athletes who are already fully trained. When the body is subjected to a hypoxic environment, adaptive processes attempt to facilitate the intake, transport, and use of oxygen. Ventilation is markedly increased on exposure to reduced oxygen pressure. The body attempts to move greater amounts of oxygen into the lungs by increasing respiratory rate, tidal volume, or both. Breathing is heavier, that is, the athlete works harder at breathing.
Opinions differ as to whether cardiac output is initially elevated or unchanged.
The second phase of adaptation deals more with the body's adaptations to increase the oxygen transport capacity of the blood. Changes in ventilatory, cardiovascular, and hematological variables occur. Cardiac output is reduced when compared to sea-level variables several days to weeks after exposure primarily through a reduced stroke volume. Once depressed, cardiovascular function remains depressed and does not recover with increasing length of exposure. Hematological adjustments occur to augment the oxygen content of the blood.