Altitude training refers to a situation where athletes are competing at higher altitude than what they are accustomed to. Even an elevation as little as 7500 feet can lead to a 5% deterioration in performance. The reason for this is because there is a decrease in the amount of oxygen (lower barometric pressure) that can be taken into the body and transported to the muscles. More specifically, a decrease in the maximum oxygen consumption.
People who are native to altitudes have shown to have larger chest capacity, more red blood cells (oxygen carrying cells), and more capillaries and more alveoli in the lungs. Some evidence indicates that this is some kind of genetic adaptation and much more than experienced by people who may relocate to altitude but are not native to it.
Someone who moves to altitude, even just after a months, show a conservation of glucose, more mitochondria, and more red blood cells (hemoglobin).
During the struggling phase, when a person first arrives at altitude, the following can be experienced; increased breathing, increased heart rate, increased hemoglobin in the blood, and changes in blood flow distribution (e.g. more blood being sent to the muscles as opposed to the digestive system).
The more time someone spends at altitude, e.g. before the competition, the better they will acclimatize. Some experts believe that 3 days is adequate, while other believe that 3 weeks is needed. Obviously the greater the altitude the more time will be needed.
Beware that there are problems that can occur. If the altitude is too great, or not enough time is allowed to acclimatize, the person could experience sleep problems, nausea, headaches, and dyspnea. This may happen at around 8000 feet. Pulmonary edema is much more serious and may occur at approximately 10,000 feet. This is when blood begins to pool in the lungs. As you can imagine if this continues it can become life threatening. The best treatment is to administer oxygen and to take the person to lower altitude.
In severe cases of altitude, called altitude sickness, the lungs are simply not able to absorb any of the oxygen in the air. This occurs because the oxygen pressure at those levels is very low. Bleeding can also occur in the lung tissue and may show up in coughs. If this continues death is imminent.