At any fitness level, increased altitude causes a decrease in physical performance according to the U.S. Army Public Health Command. At 6,000 feet or more above sea level, air has less oxygen, causing shortness of breath and increased heart rate during exercise for the first few days. Other problems that can reduce athletic performance and may last longer are decreased night vision, nausea, insomnia and dehydration, reduced memory, attention and judgment, and mood swings.
Your Brain Under Pressure
Traveling to 6,000 or higher feet above sea level can cause altitude sickness, also known as acute mountain sickness, in the first two or three days. The reduced oxygen in the air and, therefore, in your blood may cause the blood vessels in the brain to dilate, which causes brain swelling and sometimes headaches, according to the Institute for Altitude Medicine. In theory, pressure on the brain is also what causes dizziness, tiredness, nausea, chills, irritability and the other possible effects of high altitude that can negatively impact your athletic performance.
High Altitude and Cardio
VO2 max is a measurement of the maximum amount of oxygen your body can absorb and use in one minute. Physically fit people have higher VO2 max than less fit people, which allows them to perform cardio sports faster and longer. At 5,000 feet above sea level, your VO2 max should be close to what it is at sea level. Going up from there, your VO2 max drops 3 percent with each 1,000 feet of higher altitude. Acclimatization -- allowing your body time to adjust to a higher altitude -- will help, but you will not be able to perform cardio exercise at the same pace that you could at lower altitudes. The Institute for Altitude Medicine recommends 10 to 20 days of acclimatization for athletes before performing cardio exercise. If you are performing aerobic sports at an elevation of 12,000 feet or more, you should also first acclimatize at a lower part-way-up elevation before you exercise at the top.
Acclimatization and Athletes
During acclimatization, the hormone erythropoietin -- or EPO -- is released and causes your bone marrow to produce more red blood cells. This increases the ability of your blood to carry oxygen to your body tissues, but the process takes several weeks. The end effect of increased EPO is improved sports performance for endurance athletes. Anaerobic athletes who perform sports like weightlifting and sprinting might not need weeks of acclimatization, but can still suffer from some of the longer-lasting effects of high altitude.
Sleep High, Train Low
Serious endurance athletes use EPOs to get the edge on their competition. Sleeping at a high altitude increases red blood cells and VO2 max. Most, but not all, athletes living at a high altitude and training at a lower altitude experience increases in speed and stamina, according to an article by Edmund R. Burke, Ph.D., Director of Exercise Science at the University of Colorado. High-altitude training camps are one of the ways that some professional athletes experience the EPO-enhancing benefits of sleeping high and training low. Sleeping in a hypoxic tent, which creates an environment similar to high altitudes by reducing the oxygen content in air, is a lower-cost, less time-consuming alternative to traveling up and down mountains.