Exercise and sport performance at altitude

The difficulty of demanding physical exertion at high altitude has been described by many climbers. In 1925, E.G. Norton gave the following account of climbing without supplemental oxygen at 8,600m(28,208ft): “Our pace was wretched. My ambition was to do 20 consecutive pases uphill without a pause to rest and pant, elbow on bent knee, yet I never remember achieving it – 13 was nearer the mark.”

Maximal oxygen uptake and endurance activity

Maximal oxygen uptake decreases as altitude increases(picture below). VO₂max decreases little until the atmospheric PO₂ drops below 131mmHg. This generally occurs at an altitude of 1,500 to 1,600m(approximately 5,000ft) – about the elevation of Denver, Colorado. At altitudes up to about 5,000m(16,400ft), the decreased VO₂max is due to the reduced arterial PO₂; at higher elevations, a decreased maximal cardiac output further limits VO₂max. VO₂max decreases approximately 8% to 11% for every 1,000m increase(or 3% for every 1,000ft increase) in altitude abouve 1,500m. The rate of decline may become even steeper at very high altitudes. When men and women are matched for their initial aerobic fitness level, there are no sex differences in the rate of decline in VO₂max.

As shown in the figure below, men climbing Mount Everest in a 1981 expedition experienced a change in VO₂max from about 62 ml x kg^-1 x min^-1 at sea level to only about 15 ml x kg^-1 x min^-1 near the montain’s peak. Because resting oxygen requirements are about 3.5 ml x kg^-1 x min^-1, without supplemental oxygen these men had little capacity for physical effort at this elevation. A study by Pugh and coworkers showed that men with VO₂max values of 50 ml x kg^-1 x min^-1 at sea level would be unable to exercise, or even to move, near the peak of Mount Everest because their VO₂max values at that altitude would decrease to 5 ml x kg^-1 x min^-1. Thus, most normal people with sea-level VO₂max values below 50 ml x kg^-1 x min^-1 would be not be able to survive without supplemental oxygen at the summit of Mount Everest because their VO₂max values at such an altitude would be too low to sustain their body tissues. Enough oxygen would be consumed to barely meet their resting requirements.

Obviously, activities of long duration that place considerable demands on oxygen transport and uptake to the tissues are the most severely affected by the hypoxic conditions at altitude. At the summit of Mount Everest, VO₂max is reduced to 10% to 25% of its sea-level value. This severely limits the body’s exercise capacity. Because VO₂max is reduced by a certain percentage, individuals with larger aerobic capacities can perform a standard work task with less perceived effort and with less cardiovascular and respiratory stress at altitude than those with a lower VO₂max. This may explain how Messner and Habeler were able to reach the summit of Everest without supplemental oxygen in 1978 – they obviously possessed high sea-level VO₂max values.

Anaerobic sprinting, jumping, and throwing activities

Whereas endurance events are impaired at altitude, anaerobic sprint activities that last less than a minute(such as 100m to 400m sprints) are generally not impaired by moderate altitude and can sometimes be improved. Such activities place minimal demands on the oxygen transport system and aerobic metabolism. Instead, most of the energy is provided through the adenosine triphosphate(ATP) and glycolitic systems.

In addition, the thinner air at altitude provides less aerodynamic resistance to athletes’ movements. At the 1968 Olympic Games, for example, the thinner air of Mexico City clearly aided the performances of certain athletes. At Mexico City, world or Olympic records were set or tied in the men’s 100m, 200m, 400m, 800m, long jump, and triple jump events and in the women’s 100m, 200m, 400m, 800m, 4x100 relay, and long jump events. Because similar results occurred in swimming events up to 800m, some exercise scientists have questioned the role of lower and density in improved sprint performance.