Final answer:
A significant reduction in VO2 max in athletes at high altitude is primarily due to decreased hemoglobin saturation caused by lower partial pressure of oxygen in the air at such elevations.
Step-by-step explanation:
The reason some highly trained athletes experience a larger-than-expected decrease in VO2 max when tested at altitude is likely due to a large desaturation of hemoglobin.
At high altitudes, the partial pressure of oxygen decreases due to the thinner atmosphere, which makes it more difficult for oxygen to bind with hemoglobin.
This phenomenon can lead to acute mountain sickness (AMS) and impacts athletes' performance because their blood carries less oxygen, affecting their VO2 max, which is a measurement of the maximum amount of oxygen an individual can utilize during intense exercise.
Hemoglobin saturation may fall significantly at high altitudes, with values around 67% at 19,000 feet compared to approximately 98% at sea level.
The body can partially compensate with more erythrocytes and increased release of oxygen into tissues, but during rapid ascent or acute exposure, athletes may not have time to acclimatize, resulting in reduced oxygen transport and aerobic performance.