Final answer:
Maximal exercise decreases at altitude due to decreased oxygen availability which causes lower oxygen saturation of hemoglobin. This results in tissues and muscles receiving less oxygen during exertion, thus reducing exercise capacity and increasing the risk of acute mountain sickness.
Step-by-step explanation:
Maximal exercise is reduced at altitude primarily due to decreased oxygen availability. As altitude increases, atmospheric pressure decreases which in turn reduces the partial pressure of oxygen. Hemoglobin, the protein in red blood cells that carries oxygen, has a lower saturation level at high altitudes compared to sea level. For instance, hemoglobin saturation may only be about 67 percent at 19,000 feet above sea level, compared to approximately 98 percent at sea level. Lower oxygen saturation means muscles and other tissues receive less oxygen during exertion, resulting in quicker onset of fatigue and reduced maximal exercise capacity.
At high altitudes, though the body tries to acclimatize to lower oxygen levels by increasing erythropoietin (EPO), which stimulates red blood cell production, this process takes time. Moreover, with lower oxygen reserves in venous blood, the body's capacity for physical exertion without experiencing symptoms like headache, fatigue, or nausea, known as acute mountain sickness (AMS), is compromised.