Final answer:
The decreased PO2 gradient at high altitudes limits exercise capacity because less oxygen is available for hemoglobin to carry to the muscles, potentially resulting in acute mountain sickness during exertion despite compensatory mechanisms.
Step-by-step explanation:
The statement that the decreased PO2 gradient between blood and muscle significantly limits exercise capacity at altitude is true. At higher altitudes, the partial pressure of oxygen is lower due to a decrease in atmospheric pressure. This results in less oxygen being available for the hemoglobin in the blood to transport to the muscles during exercise, which can lead to acute mountain sickness (AMS) due to the low oxygen reserves. Compensatory mechanisms such as increased BPG production and a higher concentration of erythrocytes can partially mitigate these effects at rest, but during physical exertion, the limited oxygen availability can indeed be a limiting factor for exercise capacity.