[ProtectOurFreedom]

The pressure in a U-2 cockpit at typical mission altitude is equivalent to the atmosphere at 29,000 feet—as high as the summit of Mt. Everest.

Yikes! My company had a woman speak about her expedition to the top of Everest. She said at 29,000 feet, even with oxygen bottles, it was very hard to move one foot forward. You have to move the foot, stop for a minute and pant, then move the other foot forward one step. I can't imagine the difficulty of being a high performance pilot at a pressure equivalent of 29k feet.

[Harley]

...at 29,000 feet, even with oxygen bottles, it was very hard to move one foot forward.

Yet many elite climbers like Ed Viesturs have climbed many 8000 meter peaks including Everest without O2.