I can't believe how long it's been taking me to write about this. Probably because the reality of it really hadn't sunk in but it is now.
For a few years, I've thought I was sensitive to altitude. At a certain elevation my muscles would just run out of energy. It always seemed to be around 9600', but once in a while it would happen at 7500' which seemed odd.
Finally I started paying attention to how hard I was working and started to wear a heart rate monitor. Then I noticed that it always seems to happen about 1 hour into a climb.
I'm used to working at a high heart rate. Two years of spin classes has taught me to love the high that it gave me. Now I'm realizing that is a bad habit. When I work too hard for longer than an hour (called working anaerobically), my muscles run themselve out of glycogen and oxygen and they "bonk." They just nearly stop. They can still function, but at around 25% of what they were.
The thing that gave me a clue to this is an experiment I did when I climbed Mt Hoffman in Yosemite. Whenever my heartrate wents above 150, I stopped and let it slow down. Climbing it took forever, but hours in I never once had an issue with my muscles just quitting.
So these days I try to not work above 155 bpm. It makes me really slow. which is crazy making as I'd be happy working at above 165 bpm, but at a certain altitude that is not a sustainable rate.
This took me a long time to work out as apparently when I'm at sea level there is enough air pressure to help oxygen still get to my muscles.