What do you guys think it is about exceeding a certain heart rate that makes us worse yet healthy people should want to exceed their heart rate to be healthy and keep fit. So bizarre.
Also, I don't understand why I am "fat burning" and in exercise zones when I mainly lay around all day. It's pretty scary that going down a few stairs really jets me up as if I am running a marathon.
IIRC,
@Sushi is right about the lowered anaerobic threshold. I believe Stevens, Snell et al. found that ME/CFS patients had a lowered AT compared to sedentary controls, though it was the decline in capacity on Day 2 of the exercise test that showed PEM physiologically. People with primary mitochondrial disease can also have lowered ATs.
My AT is 89 (per the 2-day CPET), which is low even for ME/CFS, I think. My resting HR is about 65-70. That doesn't leave a lot of leeway before hitting the wall! Just getting up will do it sometimes. My guess is that limited leeway is true for a lot (all?) of us, probably to different degrees. So some people can walk for awhile, even do some exercise, some people can't even sit up without going over. After I had the test, they explained it to me in that way--stay under the AT, so you don't hit "the wall," because that will result in PEM.
As far as the "fat burning" and other zones are concerned, FWIW, I ignore those. They're based on parameters for healthy people, and I'm not sure they're too meaningful anyway. They are making assumptions about some really complicated and individualized biochemistry. But that's my curmudgeonly opinion of exercise apps.
And they might loosely correlate to the work your body is doing physiologically--i.e., it IS exercise for you to go down a few stairs.
-Vasha