Some time ago I picked up a pulse oximeter to check blood O2 levels during workouts. That was a bust. Started wearing it at night…just before going to sleep I like to do some Wim Hoff breathing exercises because those things work better than any other sleep aid I’ve ever tried…and maybe there is some third order benefit on the bike, who knows.
I notice some correlation between how my blood O2 & pulse vary during those breathing exercises & how proximal workouts go. I can feel fatigued but if it’s hard to move my blood O2 out of the 90’s I’m good to go. On the other hand I can feel OK but if the bottom drops out of my blood O2 & it easily drops into the 70’s or 60’s tomorrow’s workout is going to be a giant sufferfest.
Doesn’t work if you have had a drink. For whatever reason that keeps O2 up there no matter what.
Anyhow, maybe useful to some on the forum as a cheap & (maybe more important?) off-the-grid way to keep track of your recovery. Might also be that this interaction is unique to me.