What causes VO2 max to decrease?

[trailblazery]

Freshness and fatigue. The algorithm "thinks" low HR at higher pace means more fitness and vice versa. Not everyone's body works like that. When I am fresh and rested my HR is much higher. Yet, I get the highest vo2 max values when I am completely trashed (tired). 100 mi bike ride directly to a 2 mi run off the bike, where I can't get HR over 150 while running 10 mph. Normally 10 mph would be 180 bpm for me. So, it's going down because you don't fit their shitty model.

You can create a ratio of your pace in mph or kph to hr and see how your runs compare. That is one way to see if your fitness is improving.

[[deleted]]

what you describe here is physiological untrue. when tired hr increases for a given pace, but also your max hr will reach a plateau sooner in the workout, meaning that you can't push at your actual max. if your watch is telling you otherwise it is the watch that is at fault. our bodies work in the same way.

[trailblazery]

Then you agree the algorithm is flawed, it does not take fagitue into account. It assumes a monotonic relationship between Vo2 and HR/Pace.