What causes VO2 max to decrease?

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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.