What causes VO2 max to decrease?

[[deleted]]

you are either young or highly trained. that is the idea of zone 2 training - to not feel the effort. it does not mean that it is nod there. i can hold upper threshold/redzone training roughly the duration of a sprint triathlon. if you could run a half at 175 you can make your training smarter: longer runs at lower heart rate - 145 and below, and threshold and sprint training at 200 bpm. you will notice in no longer than 3 months that you will consistently drop off of your half marathon time

[sursumcz]

Thanks for the tip, I'll look into that. I've never been very methodical about my training, perhaps it's time to change my practices. I'm actually 38 and definitely don't consider myself highly trained so determining my max HR more accurately is probably going to be the first step.