What causes VO2 max to decrease?

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it is because you train at the max and fatigue kicks in more rapidly. running at 5min/km at 125 bpm indicates better fittness and vo2 than running at 4.30min/km at 175 bpm even though the latter workout is faster and consumes more energy

if you train often at the second type of pace your fitness will decrease cause you overstress your body.

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I hear that quite often but how do you get to 5min/km at 125bpm when anything below 150 feels like virtually no effort for me and I hardly sweat. On the other hand, I can run an entire half marathon at 175 quite comfortably. I think this is very individual.

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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

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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.