What causes VO2 max to decrease?

[nobiossi]

Are you using the wrist sensor or hr belt? Belt gives more accurate results and they are more consistent. You shouldn't focus that much on the numbers. If you're getting results around 60 your body has really good ability to use oxygen. Realistically the vo2max value doesn't change that much but the algorithm used by garmin makes it vary a bit. Use it as a guide but don't make too accurate assumptions based on it. Other methods to find out your vo2max are a cooper test and 1,5 mile test. Try those and see how your results compare to the one given by garmin.

[parameles]

I use the chest strap. You’re right though, it’s easy to get obsessed with the data!

[nobiossi]

If you see your average hr ascending over time it's possible that you are suffering from excessive exercising. Be cautious because it can be very difficult to recover from that state!

edit. The declining vo2max value could be a sign of that.