Many of Polar's fitness watches, to include the Vantage M model, have the capability to estimate V02 using heartrate variability as a proxy. There are at least a few peer reviewed papers that have concluded that these estimates are sufficiently precise to be used to track trends in a user's fitness over time even if they are not completely accurate. Indeed, in my own experience the estimates appear internally consistent in that the values produced don't vary much over the short term, but do vary in an explainable way over long periods of time during which my fitness regime has changed. That observation is based on years of data.
However, I recently have observed that my estimated V02Max has dropped dramatically. It had for several years been reporting a values in the mid to high 60s. It now reports values in the high 30s. There's a startling inflection when graphed over time. My exercise regime was consistent over the period when I observed this change.
I concede that values in the mid to high 60s are probably an overestimate, so either the heart rate sampling or the proprietary algorithm used to deduce V02 Max (or both) probably have something wrong. Even, so I'm inclined to believe that something has dramatically changed and I'm not sure how to account for this. Are there any conclusions to be drawn? Where should I look for explanations?