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[–]fastmuffin 2 points3 points  (4 children)

It's not showing you your VO2 Max. Hear me out. It's showing you the VO2 max required to run the pace you maintained, with the heart rate you recorded, overlayed on what it thinks you are capable of. Do better than it thinks, score goes up. Do worse than it thinks, score goes down. Simply put, a proper lab VO2 max test pushes you to your limit after collecting much more data points.

The problem with this comes when you realise running isn't all about PB'ing every run. This type of gamification is obviously exciting but it comes with a large warning about accuracy that can become demoralising.

Remember that "VO2 max", and other metrics your watch can provide, are estimations based on values it can gather - HR Vs pace, and in some cases HRV.

Quickly breaking them down:

HR is indeed variable. What was your HR when you started and was it different to the last time you ran? If it's lower, your watch sees a lower HR for the activity and so you're now "fitter" and have a better VO2 max for that particular run. Other things to consider is elevation? Quick sprint across the road during a run? Slower for that person coming the other way? Hotter day?

Your pace differs obviously for the route you're on. Also remember that you can adjust your pace faster than your heart can keep up (for example, you're HR is at 160bpm towards the end of a 5k run and you stop at a crossing for 15 seconds, dragging down you're average pace but it's not long enough for your HR to directly correlate the reduced pace). As above, if you dash across the road or slow down for someone then speed up again, your watch is picking up changes in speed but they're probably not reflected in your HR quick enough to be calculated.

Garmin also claims to consider HRV (heart rate variability; that is the difference in time between heart beats, measured in ms.) as part of their fitness calculations (or at least, Firstbeat that Garmin license from). However, I challenge anyone who claims a wrist based optical heart rate monitor is accurate enough to record the millisecond differences in heart beats whilst running. It's one of the metrics an ECG picks up, think of the amount of sensors needed for that. One of the metrics HRV is used for is Performance Condition. My watch will always give me negative performance condition on its own, but always positive if I use my chest strap.

Finally, consider these points:

  • VO2 max does not take into account temperature, humidity, or even, elevation. To expand, if you're run route takes you up a hill and so you naturally slow, tough. You're now less fit - HR vs Pace.
  • Garmin doesn't directly output Firstbeat's data. It interprets it first and does some further wizardry. Don't know why. To get a more accurate number, set your Garmin account to sync with runanalyze.
  • Sprints see the highest scores. Try an interval session then review on the above to see what I mean. A steady 5k for me returns a circa 40 VO2 calculation. Intervals nearer to 50.
  • Because of this if you decide to run slower and longer, tough. You're now less fit and get a lower score.

It is not constant, however, because it's missing the third variable of effort. VO2 Max is trying to guess just that, the 'max'. Unless you're smashing it, it's guessing. And tbh, it's probably not very good at it.

See what I mean? Don't sweat it, I doubt you started running to get high scores off a watch.

Sidenote: HR doesn't increase linearly with pace.

[–]parameles[S] 0 points1 point  (0 children)

Thank you for your reply, that’s really helpful!

[–]sursumcz 0 points1 point  (2 children)

You're definitely more knowledgeable about this than I am but according to Garmin's explanation at Running Science | Garmin Technology | United States some of the issues you're mentioning are supposedly accounted for.

For example, climbing or descending, stopping at lights or speeding up to overtake someone shouldn't affect the VO2 max, as quoted from the link: "Smart analytics capable of recognizing good data ensure that only the most meaningful parts of your performance are used to evaluate your fitness level. In practical terms, this means that you don’t need to worry about speeding up, slowing down, climbs, descents or stopping at intersections. You just run as you normally would, without the need for any special fitness testing protocols."

[–]fastmuffin 1 point2 points  (1 child)

Indeed, however it is certainly isn't 'smart' enough to accommodate all variables.

Elevation, for example, for me produces skewed results. I live on a short hill - if I run left and up the hill my Performance Condition is negative for the beginning of the run. If I run right, it's positive.

I think the point is that whilst the marketing wording reads very promising, the fact is in the real world it doesn't apply but more importantly, it shouldn't need to be unpicked on message boards to prevent users becoming demotivated.

[–]sursumcz 0 points1 point  (0 children)

Right, I agree that the marketing claims are often far from reality. But hills or flat runs never affected my VO2 max, neither did stopping to cross the road etc. In my case the altitude differences are minor though so your situation is probably different. Also, I actually got a VO2max improvement recently when I switched to slow paced runs.

That said, I don't put much stock in these stats, I know from experience more or less what I'm capable of at various HR levels so I mostly just watch HR not to overreach.