Trail Running VO2Max / Performance Condition - correlates with how many trees are around

Yes, agree. And more specifically I suspect that either pace or distance are still mixed up into the algorithm even in the case of Trail Running. And my experience is that both pace and distance are underestimated when running on trails, and especially on trails with dense tree coverage, which may result in the algorithm underestimating  performance condition and negatively impacting VO2max calculation. 

That may not be the case for everyone, because what I mentioned above greatly depends on a number of factors. Trails in my area (Pacific Northwest) tend to be quite wavy and switchbacky, and tree coverage tends to be very dense, and based on some anecdotal evidence GPS devices tend to struggle in this area more than they do in other areas. That was one of the reasons Suunto Ambit series watches were so popular among trail runners in this area several years ago.

I wonder if using a Stryd or Runscribe footpod would help with the tree issue. I'm in Northern California and often run through redwoods, so I have the same issue. When I use a footpod I at least am getting an accurate distance/cadence measurement when I'm in the trees. Not sure if Garmin would use that external data or rely on its own GPS data.

shandel23 said:

I wonder if using a Stryd or Runscribe footpod would help with the tree issue.

Yes it does.  I too run the trails in the Pacific Northwest and I don't have any problems when using my Stryd. 

So guess what, I have upgraded to Fenix 7 and my VO2max does no longer appear to be punished by running on trails. 

Furthermore, before the upgrade, my VO2max was bouncing between 49 and 50 for a long time, occasionally touching 51 for a short time and bouncing back down. Now two weeks after the upgrade it moved to 51 and then to 52. Coincidence? I don't think so. I think the VO2max algorithm depends on how accurately the watch measures pace or distance even in Trail Run mode. Higher GPS accuracy of Fenix 7 is what makes the difference. I should mention that I used exactly the same chest HR sensor with both devices. 

silentvoyager said:

I think the VO2max algorithm depends on how accurately the watch measures pace or distance even in Trail Run mode.

If so then people using a F6 and a Stryd/footpod combo should get at least the same VO2max accuracy as a F7 owner.

Tisztul_A_Visztula said:

If so then people using a F6 and a Stryd/footpod combo should get at least the same VO2max accuracy as a F7 owner.

What makes you think that VO2max algorithm trusts the footpod distance? The footpod may be uncalibrated. Is VO2max measured on a treadmill? 

I just know that if you use a footpod/Stryd and chooses them as the source of pace/distance always (not just indoor) then these data are used and saved into the fit file. There are no two pace or distance data tracks in the fit file, just one and one, respectively.

So if you were right when you said what I quoted then I have to be right, too.

But honestly I dont know how exactly Vo2max is estimated by the Fenixes. I just said that a footpod, and especially a Stryd can improve it if it heavily relies on pace/distance.

I guess Garmin trusts those other speed/pace sources, since they use Firstbeat method for Vo2max estimation, too. And Firstbeat relies on footpod data, see “Technology (such as GPS sensors and foot pods in wrist devices or mobile phones) enables reliable measurement of running speed along with heart rate (HR).” from https://assets.firstbeat.com/firstbeat/uploads/2017/06/white_paper_VO2max_30.6.2017.pdf

Moreover they seems to prefer footpods and indoor tests: “A study examining the accuracy and replicability of results by the Firstbeat method (used foot pod to measure running speed and laboratory tests for reference) found that …”

silentvoyager said:

Technically, a footpod is more precise than GPS but not necessarily more accurate, if you know the difference between these two terms.

Here lies the debate since the inception of the footpod.  Precision should be kept out of the conversation as it means nothing in relation to the topic.  If a footpod records 4.69 miles every 5 mile run, we would have to agree the precision is exceptional, but the data is still incorrect.  A properly calibrated footpod has shown to be more accurate (1-2%) when running on a flat smooth surface in the open versus GPS.  The degree of accuracy really isn't that much and it's only noticeable the further you run as the gap between errors becomes more noticeable.  When it comes to trail running with a lot of switchbacks, uneven terrain and compromised GPS due to vegetation both GPS and a footpod suffer.  I have found even with the challenges trail running presents, the degree of error I have is still less with my Stryd, though still not very accurate.  My road and trail runs are fairly equal and my VO2Max never changes by more than 1. Now that being said, this is when I was trail running in your neck of the woods. I recently left the Seattle area and moved to the coast of Corpus Christi, TX.  I still trail run minus the hills and thick overhead cover. My Stryd provides better data, though it's still not 100% accurate, but my VO2Max has taken a 4 point hit. I believe this is due to the way I'm running because most of the trails are now very sandy compared to the harder packed trails I ran in the NW.  This leads me to believe it's an individual's running style on trails that affects our VO2Max.  I'm just one and in no way have I proven anything; just sharing my experience and thoughts.