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

silentvoyager said:

One year ago I did road runs infrequently, so my VO2max wasn't tested most of the time. It was 55 at the peak. Now it gets tested on every run, and it went down to 49, and I feel it is about to hit 48.

Just speculating here since I don't know all the facts, but it sounds like if you ran too infrequently to get a VO2 Max most of the time, but when you did, it was 55, perhaps the reason for that is because of the sample size being too small. It is well known that the more data the algorithm has to work with, the more accurate it is. Therefore, as much as you probably don't want it to be true, I'm speculating that your VO2 Max probably really is in the high 40's, and now that you're running a lot more frequently it's starting to correct itself on the watch accordingly.

Back to your tree cover question, though - you mentioned that you're recording most of your runs as trail runs, but are these in fact off-road (dirt) trails? Or are they paved? If it's the latter, then you might want to go back to recording them as normal runs, and see if that affects your performance condition stats.

I mostly do trail-runs, have not had any big VO2 issues/variations yet. Since vegetation and trails in some "GPS-difficult terrain" might make the GPS a inaccurate on distance/speed, I have started using Stryd. This makes the distance very correct, and I could immediately see that on some of the trails.

Maybe this could make the VO2 even more accurate.

My performance condition has a direct correlation with my heart rate.  I don’t get a reading off in doing a recovery run.

Since Garmin released the last big update to include Trail Running VO2max, my numbers have been steadily going down for no reason. I was at 67, then next run 66, 65, 64, 63... all the way to 59 on Sunday after a trail run. I was at a consistent 65/66/67 for the last few years and my training is pretty much the same. Same time/HR on familiar courses. The only thing that changed was the Garmin algorithm.

If you were ‘screening’ your trail runs by using the trail run activity to avoid estimating a VO2max then it is quite possible that your VO2max was overestimated because you only used road run data. Now, with the Trail Run activity estimating VO2max it is quite possible that an alignment is taking place.

Before the Trail Run VO2max I recorded all my runs using the Run Activity whether or not hey were road or trail. For many years my VO2max has hovered around the mid-40s. Since the advent of the Trail Running VO2max it’s climbed to 49 whereas it’s never been higher than 46/47 previously. Not a huge change but nonetheless a trend that is heading up not down and opposite to your experience.

I’m not sure it works that way. VO2max for a trail run or flat road run should be the same number if the algorithm is accurate because it’s measuring oxygen uptake. That doesn’t vary considerably with grade of the hill.!

silentvoyager said:

I am recording most of my runs as trail runs, and I've been watching changes in VO2max and looking at Performance Condition graphs from most runs, and my impressions is that the biggest influencing factor is how many trees are around.

If the trees causes a weak GPS signal, this section may be excluded for the estimation of VO2max.

Please have a look at figure 1 of this paper from firstbeat

https://assets.firstbeat.com/firstbeat/uploads/2017/06/white_paper_VO2max_30.6.2017.pdf

shandel23 said:

t’s measuring oxygen uptake.

The algorithm is estimating oxygen uptake. You can only measure the uptake in a lab.

shandel23 said:

VO2max for a trail run or flat road run should be the same number

Yes the VO2max estimate should be the same because physiologically your VO2max does not care where you run.

An assumption is made that running on the road is 'easier' than running on trails. In other words for a given heart rate velocity is faster when running on the road than when on trail. Therefore a 'calibration' factor of some sort needs to be introduced when estimating a VO2max on trails to being the two estimates closer together. Therein lies the problem. Trail runs can be very different to road runs or very similar. For instance, a lumpy trail run on easy trails can be as fast as a lumpy road run but the trail run algorithm doesn't know that. Therefore the Trail Run VO2max is likely to get up-rated.

What trail runners did not like in the past was that their estimated VO2max would reduce when on trails because the velocity was usually lower for a given heart rate than when running on the road. They felt 'penalised' so many chose to use the trail run activity and not have a VO2max estimated. Therefore, VO2max solely from road runs for a trail runner might be higher and now correcting as VO2max is being estimated for running from two different activities using two different algorithms especially if Trail Running is the predominant activity.

Physiologically your VO2max doesn't change depending on whether you run trail or road.

Agree that it's an estimate, not a measurement of VO2max. Good clarification.

I suspect that their algorithm is "underestimating" the difficulty of trail runs since most people who have this issue are seeing their numbers go down and not up once trail run VO2max estimate was added.