tracking fitness only from run mode (Forerunner 945)

Yes that does work. I'm glad VO2 max can be turned off! For those who combine trail running with road/track etc. it makes little sense to have both trail and road tracked at the same time. Thanks!

The trail run VO2 Max uses a bunch of algorithming to compensate for trail conditions in an attempt to provide a score that would correspond with a road or track based VO2 max score. I run on road, track, and trail and my VO2 max score has remained consistent while using the new trail run feature. The only time I turn off VO2 max recording is when running with the dog or a stroller.

Of course the feature obviously won’t work well for everyone, hence the ability to turn it off. If you’re interested, DC Rainmaker has a detailed response from a Garmin employee about how the feature works in his review of the Enduro. It’s way down under the “Sport Usage” section.

www.dcrainmaker.com/.../garmin-enduro-gps-watch-in-depth-review.html

Why? For years I’ve been tracking all my runs with the Run activity both on and off road. Even now I usually forget to switch to Trail Run but stil my VO2max varies little. The reality is that everything you do affects your physiological VO2max. The estimated VO2max is different and affected by every activity. However, over time the trend is what matters not one or two wayward calculations. 

I don’t think it is as simple as this for everyone. I agree that one or two wayward calculations do not matter but if a large % of runs are very technical trails (hilly, rocks, roots, muddy etc) then the algorithm will not compensate well. Better examples are activities that really mess up  VO2 calculations such as snowshoe running.   In the “old days” I would watch my V02 numbers drop significantly every winter as most of my running would be with snowshoes (which can require over twice the effort for a normal running pace).  I also Orienteer and this would mess up results. 
so I fully agree that the trend is more important but the ability to turn it off for types of activities is still needed for many people.  

razmichael said:

I also Orienteer and this would mess up results

That depends a lot. Like last weekend Jukola: https://connect.garmin.com/modern/activity/7348558670 
It was just -1 or -2 in the performance condition. Then more relaxed training: https://connect.garmin.com/modern/activity/7274361886 it hits -9, but still "recovers" a lot in the end :D 

I haven't noticed it much messing up the VO2max.

Yeah but when you start looking at long term trends (yearly view) and you see a good trend during training 59-59-60-61...then trail run 58! - 60 - 58(?!?) - 62-62-62  you start loosing a trend line, due to bad data.  When it starts happening every few months its a mess, or if you happen to sit at a lower vo2 for a couple months you look back and think, oh what happened, must have been training less...

it is a pain to have an entire months vo2max set near the end of the month due to gnarly train run with a lot of loose sand/mud, driving HR high for a slow pace.  Similar to a a snow or beach run - where for the same pace HR will be higher by 10+bpm.  Even just running in grass drives pace way down for a given effort.  Maybe very experienced trail runners don't experience such a drop in pace at a certain HR... but I sure do, even have done a couple 50k trail races.

Now if you have time to run a good normal run after... it will hopefully go back up before end of month.  But what if it's the 26th of a month, you have a easy recovery run the next day (won't trigger vo2max probably due to low HR)... then maybe rest day and bike rides to end the month... that 'bad' data point will stick!   unfortunealty the montlhy vo2max data point is just the last one!  which doesn't make a lot of sense, should be average I think to wash out those high/low data points and get the 'real' trend captured.  Runs while hungover also mess this up often! lol  but the turning off vo2max option is goodfor all these weird use cases, to have clean data trends to track.  Editing or removing a data point would also be a good option.

If you only run and do it frequently, yeah pretty easy to update and get new data points often.  but if you do a lot of super slow recovery runs without vo2max updates, bike rides, cross training, run infrequently... a bad data point can stick for quite a while.  or often vo2 will drop 3-4 points!!!! but then only raise 1 on the next 'good' run?! I have never seen it rise by more than 1, but it will drop many!  Preventing a return to the trend line / average vo2.

Another use of the no-VO2-max trail run "feature" is that can serve to offset problems I have with the wrist HR. For me, I get accurate wrist HR only on warm summer days. So when the weather is cool including all winter, unless I wear the chest strap, wrist HR is really unreliable  - next to useless. That's where a run setting with no VO2 max is also useful.

I wonder if this is a setting you can change on the fly during a run?  Like if you see some crazy HR readings during a run you could at some point during the end of the run or cooldown switch off the calculation for vo2max... lol   that would be sweet!   Wish it asked you to accept new Vo2max readings similar to FTP settings for biking!