Does anyone know how Garmin (Fenix 6) calculates VO2Max for cycling? Does the speed have an influence? I cycle on the indoor trainer (Flux S) in the lightest gear (because of the noise). Since I’ve been doing this, my VO2Max has dropped 10 points 
(from 54 to 44) - even though my FTP is going up 
. Does anyone here have any experience?
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Nothing to do with speed, it estimates it from power and HR. Are you actually recording your TR sessions on a garmin device? As it won’t estimate VO2 from third party data. Mine tends to vary quite a bit based on all the factors that cause HR to vary (temperature, time of day, caffeine, fuelling, etc) but more in a range of maybe 5, never seen a drop anything like 10. Did you change your HR zones or weight on your garmin recently?
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There are quite a few possibilities, and most likely a combination of changes to several metrics.
Very broadly:
Power - Lower power output at a given heart rate would result in a decrease Vo2Max
Heart Rate - Higher heart rate at a given power output would result in a decrease Vo2Max
Weight - An increase in the weight setting would result in a decrease in Vo2Max
Max HR/HR zones - An increase in the Max HR setting would shift your HR zones around and result in a decrease in Vo2Max
If your FTP is increasing, it’s most likely one of the later two. I’ve done this myself having forgotten to update my weight after a long period of substantial change.
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i use the automatic upload from TR to Garmin. And it does estimate VO2 from third party: Garmin Training Status Now Includes Zwift, TrainerRoad, The Sufferfest and Tacx App Workouts | DC Rainmaker
That’s why I’m therefore unsure. I always train in the evening after work. Similar diet, no weight gain, constant resting pulse. No change in the settings in Garmin. I’ve been very consistent now for a long time between 48-54. And my FTP is going up - that’ why I ask the question
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Garmin compares your heart rate (as a % of MaxHR) to your power output, speed has nothing to do with it.
If you have recently switched from riding outdoors to riding indoors, then your HR will likely be higher due at any given power level to the lack of cooling from natural airflow riding indoors. This higher HR will lead to a lower VO2Max estimate, even if your fitness has improved
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I’d take Garmin’s VO2max calculation with a pinch of salt. It has me down as superior and there is no way I am. It seems to go up a bit when I am do higher intensity stuff on the turbo but falls when I do Z2 or leisure rides. There’s also a bug in it at the moment (which Garmin know about) where the trend graphs update but the summary page doesn’t.
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“superior” depends if you are comparing yourself to other passionate cyclists, or the general public.
Also, the accuracy of the calculation is significantly improved if you use your PERSONAL maxHR, rather than the age-based default.
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Just FYI - this update still doesn’t pull across the Aerobic and Anaerobic scores that Garmin uses for their Training Status metrics. You need to run Garmin side-by-side if you want to use those metrics.
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My number has started to fall with age but its always lied in the top 1% of everyone on Garmin which includes passionate cyclists. I’d take that with a pinch of salt. I consider myself a decent cyclist but there’s no way I would fall into the top 1% of all garmin users.
My max HR is based on what I regularly can actually achieve and not the age based default, going by that I am 22 year old and not 46 
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Yep, my garmin VO2 is so off it’s comical - lab tested VO2 relates to WKO5 quite nicely OOTH.
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My TR to Garmin integration seems pretty hit and miss. Last 4 weeks contains a mix of outdoor workouts recorded on Garmin and indoor workouts recorded on TR iOS app. When I look at my VO2 history on Garmin Connect there’s a data point on every day I rode outside but only maybe a third of the days I rode inside. And as above the Aerobic/Anaerobic contribution for those indoor rides mostly doesn’t update either (sometimes it does but appears to be based on average power not time in zone, I.e. A TR VO2 Max workout shows up in Garmin as a Tempo ride despite having 0% of time in the Tempo zone).
Have those 10 points been a steady drop? Or is your estimated VO2 spiking up and down from one ride to the next? If it’s just steadily dropping as your FTP steadily rises with no setting changes that’s really weird!
That’s not correct. I’ve been training for weeks now via trainerroad exclusively, and only record my workouts through Trainerroad. My Training Status on Garmin shows the categorized training load (anaerobic, low aerobic, high aerobic). It also updates my cycling based VO2max on most trainerroad workouts. This however only happens after Garmin has synced the trainnerroad recorded workout to a Garmin device. So if you only have a cycling computer, it is enough to turn on the Garmin for a few minutes after your ride. Since I have a watch, I don’t have to do anything to make this happen. Trainerroad uploads ride to Garmin Connect. Garmin Connect syncs with Garmin device. After GC has synced with a device, your training status and updated vo2max is available in Garmin Connect. You can’t see the focus of your workout in the workout details, but you can see it on the training status page.
the drop was instant. and has been stable ever since. (4 weeks) But I also have to admit that there were no more outdoor rides. I will probably ignore the Garmin VO2Max over the winter and be happy when it goes up again in spring.
I hope that Garmin will improve the third party integration.
Would you take a screenshot of the ride and show me the Garmin Anaerobic and Aerobic values? My TR rides don’t show those, only those recorded with a Garmin device.
Training status updates as normal. So you’ll be able to see the stuff like your training load, split out into its three zones.
It’s training effect that doesn’t pull through when you do the TrainerRoad auto-uploads. So you won’t get the thing that gives the workout a score out of five on aerobic and anaerobic, or decides what kind of a workout (base, tempo, etc) it was.
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Go to training status in GC: https://connect.garmin.com/modern/report/activity_training_status/all/last_four_weeks
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Hover your mouse over one of the workouts in the exercise load chart (without hovering you won’t see it)
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The workouts should now be colored according to the focus. See attached screenshot.
Right, and since Garmin doesn’t know what the effect was (was it aerobic, was it anaerobic?), the data it gives you on “productive, unproductive, etc.” is going to be incorrect.
I’m asking to see the values for Aerobic and Anaerobic, like this. The TR file doesn’t have this info, which is why you have to still run side by side.
As far as I know, that productive/unproductive doesn’t need the training effect data.
It’s just a function of load (TSS, which is up, flat, or down) and fitness (VO2 Max, which is the same).
The productive/unproductive/peaking/recovery/etc are all just descriptors for that. Unproductive is when load is flat or up and fitness is down, for instance; peaking is when load is down and fitness is up.
Your training status descriptor should be able to change fine through TR workouts, since it has load and it has VO2 Max. Or mine does, anyway.
Like I tried to explain, you won’t get that in the workout details. But GC shows those values when you hover the workouts in the chart I mentioned. In addition to the colors shown, it also adds a text labeling them as threshold, tempo, basis, etc. just above the chart during hover.
So not visible in the workout details, but definitely (for me) visible in the training status charts.
Below chart shows the category (tempo in this case), and the labeling based on colors (high aerobic for the hovered workout):
