Fitness Score Dropping

[Caroline97]

I've been using my Fitbit for a few years now, and early last year I started running. I always track my runs with gps and apart from doing longer runs over time, nothing had changed. Except that after an initial period of steady increases in my fitness score, it has suddenly started dropping. I'm running further and faster than ever and yet it continues to drop. I'm telling this because the service seems to either be track your runs or that it's given false readings at the start. Neither of these things seem to be the case, and I'm hoping there is an actual fix for this.

Thanks for your help

[LZeeW]

Hi @Caroline97   I don't mean to insult your intelligence with this explanation.  Your cardio fitness score is an estimate of your VO2max.  Fitbit estimates this by measuring heart rate over distance.  If you have two otherwise identical people (sex, age, weight, etc.) running side by side, the one with the lower average heart rate will have the higher cardio fitness score.

Please take a very close look at your heart rate graphs for your runs.  Do they seem accurate?  I wear  Sense 2, but I am seeing a lot of error in my current cold weather.  My hip won't let me run, but I can guarantee you that my heart rate isn't 160 walking level.  I feel like my heart rate is more accurate with my Fitbit completely covered -- no cold air sneaking in

Have you added more hills?  It's my opinion that the algorithm just uses distance and heart rate.  I don't think the algorithm is 'smart' enough to use elevation data collected in the GPS data.  Cardio fitness score is more accurate on level running.

I suggest you try some test runs where you run on a mostly level surface and see if you get a more realistic cardio fitness score.

[Caroline97]

Thanks, I don't do hills and my heart rate graph shows a steady improvement over the last six months or so. My fitness IS improving, but Fitbit says it's steadily declining and has been since before Christmas. I have great sleep scores and feel well rested, I don't overtrain - so why did the fitbit say this?

[LZeeW]

@Caroline97   Fitbit's VO2 max estimate is proprietary, but is similar to other known ways of estimating it, like the Cooper Test and the Rockport Walk Test.  It uses heart rate and distance.  Your sleep score isn't a factor.  You can also use this online calculator  developed by a repeated physiology lab to estimate your VO2 max.  Ignore the advertisement for the app.  I used this calculator years ago when I ran and it was almost identical to my Fitbit Cardio Fitness Score.  It asks for an awake resting heart rate sitting quietly.  You Fitbit RHR includes sleep. 

If you think that your heart rate and distance data are correct, then it's time to look for other factors that can contribute to this drop.  I moved your post to the Get Moving forum, a better fit for something that isn't specific to a device.  I searched "does VO2 max drop in winter" and got a long list of reasons.  Try that, too.