How accurate is cardio fitness score in the heat?

[Morgan.Allen]

Hello! So I have a theory that cardio fitness score may not be as accurate as it could be and wanted to see if anyone could do some explaining for me. My cardio fitness score was 46 after doing some runs in cooler temperatures. Today I had to run in 82 degree humid weather which is not optimal. Of course, I ran at a a slower pace (30 seconds per mile slower) and my average bpm was higher (by about 20). However, after research I found out this is completely normal. As a matter of fact, the web said it is normal to slow down 30-90 seconds per mile in hot weather, so I was on the lower end of that. In addition, 20 bpm is right where your heart rate is expected to rise. To my dismay, I found out after my run my cardio fitness score has dropped to 45. I am baffled at this. If you were to just look at the statistics of the two runs, it would look like I have gotten more out of shape, but this is without the weather taken into account. This hardly seems fair and seems like maybe something that could be fixed? I was just proud of myself for still going on a run in the heat haha. Thank you!

[Guy_]

Hi @Morgan.Allen - the idea is to get you to keep fit, so that's already working!

You may be right about heat affecting readings [possibly more sweat may interfere with the readings] but if you look in the forums heart rate is known not to be very reliable when moving, for some it over reads and others it under reads, and of course heat can also vary it.

In all probability you may get to know variations over time. My heart rate measured is nearly double the real value during exercise, but it's generally pretty consistently wrong, if that makes sense.

[Morgan.Allen]

Yes I love any kind of statistics so the fitbit definitely helps me get moving because I can actually see each little bit of progress (usually, except for this case where it appears that I have somehow gotten less fit from running. Interesting) I just wish these devices could somehow be made more accurate if they are known not to be so. I feel like mine is generally very accurate but maybe it’s not. Thank you for the help!

[t.parker]

There has been research study done whether temperature affects VO2Max and indeed it does. However, that effect disappears once an athlete becomes heat acclimated. The cardio score is Fitbit's name for VO2Max. Garmin takes that into account adding "heat acclimation" metric so the VO2Max and performance are being adjusted to the level of heat acclimation. Fitbit has no such feature and doesn't know what temperature you are exercising in. Nevertheless, if you exercise in higher temperature you get heat acclimated and your VO2Max should go back to normal (at least this is conclusion of the linked research paper).

[LokeAa]

You say, " My cardio fitness score was 46 after doing some runs in cooler temperatures". I do not know why Fitbit gives a cardio fitness score from day one. Data over some time is needed to get an accurate score. Maybe it is just a coincidence, my estimate is 52-56. Another app I use gives me 54, a few years back my estimate was lower, on my journey to better cardiovascular health the two apps have been at the same page all the way. I would say it is accurate, again, maybe it is just a coincidence?