What causes inaccurate VO2 Max readings on Apple Watch?

I continually show low cardio fitness in the Health app. I believe this is due to the inaccuracy of the VO2 max calculation from my Apple Watch Ultra 2 but this has been the case since I had the Series 5. There is no way my cardio fitness is 'very low'. I work out regularly, incorporating HIIT workouts twice per week. My resting heart rate is in the mid to high 50s (I know that's not an altogether gauge of cardio fitness, but it's certainly relevant). I always warm up and cool down with a 20 min walk (each time for a total of 40 mins, with my workout in-between). Usually, I do the walks on a treadmill, but recently I've been doing my cool down walk outside, which is being logged by the Health App for cardio fitness. I believe this is what's throwing off my cardio fitness score because I immediately start my walking cool down right after a very strenuous HIIT routine that lasts any where from 20-25 minutes. So my heart rate is very elevated when I start my walk and gradually comes down.


However, I have done several outside walks on days when I didn’t work out and they are still showing very low to low fitness level.


Is there anyone with knowledge of how the watch calculates VO2 max that can confirm that this workout configuration could lead to an artificially low cardio fitness score??


[Re-Titled by Moderator]

Apple Watch Ultra 2

Posted on Jan 19, 2025 1:19 PM

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Posted on Feb 17, 2025 8:54 AM


mpambika wrote:

Same here. I have experiance similar results. My VO2 max has dropped to below 30 despite playing tennis, running and doing HIITs each week? I am puzzled as well. I wear Series 9.

Keep in mind that the only accurate way to measure VO2 max is in a laboratory on a calibrated treadmill with multiple sensors monitoring you, including measuring your respiration volume and CO2 content. More basic activity monitors such as the Apple Watch cannot produce accurate results.



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Feb 17, 2025 8:54 AM in response to mpambika


mpambika wrote:

Same here. I have experiance similar results. My VO2 max has dropped to below 30 despite playing tennis, running and doing HIITs each week? I am puzzled as well. I wear Series 9.

Keep in mind that the only accurate way to measure VO2 max is in a laboratory on a calibrated treadmill with multiple sensors monitoring you, including measuring your respiration volume and CO2 content. More basic activity monitors such as the Apple Watch cannot produce accurate results.



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Feb 15, 2025 7:21 AM in response to jkdman123

jkdman123 wrote:

i don’t have a problem with it recording outdoor walks (which as far as I’ve read is the only workout it bases your fitness level on). They are just consistently rated as low or even very low even though my heart rate does not get very elevated during the walk.

Ok, so it's a little different than I thought at first. It still makes sense though. Here are a few examples that tie in to what I'm thinking you are seeing...


This time of year (winter - cold - lots of snow) I do more exercise indoors than outdoors. I'll indoor cycle, and lift weights. The watch records the cycling at a high activity level (as it should - even though I'm stationary). I'll go immediately from cycling to lifting weights, and even though it's more strenuous, I'm not breathing as hard and my heart rate is lower. The watch records this as either low fitness level OR sometimes I get the message that the watch thinks I have finished my workout and wants me to confirm... but here I'm still sweating and straining - just low heart rate. Go figure. I'll also go to a local park that has 18 miles of biking and hiking trails - I'll walk about 6 of them. I'm bundled up, and walking through snow. The watch knows I'm working out and records it as a high fitness activity level. I'll go back to this in a minute.


Nice weather... I cycle outdoors for two hours four times a week - and the watch is accurate - high fitness activity level. However going for walks on the same 6 miles of trails - wearing shorts and a t-shirt, on clear paths. Walking FASTER than I do in the winter, but since it's not as strenuous as bundled up walking through deep snow, the watch is constantly asking me if I am finished with my workout.


Bottom line - I think heart rate is the main driving force behind the watch reading activity level. And since it's only a watch and can't always really tell what you are doing, it's not super accurate.

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Feb 17, 2025 9:30 AM in response to mpambika

Keep in mind that the data provided by the Apple Watch is just an approximation. It’s not as accurate as clinical data. If you are interested in knowing your accurate data please make an appointment with a physician. Apple tells you that the data is not very precise in the Health app. You can then input the accurate data inside the Health app ,from Add Data.

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Jan 19, 2025 1:26 PM in response to jkdman123

Mine was kind of hit or miss for a while if I was just walking instead of doing a workout - sometimes it would count the data, sometimes it would not. More often, not. I now set the workout app to record an outdoor walk each time, and it seems to record everything.


Not sure if that is exactly what you were asking, but it's similar to what I was seeing on mine.

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Jan 19, 2025 7:52 PM in response to gsyrba

i don’t have a problem with it recording outdoor walks (which as far as I’ve read is the only workout it bases your fitness level on). They are just consistently rated as low or even very low even though my heart rate does not get very elevated during the walk.

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Jan 29, 2025 7:47 AM in response to jkdman123

According to my iwatch my cardio fitness dropped from above average to below average after a cardiac procedure that certainly helped my fitness and ability to exercise? Anyone else see similar results? Dropped from high 30s to mid 20s. ?


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Feb 17, 2025 9:34 AM in response to jkdman123

jkdman123 wrote:

If it’s gonna be wildly inaccurate then they shouldn’t even enable the function. Telling people they have ‘very low’ cardio fitness when they don’t could prompt them to seek medical attention unnecessarily.

Apple is clear in the article I linked to above that it is an estimation. I think Apple also assumes a certain amount of common sense on the part of its users. "Low cardio health" is not generally something that warrants a call to one's doctor.

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What causes inaccurate VO2 Max readings on Apple Watch?

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