Are Apple Watch cardio fitness readings reliable?

Less, I’m skeptical that their “science” would hold up to peer-review, I highly doubt their methodology has validity or reliability. I’m similar to you. Currently I’m riding 90 to 110 miles a week. My average weekly activity exceeds the physical activity guidelines for Americans which means that I’ve hit a level of activity where I’m getting the maximum possible benefits from exercise and have gone into a region where the additional time only gives you training benefits. My FTP is 223 as measured by Sufferfest’s 4DP. In Zwift I am often scoring in the top 10 in the sprints, and am often in the 70th percentile on the climbs. This is amongst a population that is likely much more fit than the general public.

Another point of data, I had a series 4 Apple Watch and it was saying my VO2 Max was 38.8 to 40.1. I bought a series 6 and it instantly dropped to 34.7. When I first bought my series 4 it had me at 44.5, then it steadily declined. There’s no way that’s possible. I’m WAY more fit now than I was then. For the same power output my heart rate is lower. I’ve lost 10 pounds, my FTP and 5 second NM have gone up. I’m consistently setting PRs on the bike. I’m getting faster. This would not be possible if my VO2 Max was decreasing.

Doing over 300 minutes of exercise per week, much of it at an intense level, and given that my performance is consistently improving, I’d be really surprised if my VO2 Max was below average. It would be one thing if it was consistent with itself and you could use it as a relative benchmark but I don’t personally see evidence of that. It’s just all over the place. I’m curious to see if there actually is an independent peer-reviewed research checking the validity and reliability of their VO2 Max predictions. The other question I have would be is it discipline dependent? Of they’ve only tested it using walking/running using a sample of people who only walk or run and don’t drive the majority of their exercise from other activities like cycling, XC skiing, rowing, etc. then it may be invalid for a significant portion of the population.

@AnotherHerbert, don’t let this number get you down. The VO2max on the Apple Watch has neither validity nor reliability from a scientific standpoint. I’m finishing up a course on prescribing exercise as medicine for a graduate degree. Based on what you said I can tell you that you are exceeding the guidelines set forth by PAGA (Physical Activity Guidelines for Americans), the WHO, American Heart Association, et al. (and I’m guessing you’re not in the US but the governing bodies in other countries all seem to agree with one another on the science of what the minimum daily dose of Physical Activity should be, and you’re at more than double that dose). I can tell you with 100% confidence that even if you were to see no changes in biometric measurements that the Watch can record, that your body is realizing the benefits of exercise induced adaptations. Focus on the valid, and reliable measurements you can make. If you run a 5k, can you do it faster than you could 6 months ago if you try to set a personal record? If you walk on a treadmill, or a track at a constant 15 minute mile pace is your average heart rate lower than it was months ago? How about recovery time? You can also do something like the YMCA step test where you step on and off a 20 cm step at a specific cadence for several minutes, and then measure how many BPMs your heart drops when you stop and sit for a minute. Improvements in those numbers all correlate strongly with an increase in VO2max. Other things to pay attention to are how you feel emotionally after exercising (less stressed, less anxious), and over time (do you have more energy, less fatigue, get less winded when climbing stairs, sleep better, etc.). When you go to the docs, what are your lipid panels and blood pressure looking like over time?

Finally, the scale isn’t as important as the fitness industry, or even doctors would lead you to believe it is (not that it’s unimportant, but there’s a big picture which is often neglected). Let’s say you have type III obesity. This is a BMI of > 40. Based on the amount of physical activity you’re getting, your probability of death due to cardiovascular disease is about the same as a person with a “normal” BMI of less than 25 who is sedentary (which is most people, very few people in a given population meet Physical Activity guidelines). As a side note, BMI is useless for predicting all-cause morbidity and mortality on an individual level (it works on a population level, but not an individual level), why it is being used by clinicians, I don’t know. There are much better metrics available for predicting all-cause morbidity and mortality, but I digress. So just keep doing what you’re doing. Eat a diet that is mostly plant based (we would call this low caloric density) with some protein (preferably fish and poultry)... but don’t count calories (if you’re eating whole foods with a more caloric density... with the exception of potatoes and white rice... eat until you’re full... diets based on caloric restriction always backfire in the end).

Anyhow, this feature of the Apple Watch is pretty useless. I have some more objective measures of VO2max and they’re consistent. The Apple Watch behaves randomly.

As for “average level”, my VO2max is easily above 46 and the Apple Watch consistently tells me I’m below average. This year alone I have ridden my bike 1,678 miles, and climbed 70,280 vertical feet. I’m ahead of target to ride 4,350 miles/7000 kilometers for the year. My current weekly average is 104 miles and that will kick up to 150-200 in a few weeks. That’s just on the bike at a very vigorous pace. A few weeks ago I averaged 20 MPH on a 26 mile ride with 2000 feet of climbing. Nobody with a below average VO2max can do that, it’s impossible. Those numbers also don’t count the moderate physical activity I get. So yeah... this feature is a total joke.

I don't get it either - I read that cycling workouts aren't recorded. My cardio fitness levels are reported as low and getting lower - I do have data points that show I was around 30, I then notified the app I was on beta blockers and it dropped to around 21 from that point.

I cycle around 70-100 miles a week on roads (and at the moment on an indoor trainer) and average around 16-18mph on the real roads. I have an FTP of 216 which is respectable for a 52 year old. I'm not a 'flat tourer' either; I always include climbs & even this AM, did a Tour leg in Zwift that had 500metre climb in it. I finished about half way through the pack of athletes.

I think that the estimation it makes is based on a mostly sedentary daytime life (I'm a desk worker.. on lockdown) and walk the dog at the weekends when I get time.

In short - I think it's B*******

Reset your fitness calibration data

From your iPhone>Watch>Privacy>Reset Fitness Calibration Data>Tap this

You will just get a confirmation box that tells you that your metrics will not be as good if you do this.

Tap confirm and do it. You will get nothing else, no confirmation but it is done.

Then go for a 20 minute outdoor run or walk using the workout app at your normal speed on a flat road/path etc

This will re-calibrate the watch.

Richard, we all understand this. Their methodology for VO2max appears to not only be inaccurate, but inconsistent with itself. My VO2max is currently around 46.34, but the Apple Watch reported it as 36 on a walk today. As reported by others I’ve also experienced VO2max numbers creeping down over time, or jumping up randomly with no plausible explanation. My numbers took a dive after buying a new Watch series 6. Prior to that they peaked in October which also makes no sense at all. They claim it’s validated, but they have not to my knowledge, published peer-reviewed research.

Finally, nobody should expect these numbers to be super accurate (that requires a lab test) but they should be... not completely random (which for many of us it what they appear to be). If the results were consistent, then much like bioimpedance, it could be used s as a relative measure of progress, but the inconsistency makes it useless for even that.

I can confirm your experience Jemmej, recalibration does absolutely nothing. I think they should remove the feature completely until they can get it to behave with consistency so it can at least be accurate *relative to itself* which it currently is not. Same goes for HRV, their method of collecting it doesn't appear to be in accordance with any published peer-reviewed research, and there is no control over when it's take which makes it completely useless. Fortunately Apple HAS released the r to r reading in the API and Elite HRV is working on an updated version that for the first time will allow the Apple Watch to collect scientifically valid, and reliable HRV data.

i just purchased my Apple Watch in Dec and workout daily. My v02 max and cardio score started out above average and goes down every single day. Now I’m below average with no change in the heart rate or duration. Makes no freaking sense and my graph looks like a gradual hill going lower and lower, despite daily workouts.

Well, I think there's more to it. I'm not taking any medicine, so they are not in my profile and I have no way to delete them. However, since I got my Apple Watch in February, I've taken up sports and movement during the day far more rigorous. After a slow initial climb of VO2max starting at low and climbing up to below average, it got stuck at some still below average level and is now starting to decline. I'm far fitter than I was when I started, doing machine rowing and brisk/forced walks for at least an hour every day at 6-7 km/h (and still going up), lost a few kilos too, feeling much more healthy, resting heart rate dropped, but so does VO2max. It makes no sense whatsoever and is actually really demotivating. I never got to "average level" to begin with and see no way to stop the measure going down. Even adding my actual lower weight only seems to make the measure going down further . . .

I have a very similar problem.

I take brisk walks several nights per week. From November 2019 until January 12, 2021, my Cardio Fitness/VO2 Max reading consistently fell in a range between 33 and 36 (Above Average).

Then on my very next walk, January 19th, 2021, my score plummeted from 35.7 to 22.5, and ever since then-- 18 entries in all-- it's been in a range between 22.5 and 25 (Low).

Nothing about me changed in the week between 1/12 and 1/19. So either the reading was wrong before January 12th, or it's wrong now, and I have no way of knowing which. I don't believe I applied any iOS or WatchOS updates that week, but I don't know for sure.

I understand that the watch's VO2 Max is a calculated estimate and thus not as accurate as a test conducted under controlled circumstances with sophisticated equipment, but what's the point of this feature if it can't be relatively consistent within its own limitations?

So it's pretty much worthless. Less than worthless, actually, since it introduces doubt about my true level of fitness that I didn't have before. It actually makes me skeptical about any health-related reading the watch offers-- heart rate, ECG, Blood Oxygen-- all of it.

That was an excellent insight. I am also taking such medication, and I do remember putting that into the app at one point. I'm not certain it coincides with my VO2 Max score dropping off a cliff, but I'd be willing to bet that is the case.

Anyway I removed the medication from my profile and went on a few walks. My VO2 score leapt up by five points-- not back to where it was before, but about halfway.

I've come to the conclusion that this feature is basically just a random number generator.

OK, maybe not that bad, but I think it's clear that regardless of your fitness level or whether or not you take medication, Apple's VO2 Max calculation is highly fudged and really not something you can rely on as a way to gauge your own fitness or compare it to others.

Thanks for the tip, but it didn't do anything.

I reset my data then went for a 29 minute walk, but when it was over my reading was 22.5, in the same "low" range as it's been every day since January 19th.