I have, at my disposal, at least nine different devices that can estimate my cardio fitness. They all put it in terms of a number scientists call VO2max. But the only way to find your actual VO2max is to get a test done in a lab, so I knew what I had to do. From my results, I’ll tell you which devices gave me the best and worst readings—and what that means for my (and your) training going forward.
VO2max is a measure of cardio fitness, so athletes and their coaches have long been interested in knowing their VO2max numbers. But more recently, VO2max has become a wellness buzzword, for some reasons that make sense and some that are probably a bit overblown.
Anyway, a big reason for the buzz around VO2max is that it’s been associated with longevity. Fitter people tend to be healthier and live longer, and VO2max puts a simple number on the otherwise nebulous concept of fitness. A 2016 statement from the American Heart Association pointed out that cardiorespiratory fitness may be a better predictor of mortality than traditional risk factors like cholesterol levels.
So if your VO2max goes up over time, that’s a good sign, whether you’re interested in winning races or just living a healthy life. Smartwatches will often estimate your VO2max based on workout data, so pretty much every wearable these days will give you a VO2max estimate, sometimes labeled as a “cardio fitness” score.
How do you get your VO2max tested for real?
Credit: Dr. Michelle StehmanTo actually test your VO2max, you need to go to a lab. And that’s why I drove out one sunny Tuesday to the Human Performance Lab at St. Francis University, where Dr. Kristofer Wisniewski and Dr. Michelle Stehman put me through a treadmill test.
For the 48 hours before the test, I was instructed not to have any alcohol. For the last 12 hours, no intense exercise. For the last three hours, no caffeine or food. That last part panicked me a little bit, until I realized I had plenty of time for a normal breakfast before my midday appointment. I showed up in exercise clothes and I brought a water bottle, although I couldn’t drink from it during the test. In hindsight, I should have also brought a snack to eat afterward while I awaited my results.
There was a chest strap to measure my heart rate, which they wanted to make sure was “uncomfortably snug” and tucked underneath my sports bra band. Then there was a mask over my mouth and nose, measured to fit and secured in place with straps that went tightly behind my head. You can see this in the photo above, and it too was, by design, uncomfortably tight.
Before the treadmill started, there were lots of little things to be aware of. For example, I wouldn’t be able to see the treadmill while I’m running—that turned out to be more disconcerting than all the physically uncomfortable stuff. A sign on the wall in front of me was perfectly centered on the treadmill, so I could use that as my visual anchor. If I got off-center, Dr. Stehman would tell me to move a little to the right or left. If I wanted to steady myself on the handrail, I needed to do that with my hand palm-up, since a palms-down grip could affect my blood pressure readings. Dr. Stehman would, yes, be taking my blood pressure with a cuff and stethoscope at a few points during the test. And every few minutes, Dr. Wisniewski would ask how I’m feeling, and how hard I felt I was working on a scale of 1 to 10.
I’m going to get just a tiny bit more technical in my explanation, to make sense of why I had to be hooked up to all this stuff on a treadmill. VO2max literally means the volume (V) of oxygen (O2) that your body can use per minute, at maximum (max), during exercise. It’s measured in milliliters of oxygen per minute, per kilogram of your body weight. (Bigger people breathe more air than smaller people, even if they aren’t necessarily fitter, so the equation accounts for that.)
Why do we care about the amount of oxygen you breathe? Because it corresponds to how much work your body is doing. If you remember that respiration equation from high school biology—glucose plus oxygen feeds into a system that gives you energy in the form of ATP—knowing your oxygen consumption tells us how much energy your body is making and using aerobically.
On the other hand, an out-of-shape, sedentary person would not be able to do what the elite athlete does. They’d manage a brisk walk, maybe a little jog, but they wouldn’t be able to work nearly as hard as the athlete, and so they wouldn’t consume nearly as much oxygen. They would be measured as having a lower VO2max.
On average, younger people tend to have a higher (better) VO2max than older people, and men tend to have a higher VO2max than women. Elite athletes have been recorded with VO2max numbers in the 70s and 80s, but among recreational athletes, many of us will have numbers in the 30s and 40s, maybe 50s. (For context, Garmin has a chart that breaks down what’s considered “good” by age and sex.)
How smartwatches and fitness trackers measure VO2max
Apple Health on the phone, Garmin watch at left, Suunto watch at right Credit: Beth SkwareckiGarmin devices, for example, use GPS-tracked activities that last at least 10 minutes. Garmin can trim out parts of your activity that aren’t helpful—say, times you stopped to tie your shoe or chat with a neighbor.
Each device has its own algorithm to turn the data it collects into a VO2max estimate, and that starts with recognizing when an activity is able to give the algorithm enough data. This varies from device to device; Garmin wants a 10 minute minimum activity, while Coros wants 25 minutes. You often need to have a certain minimum heart rate for the algorithm to kick in. Here’s an example from Apple’s developer documentation that describes when and how it calculates VO2max from an activity:
These details vary from device to device. Some Garmin watches can use power meter data from a bike in place of GPS. These algorithms generally require the device to know your maximum heart rate, which they are notoriously bad at estimating, but which they can measure directly if the device is programmed to do so. For a deep dive into what one of these algorithms looks like, here is a paper published by Firstbeat Analytics, which built Garmin’s VO2max algorithm. (It’s not clear if the details described here are exactly the same as what Garmin watches currently use.)
Oura is a bit different from the other devices I tested. Instead of calculating a VO2max estimate from your regular workouts, it prompts you to take a six-minute walking test. This type of test is well known in the medical field, and has been used to estimate VO2max, if imperfectly.
Studies on smartwatch VO2max estimates generally find that they correlate with tested VO2max results—the higher the smartwatch estimate, the higher the tested VO2max for the same person—but that the exact number can be off by quite a bit. For example, this study on the Apple Watch Series 9 and Ultra 2 concluded that “For individuals with good or excellent fitness, Apple Watch demonstrated a propensity to underestimate VO2 max, whereas among those with poor fitness, there was a tendency to overestimate.”
My results, and the winners
Credit: Beth SkwareckiFor any devices that didn’t have recent data, I made sure to take them for a run or two so they could recalibrate. Where I had multiple devices of the same brand, they all fed data into the same app or algorithm, so I’m organizing the results by brand rather than device. A full list of the devices I used is at the bottom of this article.
Tested VO2max: 42.8
Fitbit: 41 (1.8 points low)
Whoop: 46 (3.2 points high)
Coros: 37 (5.8 points low)
Withings: 36 (6.8 points low)
Garmin came out on top, estimating a VO2max of 44, just 1.2 points over the actual value. I was expecting Garmin to be pretty good, since it knows my exact max heart rate and I’ve already seen that its 5K race time estimate was pretty close to my actual time.
After that, Whoop stands out with its three-points-high estimate of 46. Whoop won’t reveal exactly how it estimates VO2max, but since it supposedly doesn’t require exercise data at all, I don’t trust it very far. (I did make sure to feed it some GPS data during my testing, which it said improved the accuracy of my estimate.) If it’s a guess, at least it’s a flattering guess.
The rest are all around five or more points too low. If I had trusted my Apple Watch, I would think I’m a lot less fit than I really am. Along with Coros, Oura, and Withings, it gave a number in the 30s. I really can’t be too impressed by these.
The VO2max estimates from each device have their own parameters that I don’t necessarily know about. I did my best to have a correct (or close-enough) weight, age, and where possible, max heart rate entered in each app. But since the companies don’t all disclose what variables they use in their calculations, I don’t have a full list of numbers to go in and double-check.
How useful is the VO2max score on your device?
I’m going to be honest here: after all that science, I can condense the practical advice into about four words: “Make number go up.” Whether your VO2max comes from a lab test or a smartwatch estimate, it will tend to get higher as you do more exercise, more consistently.
(If your watch’s estimate isn’t getting higher as you feel you’re getting more in shape, I’d check it by testing your fitness another way, like timing yourself running a certain distance, or even gauging how you feel during a workout you’ve done before, and seeing if that improves over time. But normally we’d expect changes in these VO2max estimates to keep pace with fitness improvements.)
Without quibbling too much about where the borders of these ranges might lie, I think these are fair judgments given that the lab said I’m in the 96th percentile of my cohort of middle-aged women. That sounds impressive on paper, but in real life I'm a pretty average runner. That "for your age and gender" asterisk is doing a lot of work.
Your VO2max is connected to all of that, but it’s not literally the same thing. You can have a high VO2max and still have health problems. Athletes often find that their real-life race times are faster or slower than their VO2max test results would suggest. Coaches don’t just say “let’s get your VO2max up.” They’ll have runners work on their lactate threshold, their running economy, their mental toughness, their leg strength, and dozens of other things.
The specific device models I used
In some cases, multiple devices fed data to the same app or algorithm. For example, even if you have three Garmin watches linked to the same account, you only get one VO2max score that will display in the Garmin Connect app and on any of the watches. The watches will not disagree with each other in their scores.
With that in mind, the list below includes the devices I used around the time of my VO2max lab test as the primary sources for each brand’s estimate.
Coros: Pace 3 (Used less recently: Pace Pro)
Garmin: Forerunner 265S, Forerunner 570 42 mm. (Used less recently: Vivoactive 6)
Suunto: Suunto Run
Whoop: Whoop 4.0
I made sure to get an updated VO2max estimate from each device within about a week of my VO2max lab test (either before or after the test, as convenient). The only exception was Whoop, which requires 14 days of recent sleep data to give you an up-to-date VO2max estimate. My last VO2max estimate from Whoop was three weeks prior to my VO2max lab test.
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