Smartwatches from tech companies like Apple and Garmin make it easy to see a number that can more accurately reflect how old you are than your age: VO2max, the maximum amount of oxygen your body can use during intense exercise.
The higher your VO2max, exercise experts say, the better your cardiovascular fitness and possibly the longer your life. In the past, only serious athletes sought a traditional VO2max test, which involved wearing cumbersome sensors while training in a lab, but now anyone can get an estimate by wearing a smartwatch and moving around.
Is it good to have access to this kind of information? And how accurate can a wearable be? Over the past five months, falling down a VO2max rabbit hole, I learned some uncomfortable truths about my health and the limits of smartwatches.
First, let me tell you about my fitness journey. In November, while celebrating my birthday, my Apple Watch received the most unwanted gift: a high heart rate notification. That led me to look at my VO2max, which according to the Apple Watch was 32, well below average for a man in his late 30s.
Looking for a quick fix, I bought a subscription to a high-intensity interval gym, a type of workout that specializes in boosting cardio fitness. Five months and many kettlebell swings and jump squats later I felt progress. I burned fat, gained muscle and felt more energetic. The Apple Watch gave me a VO2max estimate of 40, just below average, and a Garmin watch I also wore rated me at 45.
All I needed to do was do a real VO2max test, so I found it. This is where the good news ends. A few hours after pedaling a stationary bike with an oxygen mask on my face, I got my lab results: 25, a rating of poor, well below the flattering results of the Apple Watch and Garmin. Devastating.
Dr. Ethan Weiss, a cardiologist in San Francisco who has studied wearable technology for years, said my experiment underlined the pros and cons of using smartwatch data to research health.
“On the one hand, you can give it credit for kicking you in the ass for telling you to go work out,” he said. “But then again, now you’re kind of burned out with this real test and you’re like, ‘What am I supposed to do with this song?'”
Fortunately, after studying all the data, learning how the wearable algorithms work, and talking to health experts, I came to a positive conclusion: Even if the smartwatch numbers were wrong, they were broadly correct, and I was probably better off if I didn’t wear one.
My experience can serve as a template for anyone trying to have a healthy relationship with technology that tracks many types of health data, from sleep patterns to body fat.
A deeper look at the data
Last month, my gym in Oakland, California, called Sweat, announced it was partnering with PNOĒ, a metabolic health lab, to offer clinical-grade VO2max testing, so I signed up with excitement.
The purpose of a clinical VO2max test is to measure your maximum oxygen uptake at the point of exhaustion. This measure — a person’s ability to breathe oxygen and produce carbon dioxide during exercise — is a strong indicator of cardiovascular fitness.
At Sweat, the gym owner, Cassie Hecker, strapped an oxygen mask over my face and a heart monitor to my chest. She instructed me to pedal on a stationary bike for about 12 minutes, increasing the intensity every minute as her equipment collected data. After reaching my maximum heart rate of 182 beats per minute and starting to struggle with exhaustion, the test was done.
The test was very different from the way the wearables estimated my VO2max. Apple Watch and Garmin study your heart rate and movement for at least 10 minutes while walking or running and add a score.
Apple and Garmin spokespersons referenced documents detailing their methods. To inform their algorithms on how to make these estimates, Apple and Garmin conducted studies with people who took a real VO2max test, as well as other exercises, and studied their heart rate and various metrics.
The keyword is ‘estimate’. The watches do not actually measure your oxygen uptake and therefore not your VO2max.
“It’s an imputed VO2max at best,” said Dr. Weiss. “Not only are you not wearing an oxygen mask, you’re not exerting yourself to exhaustion.”
One simple reason my wearables estimates differed so far from my actual VO2max result is that the way my body works doesn’t match the heart rate and oxygen intake patterns of participants in the Apple and Garmin studies. That is the danger of relying too much on algorithms.
That left me with no choice but to embrace the hard truth: my VO2max result in the lab test was very low. But that statistic was just one data point. The report also showed many positives, including a very high metabolic rate and fat burning efficiency, and healthy breathing patterns.
Putting all this information together, Ms. Hecker said she rated my fitness level as “average,” higher than the Apple Watch’s cardio fitness rating of “below average.” She instructed me to focus on my cardio workout. (Admittedly, in gym classes I pushed myself harder in exercises I enjoyed, like weightlifting, and tended to take it easy with the cardio exercises I found torturous, like burpees.) No doom and gloom.
Ignore the numbers; Follow the Trends
In the end, all the health experts I interviewed agreed that while the wearable data—most of which was flawed—had worried me, I had achieved a net positive. The Apple Watch encouraged me to take better care of my health, and as a result I am now healthier.
The broad trends shown by the wearables were accurate: months ago, after the pandemic had taken its toll on my mind and body, I was in my worst shape in years and watch counts were low. Now, even though the wait numbers are too high, I look and feel better, and that’s all that really matters.
That might be the best way to approach wearables — think of them as a directional arrow rather than a precise measuring device, said Steven Adams, a sports medicine physician and personal trainer in Danville, California.
Whether you’re using devices to measure your weight loss progress, getting more sleep, or taking more footsteps, you need to know if the numbers are going up or down. But don’t take them too seriously, because all sorts of factors can mess them up: a loose wristband, a faulty sensor, or, in my case, imperfect algorithms.
“It’s the trend that’s important, not the absolute number, because this stuff isn’t accurate,” said Dr. Adams.
