Wearable Fitness Data You Can (and Can’t) Trust

This post is part of Find Your Fit Tech , a fitness wearable buying guide from Lifehacker. I ask tough questions about whether wearables can really improve your health, how to find the one that’s right for you, and how to make the most of the data wearables have to offer.

When you put on a wearable fitness device, you suddenly have a ton of data about your own body. Instead of a little voice saying, “I feel tired,” you now have a sleep score, step count, and perhaps a virtual coach advising you to tailor your workout to your fatigue.

Companies try to overwhelm you with more and more data, but if you dig through the mountains of information, you’ll discover a hard truth: only some of that data is useful.

Why you can’t trust everything your wearable device tells you

The paradox of wearable technology is that there will always be a discrepancy between what you want to know and what a bunch of sensors attached to your wrist can actually provide. Your goals are probably: sleep better, run faster, lose weight and stay active. The wearable device can provide data such as heart rate, skin temperature and distance traveled in a given time. There are some pretty big jumps between these two categories.

I find it useful to separate the characteristics of a wearable device into what it measures and what it evaluates . I like to use Marco Altini’s framework for this. It is divided into three categories, which he defines as follows:

1. Measurements are defined as data that comes directly (or with minimal interpretation) from the sensor. Examples include heart rate, skin temperature, and movement.

2. Calculations or estimates of things that we could theoretically measure in another way . For example, a wearable device could tell you how much time you spend in each stage of sleep, and theoretically you could conduct a sleep study and compare these metrics. There are studies that have tried to test or confirm some of these functions, but the results have been mixed.

3. Calculations or estimates of things that cannot be verified . Many of the more complex features of wearable devices fall into this category. If an app gives you a “sleep score” of 66, there’s no way to know if that number is correct. This is a made-up metric.

Then we are left with two questions. First, is this metric accurate ? This can only be judged by the first two categories and only if we have data that someone has collected and verified in large quantities.

And secondly, is this indicator useful? It depends on the context. What do you end up changing in your real life based on your fitness tracker data? For example, if you end up sleeping more or exercising more, that’s probably a good thing. But it’s easy to end up chasing a made-up metric that doesn’t actually benefit you. Skipping a workout to improve your recovery numbers or grumpily running around your living room because it’s 10pm and you’re only getting 9,000 steps in may not make you a healthier person.

Accuracy is only part of the picture

Fortunately, scientists can compare the performance of wearable devices to known standards for measuring things. They can test how accurately a device measures heart rate, measures sleep stages, or estimates the number of calories burned during exercise.

Unfortunately, these studies are not conducted before the devices are released. They occur after the fact, usually in small numbers and with insufficient funding. By the time they were published, companies had moved on and began producing different models. I discussed the results of some of these studies in this article on calorie burning (spoiler: every device is bad at counting the number of calories you burn).

When research is conducted before a device reaches the market, the research is usually conducted or funded by the company that profits from the device. And they can only confirm direct measurements or estimates of things that can themselves be measured (as in our sleep study example above). By definition, there is no way to verify the accuracy of any made-up metrics.

So, with all that in mind, let’s take a look at the types of data you can get from your fitness tracker and what’s typically worth paying attention to.

Pay attention to sleep time (but not stage)

Sleep tracking devices have become more and more sophisticated over the years, but their basic functionality has remained largely unchanged.

The best thing a sleep tracker can do for you is provide a reality check on how much sleep you’re getting. When you sleep, you stop moving, and your heart rate and other measurements like skin temperature can help the device differentiate between real sleep and bed reading. (Some devices do this better than others.)

But wearable devices cannot distinguish between different stages of sleep. To do this correctly, you need to analyze brain waves, something a wristwatch simply cannot do. (This is why I was amused, but not bothered, when I wore Oura and Whoop at the same time and got the opposite advice from each. Oura said I wasn’t getting enough REM sleep. Whoop said I was sleeping too much.)

Some devices have become impressively good at recognizing sleep stages, but this is still just guesswork. For example, the Oura ring, touted as one of the best, found that its newest algorithm was 92% to 93% accurate for determining when someone was asleep versus awake, but accuracy in determining sleep stages dropped to 76% to 78% . The study data shows that the algorithm still overestimates its measurements for some people and underestimates others, and accuracy varies depending on the age of the person wearing the ring. As a feat of technology: very cool. As for personal data: you shouldn’t pay attention to them at all.

Even if sleep stages were completely accurate, I still wouldn’t consider them. Because how do you get deeper sleep, lighter sleep, or faster REM sleep? The advice for each of these issues always comes down to this: get more sleep. Here Woop says that if you want to sleep more deeply, you should spend more time in bed; they point readers to the Sleep Foundation’s tips on general sleep hygiene. They give the same advice to people who want to get more REM sleep. You get the idea.

Bottom line:

• Trust the amount of sleep you report (but check for yourself: does that number make sense?)

• Don’t trust sleep stages or sleep quality indicators.

Trust your heart rate during your workout (not your zones unless you’re doing your homework).

Heart rate is a relatively simple indicator: your heart is either beating at exactly that moment, or you are between beats. Therefore, it is reasonable to expect this metric to be relatively accurate.

In truth, accuracy varies depending on the model, but most popular brands are good enough for general use. For example, here’s a 2020 study that found the Apple Watch Series 4, Fitbit Charge 2, Garmin Vivosport 3, and Xaomi Miband 3 did a pretty good job of measuring heart rate at rest or during long workouts.

Devices tend to have trouble detecting an accurate heart rate when you’re moving around a lot, so if your heart rate constantly spikes and resets when you’re doing something very active, your wrist tracker may miss some of those spikes. But if you want to gauge how hard you worked throughout your entire workout—say, whether you did your entire run in zone 2—the sensor is probably good enough. If you want to make sure you’re getting accurate heart rate readings, I recommend a chest strap .

What about these zones? This is where things get more complicated. If you train with your heart rate in mind, you’re probably using zones . Zone 1 means you barely work; zone 5 is an all-out effort that you can only sustain for a few seconds. A given workout could be targeted to a specific zone: for example, a light jog could be done in zone 2. All of this would make sense, except for two things:

• “Zone 2” has no generally accepted definition; All companies define their zones differently .

• The zones are based on assumptions about your heart: your maximum heart rate is usually calculated based on your age. These calculations are often very wrong.

If you’ve ever finished an “easy” run only to find that you spent the entire time in zone 5, I can guarantee that your zones were not calibrated correctly. This usually happens because your wearable uses a formula to predict your maximum heart rate and then sets zones based on that prediction.

But all heart rate prediction formulas suck , and they become less accurate as you age. Subtracting your age from 220 has no bearing on how fast your heart can beat. The authors of the article I just linked to concluded that “people should use [stepped treadmill tests] to determine [maximum heart rate]” rather than relying on a predictive equation. Meanwhile, the American Council on Exercise teaches its personal trainers to ignore formulas entirely and either use exercise tests to set personalized zones or use RPE (essentially how hard you feel you’re working) to design clients’ workouts. .

And if you think asking a person to guess “uh… I think it’s like three out of 10” sounds inaccurate: well, it’s still better than relying on an arbitrary zone calculated by an error-prone formula. The formula is also not exact.

Bottom line:

• Rest assured that your heart rate readings will be approximately accurate throughout the exercise (the overall picture, not individual spikes).

• Don’t trust heart rate zones if they were calculated using default settings.

Trust restoration metrics such as HRV (but not readiness metrics)

Do you need to track your recovery? No. Most of us should probably stop there. But some of us like to see how our exercise and sleep habits affect each other, and understand what’s going on in our bodies when we’re sick or stressed. I’ll be the first to tell you that this data is useless, but in the meantime, you can snatch my Oura Ring from my cold, dead hands. I like looking at numbers.

When it comes to recovery, I see a huge gap between what the heart rate numbers can tell me and what the app actually determines about my “recovery” or “readiness.”

• Your resting heart rate tends to increase when you are stressed or sick. It may increase sharply if you have been drinking alcohol or not getting enough sleep. It tends to decrease over time as your cardio fitness improves.

• Heart rate variability , or HRV, measures how irregular your heartbeat is. More variability is good, and your HRV tends to be higher when you’re healthy, not under too much stress, and sleeping well.

• Readiness or recovery scores , including metrics like Body Battery, use RHR and HRV, as well as a variety of other data, to come up with a number that describes your condition. Instead of just measuring how your body responds to exercise and sleep, they also take into account workout intensity, sleep metrics, and other variables.

Because recovery metrics take into account so many factors, they can easily stray far from what you’re actually trying to measure. For example, I found that light cardio on “rest” days helped me recover faster from hard workouts, but Oura’s algorithm considered it too much activity and lowered my score. It also doesn’t make me feel like I want to work harder and take on more fatigue.

Looking back at my results, I really don’t see any significant correlation between the days I performed well in the gym or competition and the days Oura thought I had recovered well.

I’ve previously written about how to make better use of readiness data . Ignore the grades and take measurements in context.

Bottom line:

• Trust your HRV and resting heart rate

• Don’t trust metrics like recovery, readiness, or battery charge that try to cover too many things in one number.

Trust your calorie burn, but only in general

Probably the biggest feature of wearables for many people is the fact that they can tell you how many calories you actually burn every day. You no longer have to guess how much to eat – you’ll see the number right on your wrist!

It’s a shame they’re not precise enough to really deliver on that promise. As I’ve noted before, fitness trackers are notoriously inaccurate in calculating calories burned . They’re not as bad as those recumbent elliptical machines , but they underestimate some people and some activities and overestimate others. There’s no way to tell if the resulting number is too big, too small, or right – so what’s the point?

I think calorie burn numbers can be helpful in general. If you used to burn 1,800 calories a day but started marathon training and are now burning 2,200 calories a day, you might take this as a sign that you should eat a little more to fuel your running. .

What I wouldn’t do is spend nickels and dimes when it comes to specific numbers. Oh, I burned 100 fewer calories today than yesterday, so I only need to eat half a serving of salad dressing. Or I went ice skating for the only time this year and my watch says I burned 600 calories, so I can eat an extra 600 calories for dessert. Your watch is not accurate enough to confirm any of these assumptions.

Bottom line:

• Trust the general trend, using it to test the reality of whether your activity level has increased or decreased.

• Don’t rely on an exact number, especially for personal training. Fueling your body and feeling good is more important than matching the numbers exactly.

Trust the step count as long as you keep perspective

Personally, I have a love/hate relationship with this person. If I’m walking a lot or running a lot, I like to check my step count and see how it goes up. But if I’m at a point where my workouts are mostly on the bike or in the gym, my step count will be terrible, even though I’m getting a lot of activity.

Ultimately, the number of steps you take in a day is not an important metric to track . But this number is easy to track. Even if you don’t have a smartwatch, your phone is probably already counting your steps (just open the Apple Health or Google Fit app to see).

If counting steps motivates you, feel free to follow along. Just make sure you’re willing to be honest with yourself about whether it’s having a positive impact on your life.

Oh, and if you’re interested in accuracy: no two devices can agree on how many steps you’ve taken in a day. Some underestimate, some overestimate, and they all track some actions better than others. I wouldn’t worry about these differences. Just compare the readings you get day after day from the same device.

Bottom line:

• Trust the number of steps you get (it’s not entirely accurate, but it doesn’t have to be).

• Don’t believe the assumption that you need to hit number X every day to be healthy or a good person. Use the step counter if it helps you and ignore it if it doesn’t.

Trust your cardio fitness or VO2max, but take it as a rough estimate.

Almost every gadget will now give you an estimate of your VO2max, often under a label like “cardio fitness score.”

VO2max is a measurement that can be done in a lab (and professional athletes often go to a lab to get tested) that tells you how much oxygen your body can use at one time. In short: the higher your VO2max, the better your cardio fitness . People with excellent VO2max can generally run longer and faster than people with lower levels.

Research has shown that VO2max correlates with improved health and longevity, but this is not necessarily because VO2max itself makes you healthier. It is just one of the easier components of aerobic and athletic ability to measure. (Other measures of cardiovascular health also correlate with longevity .)

Wearable devices test cardio fitness in a completely different way than a laboratory VO2max test. Instead of putting you on machines with an oxygen-measuring mask attached to your face, they simply measure your heart rate while you run or walk. If you can run faster at the same heart rate, or run at the same speed as before but at a lower heart rate, your cardio fitness has improved.

Should this number be compared to VO2max charts ? No, because it is not true VO2max. But you can keep an eye on this number over time and believe that you are improving your fitness when you see it go up . Just promise me one thing: you’ll look at how your wearable measures it and put that number in context. Some watches’ calculations will get confused in hot weather , and unless you walk or run outdoors often, the watch won’t have consistent data to work with.

Bottom line:

• Trust how this indicator changes over time (the higher the better).

• Don’t trust the exact number and generally don’t put too much emphasis on it unless you run or walk outdoors all the time.

Generally trust your mileage and GPS location.

When you engage in outdoor activities such as running or cycling, your watch will measure the distance traveled. This is useful for tracking overall mileage and speed, but there are caveats.

First, GPS is not always accurate and tends to have reliability issues in urban areas where signals can bounce off buildings. We discuss this more in our running watch and outdoor watch buying guides, but if you want the most accurate location data, you’ll want a device that can handle multiple satellite systems (such as GLONASS and Galileo in addition to GPS). ) and use dual-band GPS whenever possible.

However, most running watches and smartwatches now have superior location data compared to what they were capable of five or even 10 years ago. Your watch will likely be much more accurate than your phone.

But it won’t be perfect. These rare glitches can be very annoying. For example, there is a place in a local park where two roads are very close to each other and my watch thinks I’m on one when in fact I’m on the other. When it finally realizes where I am, the GPS track suddenly moves to my actual location, making it look like I’ve teleported and ruining my split time for that mile.

So don’t worry too much if your watch tracks distance a little differently than you expected. And definitely don’t worry if you run 5K and find that your smartwatch thinks you’ve run 5.3K. Your watch doesn’t measure distance in the same way as the course certifying body , so it’s quite normal for your watch to think you’ve gone a little further. Finish time is what matters to your personal best.

Bottom line:

• Trust distance and location with a grain of salt.

• Don’t trust race distances or locations to be accurate to the inch. A good watch will be more accurate than an old watch or phone, but nothing is perfect.

It’s up to you to trust the badges and stripes

This is where I come back to the gap I talked about earlier: the gap between what a wearable device can measure and the impact it ultimately has on your life.

Wearables brands are trying to bridge this gap through gamification. You can earn badges and continue streaks as long as you continue to interact with the device and its app. If these small doses of dopamine make you use the device, and using the device improves your health, it is probably harmless.

But, of course, things are not always so simple. On the one hand, regularly wearing an activity tracker encourages people to exercise more and can help with weight loss . On the other hand, the tricks devices use to ensure uniformity can end up backfiring. If the only thing keeping you going is the fear of breaking your streak, then once you break it, you better have something else to keep you going . Bands can serve as your training wheels, but they will never replace the work of proper habit formation .

Bottom line:

• Trust that through action you will find your own motivation. (If you try, you probably will!)

• Don’t trust that the icons and stripes themselves will help you move forward.

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