One of the most important aspects of any wearable sleep or fitness tracker is its accuracy. Without reliable data, such devices would range from gimmicky to utterly useless. For this article, I wanted to take a closer look at WHOOP — the fitness tracker I’ve been using for the past 1.5 years — to find out just how accurate it really is.
Those two articles will give you a good understanding of the technology involved in fitness and sleep tracking, as well as the pros and cons of WHOOP. And if you’re new to WHOOP, you might need some of that foundational information to make more sense of this post.
To make this article easier to consume, I decided to break it up into different sections, with each covering the accuracy of specific data points and sensors.
- Optical heart rate sensor
- Calories burned
- Sleep performance
- Respiratory rate
- Other sensor data
In a nutshell, the WHOOP strap offers excellent accuracy in the following areas:
- Heart rate variability (HRV)
- Respiratory rate
- Resting heart rate
- Sleep quality and stages of sleep
However, you might experience mixed results in the areas of heart rate tracking during certain types of exercises.
While that might sound troublesome for a fitness tracker, this shortcoming is irrelevant (in my opinion) for most WHOOP users — including me. I’ll explain how I came to this conclusion later in the article. But first, let’s talk about WHOOP’s accuracy in each of the above-mentioned areas.
How Accurate is WHOOP’s Heart Rate Sensor?
The best way to determine the accuracy of a wrist-worn heart rate sensor is to use a three-lead electrocardiogram (ECG) and then compare the data. The second-best way is to use a chest strap, which doesn’t rely on photoplethysmography and instead measures the electrical signals emitted by your heart as it beats.
One thing that’s worth mentioning is that both the sensor and the open-source algorithm used by WHOOP to interpret the raw data and convert it into “heartbeats” are pretty standard across the industry.
Wrist-worn devices, such as the WHOOP, rely on light-emitting diodes (LEDs) that shine light through your skin and onto your blood vessels. Interpreting the light signal as it bounces back from the skin is an analog process, and is thus subject to interference and signal noise.
For example, a dark tattoo can absorb some of the emitted light, thus altering how much of the light is reflected back to the sensor. Similarly, dark skin has more pigments, such as melanin, and they absorb more of the laser light — which can in turn contribute to less reliable readings.
Factors that can negatively influence the accuracy of WHOOP and similar wrist-worn devices include:
- A loose strap that has insufficient contact between the sensor and the skin.
- Wrist movement that temporarily breaks contact between the sensor and your skin.
- Body motion that the sensor might interpret as the contractions of your heart.
- The ambient temperature and your body temperature, both of which influence blood flow.
- Variables that can interfere with the reading of the reflected light (e.g., your skin color or tattoos).
All the factors above make it incredibly difficult for WHOOP to accurately detect your heart rate. That’s particularly true in situations where your heart rate changes a lot, like during CrossFit or other types of high-intensity interval training (HIIT).
So let’s talk about each of these to see how they influence WHOOP’s accuracy, and explore any potentially mitigating factors.
WHOOP’s sensor needs to be in close contact with your skin at all times. Otherwise, it won’t be able to accurately measure your heart rate or other biometrics.
That’s why it’s so important to use a strap that provides a secure fit. It should be tight enough that you can place your pinky finger between the strap and your skin with no excess space.
But don’t over-tighten the strap either, because doing so can restrict your blood flow and cause issues.
WHOOP has obviously realized that the fit of its strap is a crucial factor in ensuring accuracy. That may be why the company released its ProKnit strap together with the WHOOP strap 3.0. It’s an incredibly comfortable and infinitely adjustable strap that makes it easy to keep the WHOOP sensor in close contact with your skin at all times.
If you do CrossFit or similar exercises that involve gloves, gymnastic grips, dumbbells or a lot of wrist movement, WHOOP’s bicep band might actually be the better choice.
I know that certain workouts I do negatively impact the accuracy of WHOOP when worn on my wrist. However, I don’t mind that for reasons that I’ll go into in a bit.
Skin Color and Tattoos
The optical heart rate sensor in WHOOP relies on the reflection of light that’s emitted by the sensor’s LEDs. Anything that can interfere with the frequency of the emitted light or its reflection can introduce noise and render the signal less accurate.
Examples include ambient light that reaches the sensor due to an improper fit or tattoos. So if you have tattoos, I recommend wearing the WHOOP on a spot with fewer or no tattoos.
Any time you move your wrist, you risk the sensor temporarily losing contact with your skin. If that happens, you’re likely to get some inaccurate heart rate readings. When that happens during a time where your heart rate changes a lot (e.g., dumbbell interval training), you might see a huge gap between your actual HR and what WHOOP recorded.
The best way to mitigate issues related to wrist movement is to use a bicep strap, or to wear a tight sleeve around your wrist that covers WHOOP (such as a wrist sweatband).
One thing you might not have known is that WHOOP (and other optical HR sensors) also rely on the accelerometer to detect the contractions of your heart and the force of the blood flowing through your blood vessels.
As you can imagine, the accelerometer picks up a lot of “noise” during exercises, such as steps taken, jumps, and any other movement. WHOOP has to distinguish between those different input sources and filter out the ones related to your heartbeat.
While differentiating between the motion induced by continuous movements — such as jogging or walking — and your heartbeat is relatively easy, it’s much more difficult for fast-paced movements like CrossFit.
But I’ve also heard reports that even monotonous movements like biking can throw off an optical HR sensor, because the way the muscles contract during biking can be misinterpreted as a contracting heart.
Unfortunately, there aren’t any mitigating factors that I’m aware of. It’s just a fact that WHOOP does better with certain types of exercises than with others.
Obviously, the same holds true for other wrist-worn devices, such as the ones from FitBit, Biostrap, Polar, Garmin and other brands.
Ambient and Body Temperature
Most wrist-worn sensors seem to do better in warmer temperatures. That’s because if it’s too cold, the body restricts capillary blood flow, making it more difficult for the sensor to get a good reading.
So if it’s cold outside, I recommend wearing extra layers or warming up sufficiently before you begin measuring your heart rate.
How Does WHOOP Track Calorie Burn?
Accurately tracking how many calories you burn is incredibly difficult without establishing your basal metabolic rate (BMR) first. Doing so involves a face mask that measures how much oxygen you inhale and how much carbon dioxide you exhale.
That’s not something you can easily do at home without the proper equipment.
Instead, what most fitness trackers do is try and predict your BMR based on height, weight, age, and biological sex. While doing so often yields satisfactory results, predicting the active burn is much more difficult.
WHOOP uses a heart-rate-based formula to estimate active burn, which utilizes a model developed in South Africa in 2005. You can read more about the challenges of predicting BMR and active calorie burn on the WHOOP blog.
The bottom line is that a wrist-worn device can’t accurately predict your absolute calorie burn, so don’t get hung up on the numbers reported in the WHOOP app.
However, WHOOP can accurately record relative changes in calories burned. This can be helpful if you want to compare rest days vs. workout days, or the impact of different types of workouts, on your calorie burn.
Also, WHOOP has partnered up with PNOE and is expected to launch an integration later this year. Once available, you’ll be able to use a mask (like the one my wife Kathy is wearing in the picture above) to accurately assess your basal metabolic rate and calories burned.
How Does WHOOP Track Strain?
The strain metric in the WHOOP app gives you insight into how much load you’ve placed on your cardiovascular system through exercise or other daily activities that might elevate your heart rate.
As you can imagine, the strain score is highly dependent on an accurate heart rate reading. As a result, certain types of physical activities cause slightly inaccurate strain scores.
What many WHOOP users don’t understand is that the strain score doesn’t impact the recovery score because the latter is predominantly influenced by your heart rate variability, resting heart rate, and quality of sleep.
How Accurate Is WHOOP at Tracking Sleep?
As I mentioned in my review, WHOOP is an incredibly accurate sleep tracker as confirmed by independent validation studies.
Since sleep tracking relies on an accurate heart rate reading (along with other biometrics), how is it possible that WHOOP can do that so accurately?
The reason is simple: your movement pattern is vastly different while sleeping vs. while exercising. In fact, during the restorative phases of sleep, you might not move at all. That makes it incredibly easy for WHOOP to take accurate heart rate measurements.
Considering the excellent results of various validation studies, it’s safe to assume that the hardware sensors packed into WHOOP can accurately track your biometrics while you sleep.
One thing I’ve seen other people complain about is WHOOP’s supposedly unrealistic recommendations on when to go to bed and how long to sleep.
Based on my understanding of how the “sleep need” figure is calculated, and my experience with the “sleep coach,” I don’t think the recommendations are unrealistic at all. What most people don’t understand is that sleep debt is accumulative.
So if you’ve been sleeping an hour less than your body needs for the past three weeks, you can expect a higher sleep need for the next few days or even weeks. Sleep debt doesn’t just go away because you want it to.
How Accurate Is the WHOOP Recovery Score?
The recovery score is so accurate and useful that I’d be wearing WHOOP even if that was the only feature it offered.
Many WHOOP users, and even other reviewers, don’t fully understand how the recovery score is calculated. What’s important to recognize is that the previous day’s strain score has zero influence on your recovery the next day.
Instead, the recovery score is a reflection of how your nervous system is doing. To find that out, WHOOP measures your HRV during the last phase of deep (slow-wave) sleep. That’s the time when your brain is most “disconnected” from the rest of your body, preventing movement or other influencing factors.
We already know that WHOOP is incredibly accurate when the conditions are right, and slow-wave sleep is the perfect condition to take RHR and HRV measurements.
Why I Don’t Care About Activity Tracking
Now that you know everything there is to know about WHOOP’s accuracy, let me tell you why the areas where WHOOP shows weaknesses are irrelevant to me.
I’m no longer a professional athlete. I’m a serious fitness enthusiast and I wear WHOOP to help me improve my overall health and well-being.
Having insights into the quality of my sleep, including what factors influence it and the condition of my nervous system, helps me make better decisions throughout the day.
What doesn’t help me is knowing how many calories I burned or if my strain on a given day was 13 or 14. These are nice parameters to know but they don’t significantly influence my behavior.
Of course, there are some exceptions to this. For example, if my strain is consistently high despite having exercised, I might have to work on reducing my stress — which is a common cause of an elevated strain score. But beyond that, I care more about my recovery score, my HRV/RHR trends, and how much restorative sleep I get.
These are factors that WHOOP can accurately report on. And they’re the ones that I can directly influence based on my lifestyle choices.
If you’re a professional athlete and keep tabs on strain and calories burned to plan your meals and workouts, then you might need a separate chest strap to measure your heart rate during exercises. Unfortunately, WHOOP doesn’t allow the use of third-party HR sensors; perhaps that’s something the company will support down the line.
Despite the fact that I don’t really care about activity tracking, I wanted to find out how WHOOP compares to a chest strap heart rate monitor in order to share that information with you.
WHOOP vs. Chest Strap Experiment
So I decided to do a little experiment and wear a Wahoo chest strap during a brisk outdoor walk and a typical CrossFit workout involving various movements that caused my HR to change rapidly.
Shoot me an email If you want to find out exactly how I set up the test. In a nutshell, I connected the chest strap to my Apple Watch, enabled real-time HR broadcast in the WHOOP app, and recorded WHOOP’s HR data using the Wahoo Fitness app. Both sources stored their data in Apple’s Health app, which is where I extracted it from.
The results were pretty much as I expected.
Brisk Outdoor Walk
WHOOP was relatively accurate in comparison to the chest strap during my brisk outdoor walk. On average, WHOOP was only 3.8 beats per minute off.
Here’s a better representation of how close WHOOP’s HR data was to the chest strap. I should point out, however, that the Wahoo app only recorded one HR event every four seconds, whereas the chest strap/Apple Watch combo recorded an event every second.
|Within 25 BPM||100%|
|Within 10 BPM||95%|
|Within 5 BPM||74%|
|Within 3 BPM||52%|
You can see those discrepancies in the graph below — particularly in the areas where my HR rapidly changed.
As expected, the HR deltas were wider during a high-intensity workout that involved dumbbells, push-ups, thrusters and box-step-overs with rest periods in between rounds.
In other words, the WHOOP sensor and algorithm had to deal with a rapidly changing heart rate, a lot of irregular motion, and wrist movement. It’s kind of a nightmare scenario for any wrist-worn fitness tracker.
|Within 25 BPM||96%|
|Within 10 BPM||81%|
|Within 5 BPM||58%|
|Within 3 BPM||40%|
As you can see in the table above, there’s a noticeable spread between what the chest strap recorded and what WHOOP thought my heart rate was.
Wrap-Up – WHOOP Is Accurate Where it Matters
Based on my use cases and requirements, the WHOOP wearable is accurate in the areas that matter most to me. That includes recovery and sleep tracking.
In other areas, WHOOP has the same limitations as other wearables and it might not always track your heart rate as accurately as a chest strap or an ECG.
So if you’re a professional endurance athlete who relies on super-accurate HR data for training and competitions, you will have to use a chest strap — there’s no way around that.
However, a chest strap doesn’t provide any of the other benefits of the WHOOP strap, including detailed sleep stage and recovery tracking. So you might end up with both a chest strap and a WHOOP.
I’m a hobby CrossFitter and usually don’t care about tracking my workouts on my Apple Watch or WHOOP. I just want to know how recovered I am and how I slept. Those are the factors that determine how I plan my day.
If you have any questions or concerns about the accuracy of your WHOOP strap, leave a comment below and I’ll answer them. If I don’t know the answer, I’ll get it for you!
If you want to give WHOOP a try, use this link and get 10% off your membership*.