Whoop just released it’s next generation fitness tracker: Whoop 4.0. I have not used it personally yet but in this article I will be looking at the new features Whoop recently announced with a critical lens. I will explain what I find most interesting and what I find less interesting in comparison to the previous generation, Whoop 3.0. This will help you decide whether you should upgrade or not and which band to purchase if you are considering getting a Whoop.
One of the main issues with Whoop 3.0 was the accuracy of the heart rate sensor. When you break it down, Whoop is essentially a band with a heart rate sensor and an accelerometer. The data from these two sensors is then fed into an algorithm which produces the data displayed on the app. There have been a few complaints about the accuracy of the heart rate sensor on Whoop 3.0. Given that this is the main piece of input used by the Whoop algorithm to generate data, it can be a bit of a problem.
Whoop measures heart rate by using a technology called Photoplethysmography (PPG). This is the same technology used by Apple Watch. Whoop 3.0 uses two green LED lights (light-emitting diodes) that shine light through your skin. When your heart beats, there is an increase in blood flowing through the tiny arteries or capillaries under your skin. Blood in your arteries is red. This means that it reflects red light and absorbs green light emitted from the LEDs. The light that isn’t absorbed is reflected back. The more blood flows through the arteries under the LEDs the more green light is absorbed. The reflected light is picked up by a sensor called a photodiode which converts this information into an electrical signal which it can then use to calculate the metrics displayed on the app.
Whoop 4.0 has five of these LEDs instead of two which means more light is emitted into the skin and thus it has more photodiodes to pick up the light reflected back. This in theory sounds like it would be more accurate. For comparison, the Series 6 Apple Watch uses 4 LEDs. But more sensors doesn’t necessarily solve the issues with the PPG technology these devices use. The gold-standard for heart rate monitoring is ECG (electrocardiography) which detects electrical signals emitted from the heart which reflects the expansion and contraction of heart chambers during heart beats. This is the technology used in medical-grade monitors.
Compared to ECG, PPG is accurate in measuring resting heart rate but becomes less accurate when measuring real time variations in heart rate (during exercise, for example). However, PPG has been shown in validation studies to be a reliable tool to measure heart rate variability (HRV) especially when used for periods of measurement longer than 5 minutes, but not so much for short-duration measurement. Another issue with PPG is that it can only be used in areas of the body with a high concentration of capillaries. This can be challenging when being used on the wrist.
This is why I think the issues of accuracy may not improve as much as we would like with Whoop 4.0. They did however announce that Whoop 4.0 can be used on other areas of the body that may provide higher accuracy with the new Any-wear products.
Whoop 4.0 comes with infrared light emitting LEDs which allow the band to estimate oxygen saturation or SpO2. This is a measure of how much oxygen-carrying hemoglobin is in the blood compared to the amount of hemoglobin not carrying oxygen (hemoglobin is the main carrier of oxygen in the blood).
I am less excited about this feature. Outside clinical settings, the vast majority of cases and conditions will show a normal SpO2. Normal SpO2 level ranges from 95% to 100%. SpO2 rarely changes in normal conditions unless the person has a significant health problem in which case I would expect them to know they had a problem based on their symptoms way before being alerted by a low SpO2.
In some cases, you may see a change in SpO2 during exertion such as in high altitudes due to a lower concentration of oxygen from the atmosphere entering the body. I have also come across a few cases of people with erroneous readings due to conditions that cause poor circulation in the extremities.
Whoop 4.0 includes a skin temperature monitor. I have not found a source that specifies what kind of skin sensor they use, but it is most likely the same technology used in other wearable devices such as the Oura ring. These devices most commonly use thermistor configuration (thermistors are special types of resistors that are highly sensitive to temperature changes). There are a few challenges that have been shown in devices using this technology to measure skin temperature especially at the wrist. The main issue is that normally, skin temperature is a few °C less than core body temperature. The skin surface is also more sensitive to changes in environmental temperature. Thus the accuracy is dependent on the reliability of the algorithms used to correct for these issues.
This feature is interesting because it gives us more information about what is happening in our bodies day-to-day. There have been many studies showing that skin temperature changes during different stages of sleep and may be used as a gauge to identify causes of poor sleep quality as well as a guide to our sleep behaviour. Skin temperature can also be used to detect stress and emotional variations. When stress levels are low for example, average skin temperature goes up. This adds a useful level of insight into our bodies inner workings when taken together with the other metrics Whoop provides. I will be writing a separate guide on how to use skin temperature as a health metric.
It's great that we can track all these new metrics to help us better understand ourselves. A decade ago it was exciting just to be able to track our steps with a wrist band. It’s important however to keep in mind that what’s more important than how much data you track is what you do with that data. The more data we have, the more confusing things can get. This is why relying on the right guidance and evidence- based self experimentation is key to not just understanding our health but improving it.
What I didn’t get into here is the price difference and the esthetics. These factors vary a lot from one person to another. Whoop 4.0 potentially provides higher accuracy which was an issue with Whoop 3.0 for some users. This remains to be validated by a third party. The temperature monitor is a significant addition and is overdue when compared to Oura and Apple Watch who already offer it. Compared to others, Whoop remains my favourite wearable when it comes to accuracy and data visualisation. The updates to Whoop 4.0 definitely make upgrading worth considering. I know I will.
If you’re interested, I wrote more about the accuracy of Whoop vs Oura when measuring sleep here.
A comparison of photoplethysmography and ECG recording to analyse heart rate variability in healthy subjects
https://www.dpag.ox.ac.uk/publications/95239
Comparison Chart: Electrical (ECG) vs. Light-based (PPG) Biosensors in Wearable
http://neurosky.com/2015/01/ecg-vs-ppg-for-heart-rate-monitoring-which-is-best/
The WHOOP Technology and Approach to Measuring Sleep
https://www.whoop.com/thelocker/the-whoop-approach-to-measuring-sleep/
WHOOP 4.0 vs. 3.0: What’s New With the 4.0?
https://www.whoop.com/thelocker/whoop-4-0-vs-3-0-whats-new/
Effects of season on sleep and skin temperature in the elderly
https://link.springer.com/article/10.1007/s00484-009-0291-7
Skin Temperature Rhythms in Humans Respond to Changes in the Timing of Sleep and Light
https://journals.sagepub.com/doi/full/10.1177/0748730417702974
Sensors and Functionalities of Non-Invasive Wrist-Wearable Devices: A Review
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6021794/
Towards an automatic early stress recognition system for office environments based on multimodal measurements: A review
https://pubmed.ncbi.nlm.nih.gov/26621099/
