How WHOOP 4.0 measures health metrics and improves wearability

Podcast episode originally published on September 10, 2021

WHOOP 4.0 introduced six major changes to continuous health monitoring: more places to wear the sensor, more nightly health metrics, baseline-aware alerts, haptic wake-ups, smaller hardware, and a rebuilt optical system. In this episode of the WHOOP Podcast, WHOOP Co-Founder and Chief Technology Officer John Capodilupo joins Will Ahmed to explain how those changes were engineered, why they mattered for signal quality, and how WHOOP members could use them in daily training and health decisions. The episode is especially useful if you want to understand the mechanics behind SpO2, skin temperature, Health Monitor, WHOOP Body, and the design trade-offs that shaped WHOOP 4.0.

Note: This article covers WHOOP 4.0. For the latest hardware, see the current WHOOP product page.

For Capodilupo’s full walkthrough of the hardware and product decisions behind WHOOP 4.0, listen to Episode 139 of the WHOOP Podcast on Spotify.

How did WHOOP 4.0 move beyond wrist wear?

WHOOP 4.0 was designed to work away from the wrist, and WHOOP Body made that possible by placing the sensor into garments that hold it close to the skin. Capodilupo said the technical goal was simple to describe and hard to execute: keep signal quality high even when the sensor moves to different parts of the body.

The core challenge comes from how optical heart rate sensing works. WHOOP uses photoplethysmography, or PPG, a light-based method that estimates pulse from blood flow changes in tissue. During exercise, the signal can get noisy because several things move at once: blood, tendons, muscle, and the sensor itself. Capodilupo said the biggest problem is often motion artifact, which means the sensor shifts relative to the skin. That shift makes it harder to separate the true cardiovascular signal from everything else happening around it.

That problem is also what made WHOOP Body possible. Earlier research had already shown that the bicep could sometimes produce cleaner readings than the wrist during certain activities, so the team widened the question. Instead of asking how to improve wrist wear, they asked where else on the body the sensor could live. The answer involved raw data collection, garment design, and a large amount of trial and error. Capodilupo described engineers and apparel designers working side by side so the sensor, fabric tension, placement, and algorithms all matched the site where the device would be worn.

Capodilupo put the scope of that testing in concrete terms:

“We’ve mapped out 10 to 20 different locations and done similar data collections.”

In practice, that work led to WHOOP Body, plus sensor-compatible garments and accessories that let the sensor move from the wrist to sites such as the bicep, torso, waist, and calf. The broader launch also included a redesigned SuperKnit band and easier swapping between form factors, all described in the WHOOP 4.0 and WHOOP Body announcement and the WHOOP Unlocked 2021 recap.

The important idea for members was not just style or comfort. Different activities create different kinds of motion. A sensor location that works well for one person or one sport may not be the best place during another activity. WHOOP 4.0 gave the platform more room to handle that reality.

If you want to hear Capodilupo unpack the testing behind WHOOP Body and Any-Wear support, the full episode on Spotify covers that process in detail.

What you should take away

• WHOOP 4.0 expanded beyond the wrist by pairing sensor design, garment design, and algorithm work
• Motion artifact, or movement between the sensor and your skin, was the main technical problem the team had to solve
• WHOOP tested 10 to 20 body locations before narrowing down the placements used in WHOOP Body

What new health metrics did WHOOP 4.0 add?

Once WHOOP 4.0 could be worn in more places, the next question was what additional physiology the smaller sensor could capture. The two major additions were SpO2 and skin temperature, both of which add context that goes beyond training load alone.

SpO2 is blood oxygen saturation. Capodilupo explained that it reflects how much oxygen your blood is carrying, and he noted that most healthy people sit around 95 to 100 percent oxygenated. For athletes, that can be useful during altitude exposure because blood oxygen typically falls at elevation and can rise again as acclimation improves. For general health awareness, it adds another nightly signal that can be watched alongside resting heart rate, heart rate variability, and respiratory rate.

The measurement method matters here. Capodilupo said WHOOP 4.0 uses the same basic optical family as heart rate sensing, PPG, but applies it differently. Instead of focusing on pulse frequency, the device measures how much light gets absorbed. That is where the red and infrared LEDs come in. Oxygenated and deoxygenated hemoglobin absorb those wavelengths differently, which is the basis of pulse oximetry.

Capodilupo gave the mechanism in one sentence:

“Oxygenated hemoglobin and deoxygenated hemoglobin absorb red and infrared light differently.”

The second addition, skin temperature, is easier to misunderstand. Capodilupo drew a clear line between skin temperature and core body temperature. A wrist-worn sensor is not measuring the 98.6 degree Fahrenheit number people usually associate with a thermometer reading at a medical visit. It is measuring the temperature at the surface of the skin. That signal still matters, especially overnight, because fluctuations can line up with changes in wellness, illness status, or hormonal shifts.

He also connected skin temperature to female physiology research. Earlier WHOOP work had already explored how menstrual cycle phase can affect training response, and the WHOOP Journal discussion with Kristen Holmes, Global Head of Human Performance, Principal Scientist at WHOOP, and Emily Capodilupo, Senior Vice President of Research, Algorithms, and Data at WHOOP showed why cycle-related context can sharpen training interpretation. Skin temperature gave WHOOP 4.0 another way to observe those patterns over time.

For a full explanation of how Capodilupo describes SpO2, skin temperature, and the optical hardware behind them, listen to the full episode on Spotify.

What you should take away

• WHOOP 4.0 added SpO2 and skin temperature to give members more nightly health context
• SpO2 uses red and infrared light because oxygenated and deoxygenated hemoglobin absorb those wavelengths differently
• Skin temperature on WHOOP 4.0 is a surface measurement, not a core body temperature reading
• Overnight skin temperature patterns can help add context to illness awareness and female physiology trends

How does WHOOP Health Monitor use baselines and alerts?

Those added measurements became more useful once WHOOP placed them into a daily decision surface. Health Monitor was the feature that turned separate vitals into a single baseline-aware view inside the WHOOP app.

The practical point of Health Monitor is straightforward: your numbers make more sense when they are compared with your own normal range. Capodilupo said many of these readings are familiar from an annual physical, but WHOOP can check them every day instead of once a year. That changes how people spot deviations. Rather than relying only on a population threshold, WHOOP looks at whether resting heart rate, HRV, respiratory rate, SpO2, and skin temperature have moved away from your personal baseline.

Capodilupo framed the difference like this:“All of these stats, you probably get a readout once a year when you go for your physical, but now we’re giving it to you every single day.”

Will Ahmed added a concrete threshold from the feature itself: Health Monitor can flag an SpO2 reading below 94 percent. He also pointed to the earlier case of PGA Tour player Nick Watney using a respiratory-rate deviation to recognize that something was off before a confirmed COVID-19 diagnosis. In that sense, Health Monitor was built to surface the same kind of signal detection inside the app, in one place, without making members hunt through multiple screens.

Just as important, baseline alerts are not diagnosis. They are awareness tools. If a number is out of range, the useful next step is context: altitude, alcohol, sleep loss, travel, heavy training, illness, or a menstrual cycle shift can all help explain why a value moved. That baseline-first logic fits the older WHOOP idea of continuous measurement, discussed in What is WHOOP? and in The Day You Became a Better Athlete, where ongoing data is more informative than a single isolated reading.

The full conversation of this episode on Spotify is useful if you want to hear how Capodilupo and Ahmed think about daily baseline tracking rather than one-off measurements.

What you should take away

• Health Monitor compares key vitals with your own baseline, not only with broad population cutoffs
• WHOOP 4.0 Health Monitor brings resting heart rate, HRV, respiratory rate, SpO2, and skin temperature into one view
• An SpO2 reading below 94 percent was one of the alert thresholds described at launch
• Health Monitor is designed to surface changes for awareness and follow-up, not to provide a diagnosis

How does WHOOP Sleep Coach use haptic wake-ups?

After adding new sensing and daily alerts, WHOOP 4.0 also changed how wake-ups work. The new haptic motor let the band vibrate on your wrist, and Sleep Coach used that motor to wake you based on time, sleep need, or Recovery status.

Ahmed said the Sleep Coach had three wake modes at launch. The first was a standard exact-time alarm. The second was goal-based, where the device wakes you when you hit a chosen portion of your sleep need. The third was based on Recovery, with an option to wake once your body is in the green while still respecting a latest possible wake time.

Ahmed summarized the setup clearly:

“The new Sleep Coach is effectively gonna have 3 modes. You can wake up at an exact time. You can wake up based on your sleep goal. Or the third mode, which I think people are really gonna love, is the in the green mode.”

That feature sounds simple from the outside, but Capodilupo said the hardware trade-off was difficult. The wrist is one of the less sensitive parts of the body for this kind of vibration, so the motor needed enough force to wake a sleeping person. A stronger motor, however, uses more energy and takes up more space. That put it in direct tension with two other goals for WHOOP 4.0: smaller size and multi-day battery life.

The team tested multiple motors, different vibration profiles, and different power budgets before settling on a version that could do the job without draining the battery too quickly. That is the recurring theme across WHOOP 4.0: a new feature was only viable if it fit the rest of the system.

If the sleep side of the launch is what interests you most, the WHOOP Unlocked 2021 recap adds product context, and Episode 139 of the WHOOP Podcast on Spotify covers the trade-offs behind the motor itself.

What you should take away

• WHOOP Sleep Coach added haptic wake-ups so alarms could happen through vibration instead of sound
• WHOOP 4.0 launched with three wake modes: exact time, sleep goal, and in the green
• The motor had to be strong enough to wake people on the wrist without taking too much space or battery

How did WHOOP 4.0 get smaller without losing battery life?

Adding more sensors, a haptic motor, and broader wearability raised the next challenge: power and space. WHOOP 4.0 answered that challenge by reducing energy use almost everywhere and redesigning the battery system around a smaller footprint.

Ahmed said the device ended up 33 percent smaller than WHOOP 3.0 while still delivering a five-day battery life at launch. He also noted that the industrial design process went down to tenths and hundredths of a millimeter, which gives a sense of how tight the packaging problem was. Capodilupo compared the internal work to Tetris, with circuit boards, sensors, battery, and motor all competing for the same tiny volume.

The power savings started with hardware and firmware choices. Components were selected partly for energy efficiency, and the processor spent much of its time asleep between work cycles. Capodilupo explained that the sensor is always on in the user-facing sense, but many of the internal systems are active only briefly before returning to a lower-power state.

Capodilupo described that efficiency target with a specific number:

“We only want it to supply power maybe only like 20 milliseconds out of every second.”

Battery chemistry also changed. WHOOP partnered with Sila Nanotechnologies on a silicon-anode battery design that Capodilupo said allowed more energy density in the same physical space. In plain terms, that meant WHOOP could use a smaller battery without giving up the multi-day runtime needed for continuous wear.

The redesign also fixed a major frustration point from earlier hardware: the battery pack itself. By moving to wireless charging between the battery pack and WHOOP 4.0, the pack no longer needed exposed charging contacts in the same way. That made it possible to build a waterproof battery pack and a cleaner outer shape. Ahmed also noted the double-tap battery level check, which solved the common question of whether the pack still had enough charge left.

Capodilupo goes deeper on the power budget, the battery chemistry, and the waterproof charging pack in the full episode on Spotify.

What you should take away

• WHOOP 4.0 was 33 percent smaller than WHOOP 3.0 at launch while keeping a five-day battery life
• Lower power draw came from efficient components, careful firmware timing, and a processor that slept between work cycles
• A silicon-anode battery design helped fit more usable energy into less space
• Wireless charging enabled a waterproof battery pack and a cleaner device profile

How did WHOOP improve signal quality and sensor accuracy?

Size, battery life, and new features only mattered if the sensor data stayed clean. That is why WHOOP 4.0 was also a full rebuild of the optical system, with Capodilupo saying the team went back to the drawing board rather than making a small revision to the previous setup.

The target was signal-to-noise ratio. In practical terms, WHOOP wanted more usable physiological signal and less noise from movement, skin differences, and placement. Capodilupo said the team built around 90 different sensor configurations to test how LED number, LED brightness, photodiode count, geometric placement, and the mechanical structure on the bottom of the device affected the final signal.

He described the scale of that development process this way:

“We actually built around 90 different sensor configurations in different prototype WHOOP straps, looking at how different LEDs and photodiodes could affect the signal-to-noise ratio.”

The final hardware changed several pieces at once. WHOOP 4.0 moved to three green LEDs instead of two, added red and infrared LEDs for SpO2, and expanded from one photodiode to four. Those additions gave the algorithms more signal to work with. Just as important, Capodilupo said the team had already completed more than 20,000 data collections for Generation 4 and planned to keep improving performance through continued algorithm updates after launch.

Those studies were not limited to a few ideal lab subjects. Capodilupo said WHOOP deliberately recruited a wide range of participants because skin tone, anatomy, tendon placement, and vascular patterns can all affect optical sensing. The hardware was also tested with reference systems such as chest straps and ECG setups so the team could compare raw signals under controlled protocols and live activity. That focus on broad participant testing was one reason WHOOP treated Generation 4 as a new platform rather than just a smaller device.

The long-term point was equally important. Capodilupo said WHOOP Labs would continue refining algorithms and shipping updates. In other words, WHOOP 4.0 was built to improve after people put it on.

For the full discussion on sensor design, participant testing, and why WHOOP treated Generation 4 as a new platform, hear Capodilupo in the full episode on Spotify.

What you should take away

• WHOOP 4.0 rebuilt the optical stack to improve signal quality rather than making a minor revision to earlier hardware
• The team tested around 90 sensor configurations before choosing the final design
• WHOOP 4.0 used three green LEDs, one red LED, one infrared LED, and four photodiodes
• More than 20,000 data collections fed the Generation 4 development process, with further algorithm updates planned after launch

The bottom line

• WHOOP 4.0 expanded wearability by supporting sensor placement beyond the wrist, including integration with WHOOP Body garments
• SpO2 on WHOOP 4.0 is derived from red and infrared light absorption differences between oxygenated and deoxygenated hemoglobin
• Skin temperature on WHOOP 4.0 reflects surface temperature patterns during sleep, not core body temperature
• Health Monitor was built to compare resting heart rate, HRV, respiratory rate, SpO2, and skin temperature against your personal baseline
• WHOOP Sleep Coach added three haptic wake modes: exact time, sleep goal, and in the green
• WHOOP 4.0 reached a 33 percent smaller size than WHOOP 3.0 by cutting power use, reworking internal packaging, and using a silicon-anode battery design
• Signal quality work on WHOOP 4.0 included about 90 sensor configurations, four photodiodes, added red and infrared LEDs, and more than 20,000 data collections

Frequently asked questions about things discussed in this episode

How does WHOOP measure blood oxygen?

WHOOP measures blood oxygen by using red and infrared LEDs to estimate how much hemoglobin is carrying oxygen, then surfaces that result as SpO2 in the WHOOP app.

What does WHOOP do with skin temperature data?

WHOOP uses skin temperature data to track overnight changes relative to your baseline, which can add context to wellness trends, recovery interpretation, and female physiology patterns.

How does WHOOP Health Monitor decide when to alert you?

WHOOP Health Monitor alerts you when key metrics such as resting heart rate, HRV, respiratory rate, SpO2, or skin temperature move outside your usual baseline range.

What does WHOOP Sleep Coach do with haptic alarms?

WHOOP Sleep Coach uses the haptic motor in WHOOP 4.0 to wake you at an exact time, when you hit a sleep goal, or when Recovery reaches the green range.

How does WHOOP Body let you wear the sensor away from the wrist?

WHOOP Body places the WHOOP 4.0 sensor into garments designed to hold it close to the skin, which allows data collection from locations such as the torso, waist, or other supported sites.

What does WHOOP 4.0 do to improve battery life?

WHOOP 4.0 improves battery life through lower-power components, tighter firmware timing, processor sleep cycles, and a smaller battery with higher energy density.

How does WHOOP improve sensor performance over time?

WHOOP improves sensor performance over time by using the Generation 4 hardware as a platform for ongoing algorithm and firmware updates informed by continued lab testing.

Episode 139 of the WHOOP Podcast shows that WHOOP 4.0 was built as a new sensing platform, one meant to make health monitoring more useful by letting the sensor disappear into more parts of daily life.