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Tech & Wearables

Review: Whoop Strap 4 is a leap forward in accuracy of recovery monitoring

More sensors, enhancements to activity and recovery metrics, and ‘Sleep Coach’ improve upon the previous version of the recovery monitoring wearable.

Review Rating


The always-on recovery tracker offers improved accuracy through additional sensors, with new metrics added, as well as the unique “Sleep Coach”.


4x more sensors than previous version;

records and measures: respiration rate, SpO2, skin temperature, heart rate, heart rate variability, and heart rate reserve;

haptic feedback and silent alarm;

improved clasp and retention system;

modular battery is now waterproof;


optical heart rate sensor still lacks the accuracy of a chest strap;

does not allow third-party devices to connect to it;

expensive subscription model

Our Thoughts

Additional sensors, enhancements to activity and recovery metrics, and ‘Sleep Coach’ are a leap forward from the previous version.





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The Whoop Strap is a 24/7 wearable monitoring tool that measures heart rate and several other biological markers to help inform the effectiveness of rest and recovery. It offers informed predictions about the effectiveness of one’s sleep and readiness to perform on the next ride or other activity.

Version 3 of the Whoop Strap used two green LEDs to sense heart rate and award a “Strain” score – Whoop’s metric for expressing relative physiological exertion. Strain is a daily score that aggregates anything that makes your heart beat faster — rides or walking up flights of stairs — and is balanced by the Recovery score (for example overnight sleep, naps, and even just sitting at a desk behind a keyboard).

The Whoop Strap 3 was rather useful for monitoring recovery when movement was minimal and the few sensors would not be moved or disturbed. The biggest flaw in the Whoop Strap 3 was the accuracy of heart rate measurements. With the Whoop Strap 4, many of the issues I experienced with heart rate monitoring accuracy are notably improved.

Also read: Epic Ride Weather review — One of the handiest cycling apps just got better

More sensors, more data

The newer Whoop Strap 4 features four photodiodes and five LED sensors that collect data at a claimed 100 times per second. Whoop claims the Strap 4 has four times the number of photodiodes used to measure physiological markers. These sensors monitor heart rate, respiration rate, skin temperature, blood oxygen saturation, heart rate variability, and heart rate reserve. The addition of skin temperature and blood oxygen saturation sensors can be used to help predict illness, among other things.

You can look at this data on the app, or on the Whoop website, which also provides a larger view of collected data. Also added to the Whoop 4 is a vibrating/haptic feedback alarm, which can be set to wake one from a night of sleep based on meeting recommended sleep, or measured recovery status, at a specific time of day.

What all this means is that the Whoop can offer a more robust picture of recovery and readiness to perform from data previously collected and analyzed.

Automatic activity and recovery detection, with sleep analysis

One of Whoop’s strongest features, which sets it apart from other fitness trackers, is the data measured and provided to users via the required mobile app. I write required, as the Whoop strap does no onboard processing; the Whoop Strap is merely an array of sensors and a transmitter that sends data to a BLE-paired iOS or Android device.

Whoop sleep analysis data includes phases, durations, and an efficiency score. (Illustration: Greg Kaplan )

While the Whoop can be configured to automatically start/stop recording activities, I frequently have to edit the auto-detected start and stop times. This feature shows promise, but it is only marginally accurate at determining when I start and stop riding, and frequently it does not detect weight training sessions at all.

Interpreting Whoop app data

Using the Whoop mobile app Health Monitor Dashboard, I can access my activity and recovery information on the app which is always running in the background. The Whoop Strap can only store about 72 hours of data, and uploading stored data to the mobile app takes a while, so it’s more convenient to just keep the app on in the background.

Following the learning period of five days — and again at 30 days — in which Whoop gathers enough data about my physiological responses to different types and durations of activities, I’m presented with summaries and detailed information on daily, weekly, and monthly periods.

Also read: Reviewed — Whoop Strap 3

Whoop makes performance predictions through its Day Strain based on the data collected and analyzed against an ever-increasing store of benchmarks. I’ve found this predictive tool to be improved since the previous iteration of the Strap, with increased accuracy based on the better quality of data collected. When I review my Strain Score at the beginning of a day it almost always aligns with how I feel, which almost always correlates with the quantity and quality of the previous night’s sleep. I covered many of these features in my review of the previous version of the Whoop Strap.

Like the previous version of the 24/7 wearable, the Whoop Strap 4 scores your previous nights’ sleep (left), and can automatically detect activities. (Illustration: Greg Kaplan )

Where the Whoop is most accurate in measuring and recording data is for recovery and sleep; when I’m not moving, the sensors staying place and most reliably collect data. When doing short intervals in which my heart rate quickly changes — rides with a lot of accelerations, or punchy climbs, or Tabata microintervals — the Whoop optical heart rate sensor misses fewer heart rate spikes, and lags less than the previous versions.

Why this is a big deal is because the recovery needed that Whoop provides on an ongoing basis accounts for the measured exertion. If Whoop records activity exertion low, the recommended recovery—hours of sleep, successive easier training sessions—from a hard day may not be enough for sufficient recovery.

Performance prediction is based on factors like recovery duration and quality, activity intensity and duration, and long-term trends, and is more accurate with the Whoop 4 than previous versions because of the improvements to the biometric monitoring sensors.

Sleep Coach

Whoop can wake you from sleep at a predetermined time, or after it detects you've had the minimum amount of sleep, or an optimal amount of sleep.
Whoop can wake you from sleep at a predetermined time, or after it detects you’ve had the minimum amount of sleep, or an optimal amount of sleep. (Photo: Greg Kaplan )

My initial and ongoing interest in Whoop is the data collected and intelligence provided about recovery and sleep. I describe my sleep as delicate, and I’m always on the lookout for tools and methods to help improve the quality of my sleep. The Whoop Sleep Coach is a feature that allows me to set an alarm to wake me at a specific time of day, or when I reach a specific sleep “goal”, or when it detects I have reached an optimal amount of sleep based on data collected. While the idea of sleeping until I am fully recovered from a hard day is most ideal, the reality is that I need to set an alarm for a specific time of day if I am to cram all I need to do into 17 or so waking hours. But the power of the Sleep Coach is that when I do not set a specific time of day to vibrate it can nudge me awake when I have reached a minimum threshold for recovery.

Subscription pricing

Unlike Garmin or Wahoo products — which you pay up front for and get the data analysis tools for free— the strength of Whoop is not the hardware, but the analysis and recommendations made by the software. The Whoop Strap 4 is not a one-time purchase.

To use the Strap, which connects to Whoop app on your smartphone, you must subscribe to the service for a minimum of six months at a rate of $1/day. With this subscription the Strap is provided for “free.” Longer subscription terms lower the daily cost of use.

Retention system redesign with attractive bands

The Whoop 4 is available in 12 different strap colors, two materials, and in lengths for wrist or bicep. The Whoop 4 arrives with a Superknit band; a Superknit Luxe version offers a slightly more refined appearance and a retention system with premium finishes. Whoop updated the retention system — through which the band is threaded — to make swapping straps easier, as well as improving the design from the previous version, decreasing the chance that the strap will loosen while it’s being worn. For those who are really into personalization, Whoop offers an online tool that provides for complete mixing and matching of strap material and color, clasp color, hook color, and fast link color. Pricing of add-on straps is $49 – $150 depending on clasp finish, band material, and level of customization.

Whoop Strap 4 with additional bands that add length, and color. (Photo: Greg Kaplan )

Even though Whoop claims the Superknit strap is quick to dry, I still take off the Whoop while showering, as it remains damp — and transfers moisture to whatever I’m wearing — for several hours after being saturated. This, too, was my experience with the Whoop Strap version 3. While this circumvents the purpose of the 24/7 monitoring, not having wet sleeves is more of a priority for me.

The Whoop Strap 4 (right) has an improved clasp and retention system. (Photo: Greg Kaplan )
The Whoop Straps version 4 (top) and 3. (Photo: Greg Kaplan )

One feature of the Whoop Strap 4 that I did not test is putting the modular sensor into Whoop’s line of apparel — shorts, tights, and baselayers — which can stand in for wearing a strap by keeping the senor next to one’s skin.

Waterproof battery, extended time between charges

I measured Whoop 4 at 14 percent smaller than the previous version. The modular battery is actually larger than the battery for the Whoop 3, but since I don’t wear the battery pack overnight, nor when on a ride, it’s not a big deal for me. The battery technology on the Whoop 4 is also improved, and as claimed: I get about five days between charges. A handful of times I opened the always-on-in-the-background Whoop app on my iPhone 13 to discover that I had exhausted the battery, but that’s more user error than anything else.

The design of the modular battery pack, while two percent larger by volume, is improved from the previous generation. It charges quickly via a USB C cable, and when fully charged can be slipped onto the Whoop strap from either the top or bottom ends of the Whoop sensor. This modular design allows one to charge the battery on the strap without removing the strap which would defeat the purpose of the always-on device. The Whoop 4 battery is waterproof — just like the Whoop sensor unit — and features an LED on the top of it to indicate charging status.

On the top side of the Whoop, a single LED shows red or green, indicating battery status. This is no change from the previous version of the wearable monitor.

Whoop Strap 4 battery — which is now waterproof — indicator asking to be charged. (Photo: Greg Kaplan )
The white LED on the side of the Whoop strap indicates its functioning. The green LED is part of the sensor array. (Photo: Greg Kaplan )

Where Whoop can improve

I still struggle from time to time with the accuracy of the heart rate monitoring of the Whoop Strap, but not as much as I did with the previous version. To help ensure that data collected is the best possible, I wear the Whoop Strap 4 on my bicep most of the time, and not my wrist where it can slip, gap if I flex my hand, or be occasionally knocked by the cuff on a long sleeve. Even so, I still occasionally experience data that does not align with the Wahoo Tickr heart rate monitor strap I use as a baseline comparison, and on which I rely for transmitting heart rate data to my bike computer.

My experience with wearing the Whoop on my non-dominant wrist (I’m a “righty”) is similar to my experience with my Garmin Fenix 6: The data collected while sleeping is accurate, but the data collected during activities is sometimes susceptible to how well the wearable collects data from my wrist, which is often a factor of how tight the band is around my arm. And to collect the most accurate data, I sometimes feel like I have to make the Whoop so tight that it leaves impressions in my skin. In certain hand positions on the tops, the drops, and brake hoods—as well as when doing other activities like when I clasp a dumbbell or Olympic bar while weightlifting, the wrist-wearables do not provide data collection that is as accurate as a chest strap. What all of this means is that the addition of sensors and improvements Whoop made to the measurement and monitoring technology has notably improved with the introduction of the Whoop Strap 4.

Whoop Strap 4 heart rate graph for a series of short intervals while riding indoors. (Illustration: Greg Kaplan )
Garmin heart rate graph (measured with a Wahoo Tickr chest strap) for a series of short intervals while riding indoors. (Illustration: Greg Kaplan )

When I reviewed the Whoop Strap 3, one of my wishes was for Whoop to allow third-party heart rate monitoring tools integration with the Whoop device. Straps worn around one’s chest have been proven to be most accurate in measuring heart rate during activities. I’m still hopeful that at some point Whoop will allow this kind of cross-device integration — that the strap could receive a BLE signal for heart rate during activities. But with the inclusion of additional sensors for oxygen saturation and temperature on the Whoop Strap 4, it seems that Whoop is not yet headed in this direction.

What Whoop gets right

Whoop continues to innovate its hardware with additions and improvements to its sensors. The Whoop Strap 4 is a 24/7 monitoring tool that leads the way in providing actionable rest and recovery information. The innovations to the modular battery charging block are welcome, as are the improvements to the fit of the band and clasp retention system

Using the questionnaire and log that’s part of the Whoop mobile app, I’m able to review and correlate some external factors — the timing of the last meal before bed, amount, and timing of caffeine and alcohol consumption — that may impact recovery. The daily, weekly, and monthly reports even suggest which factors may be having an impact on my recovery.

The data collected by the Whoop Strap 4 adds and improves the robust data analysis tools already available from Whoop Strap 3. The updates to the optical heart rate sensors, an onboard thermometer, and the addition of SpO2 monitoring leapfrog Whoop ahead of Wahoo and Garmin which dominate the wearable activity monitoring market. The Whoop collected data points give a more complete picture of recovery status the the competition.

Where Whoop continues to be the best in class is the sleep and recovery analysis. My experience with the Whoop is that when it indicates I’ve had a poor night’s sleep I should not expect a high-quality training session, and nearly all of the time the Whoop recovery analysis is correct.