WHOOP Review: a good idea fatally flawed?

The current iteration of WHOOP is deeply flawed, possibly fatally flawed. There may be some usefulness for some athletes, as long as you understand the problems.

1 What is WHOOP, and why might you want one?

This is not a full review of WHOOP, and I'd recommend checking out something like the 5K runner review as background. For our purposes, we going to assume that you are an athlete, and you are looking to optimise your training and recovery. Knowing how hard to push your training, and how much to recover is one of the objectives of good training. and this is the primary purpose of WHOOP, so you'd hope they do a better job than alternatives like Garmin. (I've found little to no value in Garmin's training advice. It's not quite bad enough that you can simply do the opposite, but it's sometimes close.)

2 Overview

The principle of WHOOP is to measure things about your body and use that to understand your training stress and recovery. WHOOP primarily uses heart rate, combined with movement. It also seems to use skin temperature and blood oxygen. The details of how it uses this data is a black box, proprietary model. Anyone who has used to heart rate monitor for any length of time will know that bad readings are inevitable. even a good quality chest strap will sometimes give bad data. Optical heart rate monitors, like WHOOP are especially vulnerable to bad data. The latest generation of optical heart rate monitors, like the polar Verity sense, have become good enough for most non-critical usage. Unfortunately, WHOOP has a number of compounding problems.

• Band Design. The design of the band and its locking mechanism seems quite clever at first glance. However, I found it made it quite tricky to get the tension right, and the mechanism tended to pinch my skin and it was hard to open and close. The band needs to be taken off to change the length, making it tedious to get right and impractical to move it around as each new position requires readjustments. The band isn't stretchy, so arm movements change the pressure, which makes it both uncomfortable and unreliable. If you compare this with the Polar Verity Sense, the best in class, you can see how it should be done. The polar has a stretchy band so it works with arm flexing, and it has an attachment that means you can just close it with the right level of tension.
• Sensor Design. The focus of the sensors appears to be long battery life, rather than data quality. The internal components talk about "Ultra Low-Power" rather than quality. The device only has three green HR LEDs in a line, close together. Modern optical HRMs use more LEDs and have them in a circle around the sensor.
• Quality. My overall sense of quality with WHOOP is poor. While the bands are expensive, they feel cheaply made to me. I don't get the sense that WHOOP has put the effort into creating a premium product. For instance, I have several bands, and some fit easily onto the sensor, well another requires inordinate force to remove it. This suggests variable tolerances and poor quality-control.
• Placement. WHOOP seems to have been created to be worn on the wrist, which is the worst place on the arm for heart rate sensing. The company does recommend you wear the band high up on your bicep, just below the deltoid muscle. This is a strange recommendation as the flexing of your bicep tends to increase and decrease the tension and make accurate readings even harder. Positioning an optical heart rate monitor on the inside of the upper arm just above the elbow seems to be the optimum position, as it is for most optical HRMs.

3 The possibly fatal flaws

I'd really like WHOOP to work, and I was hoping that the flaws would be small enough that there was still value. Sadly, I think I was wrong.

• My first WHOOP gave appalling heart rate readings, typically being out by 20 to 40 BPM. This seems to be a faulty sensor, but I'm concerned that it might be indicative of variable quality rather than a failure.
• Having used many different optical heart rate monitors, the data I get from WHOOP is comparable with an older generation, not the latest technology. WHOOP seems especially sensitive to positioning and tension on the strap. It seems that WHOOP requires far more tension than a modern optical HRM, to the point of being uncomfortable. Like older Optical HRMs, WHOOP really struggles at the start of a run, often reading 20-80 BPM too high. Sometimes WHOOP is able to lock back on, but other times it requires me to turn the sensor around so it face away from the body, completely loses signal, then turn it back again. I've even had the problem where WHOOP does okay during a Parkrun warm up, but goes wrong during the 5K itself.
• Because WHOOP doesn't have a display, it can be far less obvious when a reading is bad. You can overcome this by having broadcasting the WHOOP heart rate and displaying it on a watch. However, that means the bad heart rate data from WHOOP will be stored in the watch. My solution was to have a chest strap heart rate monitor sending good data to my main watch, then have a second watch that displays the WHOOP heart rate and keep an eye on the difference.
• Sadly, WHOOP has no ability to use a better heart rate monitor. There's no way of linking it to a chest strap or getting it to take heart rate data that's been stored in Strava. So even when you have good data, you can't make use of it.
• If the bad heart rate data WHOOP detects is above your maximum heart rate, it will reset your heart rate zones and then underestimate the impact of any future activities until you notice. You can minimise the impact of this by setting manual heart rate zones, but it doesn't seem to be possible to get WHOOP to ignore excessively high heart rate data.
• Where all this becomes a potentially fatal flaw is there is no way of telling WHOOP that a workout contains bad data to ignore it. So, if you do an easy run where WHOOP records it with a wildly high heart rate, the strain will be massive. This will then confuse the model, as it looks like a very, very hard day rather than an easy day. WHOOP claims that bad data will only impact their model for three weeks. It strikes me as very strange that data only impacts their model for three weeks, as most training models have a much longer time horizon. But even three weeks can render WHOOP useless, as it's hard to get a three week stretch without bad data.

4 Other reviews of WHOOP

Lots of reviewers seem to respect WHOOP and think it's worthwhile. Are they able to get much better data than I am? Are they getting "reviewer special" devices that perform much better than retail units? Is WHOOP providing incentives that bias reviews? I'm not sure, and I read enough reviews from people I believe are credible to pay for WHOOP. There are some contrary views, such as this analysis Data Scientist Breaks Down why WHOOP doesn't Work, but they are few and far between. I should have listened to the many individuals that don't write reviews, but found WHOOP to be unhelpful at best, rather than the bigger reviewers. Caveat Emptor, Your Milage May Vary, etc.

5 Is there any value in WHOOP?

Maybe. It seems like the sleep tracker is reasonable, and maybe if you're a cyclist the data quality issues won't be so bad. Personally, I'm finding WHOOP like a sick cat that I need to look after rather than a valuable training tool.