Whoop became popular as a strap that NBA and UFC players have been spotted tucking under their sweat pants, but so far LeBron James' technology has received mixed reviews in the crowded fitness tracker market.

Some users liked the detailed recovery data, which helped them know when to skip a workout and when they could really push themselves, while others questioned the accuracy of the fitness tracker's heart rate data. Read our review of Whoop 4.0 here.

According to Whoop, the new fitness tracker is 33% smaller and more accurate than before thanks to a new sensor. Also announced was Whoop body, a line of smart apparel that allows users to wear trackers in multiple locations on the body.

Here's what you need to know about the new technology.

Unlike other trackers on the market, the Whoop bands themselves do not have a fixed price. Instead, users sign up for a $30/month/£30 subscription and get the band for free; in September, when Whoop was announced, any user with at least six months remaining on their account could upgrade to the Whoop 4.0 band for free. New users can also sign up now to get Whoop 4.0 at Whoop's website.

As for the release date, here's where things get a little confusing: Whoop has said that it is allegedly slow to get new bands to consumers and is prioritizing existing members. The company said on Twitter, "Due to global supply chain issues and chip shortages, we are currently experiencing longer than normal shipping schedules. We apologize for any inconvenience this may cause. We are working around the clock to deliver 4.0 as soon as possible."

Like Whoop 3.0, the new tracker still provides users with detailed sleep, strain, and recovery data. Like its predecessor, the tracker has no screen and does not have the ability to read smartphone notifications or listen to music; it simply tracks data. Designed to fit comfortably on the wrist year-round, the new Whoop 4.0 is smaller and lighter than the older strap.

Upgraded sensors

To address the shortcomings of the older product, Whoop says the Whoop 4.0's sensors have five LEDs (three green, one red, and one infrared) and four photodiodes for more accurate heart rate tracking and Advanced algorithms are now included. The new trackers will also be able to calculate blood oxygen levels.

These new sensors are designed to give users enhanced health insights. For example, skin temperature can be used as an indicator to know when core body temperature rises. Aside from illness, skin temperature can provide more detailed sleep data, since core body temperature should drop at bedtime as part of the natural circadian rhythm.

While skin temperature is not something that can be noted in isolation (e.g., environment affects body temperature), it can be combined with blood oxygen levels and heart rate to provide a more detailed picture of overall health. Other trackers on the market are beginning to use skin temperature as a health indicator, as Fitbit did last year with its addition to Fitbit Sense.

However, the Apple Watch, which is rated the best smartwatch and is loaded with health-tracking technology, has yet to acquire a skin temperature measurement sensor. Therefore, Whoop 4.0 has an ace in the hole. Apple is said to be working on skin temperature measurement starting in 2019, but the Apple Watch 7 did not include this feature.

New Vibration Feature

Another new feature is Whoop 4.0's "Haptic Alerts," designed to "allow members to set a gentle vibration to wake them up at the optimal time based on their sleep needs and cycle." . "In other words, the new Sleep Coach feature wakes you with a vibration based on your sleep cycle, not when you set the alarm.

The Whoop 4.0 has a battery life of 4-5 days; one of the smart features of the Whoop is the ability to charge the tracker on the wrist. Why other trackers on the market, or even the Apple Watch or other smartwatches, have not adopted this is not known.

With Whoop 4.0, Whoop and the battery pack are now waterproof to 10 meters, so you no longer have to worry about wearing or charging your device while in the shower or doing laundry.

Whoop also announced incredibly detailed app changes. It will now allow members to track live heart rate, skin temperature, blood oxygen saturation, resting heart rate, heart rate variability, and respiratory heart rate in one view. They will also be able to download and export 30-day or 180-day trends to share this data with trainers and coaches.

Another feature linked to the launch of Whoop 4.0 is the "Whoop body." The "Whoop body" is a collection of apparel that allows trackers to collect data from different parts of the body. Clothing ranges from $54 to $109 and includes sports bras, compression tops, leggings, shorts, and boxers with "built-in pods" that allow the tracker to be attached to the torso, waist, and calf muscles.

There are also arm sleeves and hydro sleeves that allow users to wear the tracker in the pool; aside from a pocket for the Whoop tracker, the apparel does not appear to have any standout features. However, for athletes who also want real-time data from their sports watches, less technology on the wrist may be a useful feature. Some Whoop Body products are currently available.

Having tested Whoop 3.0, I look forward to seeing how Whoop 4.0 fares. I think it is great that the strap can be charged while on the wrist, and I am impressed that the charger is now waterproof. I'm also looking forward to seeing if the new sensors actually make the tracker more accurate, especially the skin temperature readings.