Apple Watch Ultra 4 - Ultimate Scientific Review!
In a Nutshell
The Apple Watch Ultra 4 delivers exceptional heart rate tracking with 0.99+ correlations across cycling, running, and weightlifting, though occasional signal interruptions occur during intense exercise. Sleep stage tracking matches EEG references 74-81% of the time, placing it among top performers alongside Google/Fitbit devices. GPS tracking is rated "good" (80/100) with strong consistency but noticeable smoothing that can produce unrealistic paths through buildings or obstacles.
These notes were generated by AI and may contain inaccuracies.
The testing began with heart rate tracking using the Apple Watch Ultra 4, with reference measurements collected using scientific instruments for comparison. The Polar H10 chest strap served as the reference device, proven reliable through multiple scientific studies. For stationary cycling, the match between the Apple Watch Ultra 4 and Polar H10 was nearly perfect, with all data points very close to or on the black correlation line and a correlation value close to 1.00. Individual cycling sessions showed almost perfect matching overall, with only slight deviations where the Apple Watch detected a higher heart rate momentarily in one session.
When testing running outdoors, the overall correlation remained very good at 0.99, though with slightly larger deviations than cycling. The first running round showed almost identical results to the initial review. However, the second round encountered interruptions where the device stopped monitoring heart rate for periods of time, shown as straight lines in the data. Subsequent runs the following day and two days later were almost perfect. These interruptions appear to be a minor ongoing issue with the Ultra 4, though firmware updates may have resolved some of the initial problems. The testing used developer beta software, which may contribute to these issues.
Outdoor cycling showed a correlation of 0.99 with some momentary low heart rate detections and a few slightly high readings. Interruptions occurred in some rounds, particularly at the beginning or during specific segments. Weightlifting presented the most challenging conditions due to significant wrist pressure affecting blood flow monitoring. The correlation remained very good, but moments of interruption occurred during chest press and diving exercises, with the device having difficulty capturing heart rate at peak training sets.
The Apple Watch Ultra 4's correlation performance was compared against other tested devices. For stationary cycling, it outperformed 98% of devices. For outdoor cycling, it outperformed 92% of devices. For running, it outperformed 74% of devices. For weightlifting, it outperformed 98% of devices. The overall heart rate tracking score based on correlation alone would be 99, considered an ideal rating. However, this score may be inflated because the Apple Watch drops readings when signal quality is poor, unlike some competitors.
Daily heart rate measurements showed strange results with infrequent measurements in the data export despite expectations of more frequent readings. When wearing an ECG chest strap for several hours alongside the Apple Watch, the overall correlation was very good at 0.99, with reliable measurements in ranges from approximately 50 to 127 beats per minute. The infrequent measurements may result from Apple Health deleting old values to manage data size, or data export limitations.
Nine nights of nighttime heart rate data showed good overall correlation with maximum deviation of about one beat per minute. Some anomalous values appeared stuck together in the data export. Individual measurement examples showed generally good agreement with some sudden spikes in the Apple Watch Ultra 4 data that may represent either data export issues or actual measurements during brief heart rate increases.
Heart rate variability measurements produced some of the best results, with a nice correlation across nine nights, though the Apple Watch Ultra 4 recorded slightly less variability than the reference. Individual measurements showed better matching, with heart rate variability appearing quite reliable and measured very frequently.
Sleep stage tracking was evaluated over six to seven nights using the ZMax EEG machine as the reference device, considered a silver standard in scientific studies. Deep sleep matching reached 77%, light sleep matching achieved 81%, and REM sleep matching was 74%. These results are particularly strong compared to most competitors. Examples from individual nights showed good compatibility overall, with most deep sleep periods correctly detected and REM sleep periods accurately identified in multiple cases.
When compared to competitors, the Apple Watch Ultra 4 positioned itself among the top devices for sleep stage tracking. Performance was nearly identical to previous testing of the Apple Watch Ultra 3, suggesting Apple continues using the same sleep-tracking algorithm. All Apple Watches, including the Ultra and SE models, perform at similar quality levels. The top five brands for sleep tracking include Apple Watches, Google products (Fitbit and Pixel devices), the [ __ ] belt, and the Eight Sleep Pod.
The total sleep score rounded to 77, earning a "great" rating. GPS tracking showed acceptable results for cycling, with signals becoming very close to each other after initial deviations. Some smoothing occurred, occasionally making the track pass through buildings or appear to walk across sidewalks. Multiple bike rides to and from work showed very similar tracks overall, with only minor deviations in one line. The signals remained very close to each other through city areas, though some softening around turns made tracks appear to pass through buildings.
This appears to be due to a slight displacement and possibly some smoothing. Passing through a few buildings shows a little smoothing again. The performance remains consistent overall. The performance is good but not amazing, and that's mostly due to smoothing.
The Apple Watch Ultra 4 showed significant problems in the initial review, but perhaps that was just a coincidence. Three rounds were completed in total. The third round gave almost the same result. When all three rounds are placed on top of each other, the device shows very consistent tracking of the tours. They all tend to be a little coordinated at one juncture, and the watch makes the path go across the pond with a little smoothing around that bend as well.
There is a slightly larger deviation on the left when combining all three runs, because on one occasion the runner had to run to the left due to the presence of more tourists. When the last round is removed, good consistency is visible on the left side.
Many devices struggle on the right side, but that doesn't seem to be the case with the Apple Watch. A wall is located there, and many devices receive a very turbulent signal at this location. This doesn't seem to be a problem for the Apple Watch. The tracking actually looks very good, but again, the main problem is the smoothing or the flaw.
The smoothing is a feature introduced by Apple, and not everyone may be happy with it. Overall, the GPS tracking feature is still strong, but it's not necessarily better than it was on the Apple Watch Ultra 3. It might be slightly worse, but it's hard to say for sure. The performance is almost identical.
An 80 rating is given for GPS tracking performance, mostly due to the smoothing process. Overall, the performance is really good at GPS tracking, but smoothing means some detail is lost, and sometimes unrealistic tracks are produced. This rating has not yet been included in the database, so there is no automatic ranking for it.
Any score between 75 and 85 receives a "good" rating. The GPS tracking performance of the Apple Watch Ultra 4 is good, but not excellent or perfect. Another device is needed to achieve that level of performance, because the preference is to not have smoothing and to keep all the details.
The results show good GPS tracking, and great sleep stage tracking. The variation in heart rate during the night is remarkable. The heart rate tracking is very good, but the cause of those sudden spikes is still being investigated. Based on correlation alone, heart rate tracking is ideal, but it might be lowered to "great" if those interruptions are taken into account.
This is a somewhat personal decision. What the Apple Watch measures is essentially perfect, but taking interruptions into account might make it "great" rather than "perfect," depending on use cases.
The Apple Watch Ultra 4 performs very well overall. It's a great health tracking device, and with the Apple Health update, it's really competing with big companies like Whoop and Oura by supporting a more comprehensive system for tracking health and fitness.
There is curiosity about personal experiences with the new sensor in the Apple Watch. Some positive and negative comments about the new Apple Watch Ultra 4 have already been heard. Some people have received a worse unit than others, but these are individual experiences. More experiences with older Apple Watch Ultra models are welcome, especially from those who have purchased the new Apple Watch Ultra 4.
If deciding to buy an Apple Watch Ultra 4 or Apple Watch Series 12, or perhaps a Whoop band, or the favorite sleep-enhancing device the Eat SleepPod, or anything else from Amazon, even something as simple as toilet tissues or coffee beans, there are various affiliate links in the description below, and many of them give the best discount available.
After watching this video about the Apple Watch Ultra 4, the recommended next video shows how Apple says its heart rate tracking beats the competition, and another video about the favorite sleep-enhancing device, the Eat SleepPod.
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