Fitbit Air vs Amazfit Cheetah 2 Ultra – HR Road Bike Sprint HR Accuracy Results

Yet another hot English pre-summer day at 32 Celsius. It’s time to cool down with some hill sprints and HR tests.

On test here was the Amazfit Cheetah 2 Ultra on probably its last test before the review, and set against it, I can now show the results of Fitbit Air, TymeWear VitalPro, Whoop MG, and Polar SENSE.

Unlike previous tests that I reported this week, I have nothing entertaining to say. So let’s crack on with the stats, which can be summarised thus with DCRainmaker’s latest Analyser charting software (beta):

• Fitbit Air accuracy (vs ELEMNT/Tymeware reference): Excellent. Average 134.96 bpm. Bias -0.8 bpm (slightly low). Limits of Agreement ±6 bpm.
• Cheetah accuracy (vs ELEMNT/Tymeware reference): Excellent. Average 135.35 bpm. Bias -0.2 bpm (very close). Limits of Agreement ±3 bpm (tighter).

I would say that +/- 3 bpm is accurate enough, in the sense that it will make little difference to zone-based load calculations. However, the +/-6bpm from the Fitbit Air would put it in different zones. Perhaps it’s a timestamp issue throwing the data pairs out, or, as you can see from the chart, perhaps it’s simply that Fitbit Air missed a few bits.

Let’s get some reality into this.

There is an awful lot of nonsense being said about Fitbit Air today, actually, one of the voices of reason was Marques Brownlee on YouTube (@mkbhd, one of the Brownlee triathlon brothers’ better-known cousins). He didn’t quite agree with me about the Fitbit vs Whoop thing, but his argument was similar and, at least, defensible.

When you test an optical HR device, the first sense-check is to review the configuration. From a super-simplistic level, the various diodes and sensors need to be in a circle. Fitbit Air and Whoop are not, and they are fighting against physics to get accurate results. Admittedly, both organisations have significant funds for that fight and do a pretty good job. Then you need to look at how the sensor interacts with the skin. Does it sink in? Can the band easily flip? Basically, you are looking at the sensor’s ability to stay in place and keep ambient light out. Fitbit doesn’t score well on any of those points. Garmin, Amazfit, Apple…yep, they know what to do. Whether they do it or not is a question that actual testing will tell you.

After all these tests on Cheetah 2 Ultra, I’ve been impressed. It’s produced very good results. Actually, some of the very best I’ve had for HR. Has Amazfit found the secret sauce? A: No. It was likely hot, which makes it much easier to get accurate results from a decent sensor.

FAQ

Q: Is ±3 bpm accurate enough for heart rate zone training?

A: For most zone-based training, yes. A ±3 bpm error is unlikely to shift you across a zone boundary at typical steady-state intensities. At threshold or above, where zone boundaries are narrower, it becomes more consequential.

Q: Why does the Fitbit Air show wider limits of agreement than the Amazfit Cheetah 2 Ultra?

A: Two likely factors: the non-circular sensor array on Fitbit Air fights physics to achieve good optical contact, and the band design makes it easier for ambient light to intrude. Whether timestamps are also misaligned is worth checking in the raw data.

Q: Does heat help optical HR accuracy?

A: Yes. Vasodilation in warm conditions pushes blood vessels closer to the skin surface, improving the signal-to-noise ratio for any optical sensor. Test results in hot weather tend to flatter all devices.

Last Updated on 27 May 2026 by the5krunner

the5krunner

tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors. ID