Amazfit Cheetah 2 Ultra – Hyrox SIM HR Test

My ability to break things seems to have vanished in the heat of Day 3 of the UK’s early summer. It was 33-34 Celsius today, so the obvious place to go was a hot, sweaty gym to cool down a bit. The ideal candidate for ‘a bit of cooling down’ was, of course, a HYROX Sim. Essentially, going through and simulating an entire HYROX race in a gym-equivalent setting.

This was very much a take-it-easy day on each of the stations, with lots of faffing in between and the occasional chat with people wondering what I was doing. A: “Enjoying the aircon. That’s what I’m doing.”

Anyway, back to me breaking stuff. I normally get awful performances from optical HR devices, so I was expecting the worst from Cheetah 2 Ultra and, to be frank, everything else as well. I even expected HRM600 to throw a wobbly as it has been doing a bit of that lately. Whoop MG? People say that’s inaccurate (on the biceps, I almost always find decent accuracy) and the Polar SENSE…well…to be fair, that’s pretty accurate most of the time. Except now they seem to have disabled the ability to use Flow Sync to get workouts into Flow. This causes me a lot of annoyance, but doesn’t affect the data quality.

Also on test today was the Train.Red FYER Muscle Oxygen (SmO2) sensor

Amazfit Cheetah 2 Ultra Optical HR Test Results

Looking at the chart, it’s pretty obvious that every device behaved itself. If you really want the statistical numbers to sound impressive, then here we go:

Amazfit Cheetah 2 Ultra matches reference devices closely:

• vs WHOOP MG: +0.0 bpm bias (excellent)
• vs HRM600: -0.9 bpm bias (excellent)
• vs Polar SENSE: -0.1 bpm bias (excellent)

Tight limits of agreement across all comparisons.

Muscle Oxygen

I was also using Train Red’s FYER sensor to display muscle oxygenation with a Connect IQ data field on the Garmin FR970. There are a few muscle oxygen that are perhaps best for a separate article, another day. Check out the review, though, as it covers quite a lot of the details.

Muscle oxygen levels are influenced by supply and consumption of oxygenated blood, and the interpretation of the metrics depends on understanding the current state and the rate of change, as well as the current level. This is more complex than a simple heart rate number, but it can be useful in strength training (and many other scenarios). For example, in between reps, you could wait for the oxygen levels to recover to a certain point before starting the next rep, and the amount of time you have to wait between reps will likely increase the more reps you have completed. You can choose to vary that ‘certain’ point tomorrow and, for example, target hypertrophic gains.

Explanation

First up, even though this was in a gym setting, surprisingly few of the stations involved extensive wrist movement, which, coupled with steadier movements than normal (Concept II Rower, Ski ERG), made the Cheetah’s job perhaps easier than you might expect. Also, it was hot, which may increase blood flow near the skin, making it easier to detect a pulse with optical HR. Finally, it was mostly a Z2-Z3 cardiovascular effort, so this is another aspect of the workout that can favour accurate sensing.

This was the third in a series of Cheetah 2 Ultra tests that I’ve reported individually here to provide slightly more detail than usual, rather than saving it for the review.

The bottom line was that Cheetah 2 was highly accurate at recording optical HR in a HYROX-like event in a gym setting.

FAQ

Is the Amazfit Cheetah 2 Ultra accurate for HYROX training?

In this test, the Cheetah 2 Ultra returned near-zero bias against three reference devices — Garmin HRM600, Whoop MG and Polar Verity SENSE — across a full HYROX simulation. The gym setting, limited wrist movement on most stations, and predominantly Zone 2-3 effort all favoured accurate optical HR sensing, so results under higher-intensity or more dynamic conditions may differ.

Why does heat affect optical heart rate accuracy?

Higher ambient temperatures increase blood flow near the skin surface, making it easier for an optical sensor to reliably detect the pulse. This is one reason optical HR devices often perform better in warm conditions than in cold ones, where peripheral blood flow is reduced.

Last Updated on 26 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