Amazfit Cheetah 2 Ultra HR Accuracy: HYROX Sim and SmO2 Tested

The Amazfit Cheetah 2 Ultra just passed its third consecutive clean HR accuracy test, this time inside a gym running a HYROX simulation at 33°C. That heat matters: optical PPG sensors that read blood volume changes via light through the wrist are notoriously unreliable when skin perfusion shifts under thermal stress. The Cheetah 2 Ultra held its ground anyway.

Optical HR Accuracy Under Pressure

The test compared the Cheetah 2 Ultra's wrist PPG sensor against three reference devices: the Whoop MG, the Garmin HRM600 chest strap, and the Polar Verity Sense optical arm sensor. The HRM600 uses electrical impulse detection via ECG-equivalent technology, making it the gold standard for HR accuracy. Near-zero bias against all three references in a HYROX simulation, which means burpee broad jumps, sled pushes, and rowing intervals, is a genuinely strong result. Most optical wrist sensors crack under that kind of movement variability.

For context, the Garmin Forerunner 965 and Fenix 8 both occasionally show HR lag or dropout during high-intensity strength-cardio hybrid efforts. The Polar Vantage V3 handles HIIT-style workouts well at the wrist, but even that watch benefits from pairing with the Verity Sense on the arm for functional fitness. The Cheetah 2 Ultra's wrist sensor competing cleanly with an arm-mounted optical reference is a meaningful data point, not just marketing copy.

SmO2 and the Sensor Stack

The Cheetah 2 Ultra also carries an SmO2 sensor, which uses optical light at specific wavelengths to estimate muscle oxygen saturation. This is different from SpO2, which measures blood oxygen at the wrist. SmO2 is more actionable for training: it can indicate when a working muscle is being pushed into oxygen debt, relevant for threshold intervals or Hyrox-style circuits where you need to pace effort across multiple stations. Amazfit is one of the few wrist-based platforms even attempting this outside of dedicated devices like the Moxy monitor. Whether the on-wrist SmO2 data is accurate enough to act on in real time is a separate question, but having it integrated at this price point is worth noting for data-focused athletes.

The broader sensor package on the Cheetah 2 Ultra includes a barometric altimeter for air-pressure-based elevation tracking, GPS for positioning, and the wrist PPG array for heart rate and HRV. That covers the core needs for triathlon, trail running, and functional fitness without requiring a chest strap on every session. We tested the GPS performance separately, and those results are covered in our [Amazfit Cheetah 2 Ultra GPS, Battery, and Trail Climb Feature Tested](/en/articles/amazfit-cheetah-2-ultra-gps-battery-and-trail-climb-feature-tested-2026-05-25) article.

Real-World Use Cases

For Hyrox athletes specifically, this test result is the most relevant one. A watch that loses HR signal during wall balls or rowing transitions is useless for pacing strategy. The Cheetah 2 Ultra maintained signal integrity across all stations in 33°C heat, which is roughly the ambient temperature you'd expect in a large indoor Hyrox event. For triathletes, the combination of reliable optical HR during open-water swim exits, solid GPS on the bike, and SmO2 for run pacing makes this a credible all-in-one option. The [HYROX, bike, and heat HR accuracy testing](/en/articles/amazfit-cheetah-2-ultra-hr-accuracy-bike-hyrox-and-heat-tested-2026-06-01) paints the fullest picture across all three disciplines.

For running specifically, the Cheetah 2 Pro sits below the Ultra in the lineup and was tested across road running, cycling, and swimming. That review is available at [Amazfit Cheetah 2 Pro Review: Running, Cycling, and Swimming Tested](/en/articles/amazfit-cheetah-2-pro-review-running-cycling-and-swimming-tested-2026-05-29). The Pro brings titanium and sapphire at a significant price premium over previous Cheetah generations, but the Ultra is the sensor-heavy flagship for athletes who want the full data stack.

What's missing? The Zepp OS ecosystem still lags behind Garmin Connect and Polar Flow for training load analysis and long-term trend visualization. Third-party app support is limited compared to a Forerunner 965 or even a Coros Pace 3. Recovery metrics exist, but they don't carry the depth or trust of Whoop's strain and recovery model. If you live inside Garmin's training ecosystem, the Cheetah 2 Ultra won't pull you out of it.

The verdict: the Cheetah 2 Ultra is built for athletes who want serious optical HR performance, SmO2 data, and strong GPS in a single watch without paying Garmin Fenix 8 prices. It earns its place against the Polar Vantage V3 and Coros Vertix 3 on pure sensor reliability. If your priority is Hyrox-specific accuracy or you train in hot environments where wrist optical sensors typically fail, this is now a legitimate option to test.