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There is not enough independent evidence to say that Hiyd is as accurate as an Apple Watch, Garmin, or Fitbit. Hiyd is still pre-launch, and no independent validation study or head-to-head test of its health tracking was found. A 2022 ECG-referenced study did evaluate specific models from Apple, Garmin, and Fitbit, but only in postoperative patients at rest. Its findings do not establish that every model, metric, or workout reading from those brands is accurate in every setting.
For now, Hiyd’s sensor details and tracking claims are the company’s own specifications, not proof of accuracy. Whether it is a good fit depends not only on the sensor design but also on the metric you care about—heart rate, steps, sleep, or recovery—and on how that metric is independently tested.
What the available evidence says
Accuracy is not a single score that applies to an entire wearable. A device may track heart rate well during quiet rest but perform differently during exercise; step counting and sleep estimation require different comparisons altogether. That is why a heart-rate result cannot establish that a tracker’s sleep stages, calorie estimates, or step counts are accurate.
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Evidence by device
| Device or evidence | What was assessed | What the result supports |
|---|---|---|
| Hiyd | Vendor descriptions of planned hardware and tracking features | No independent head-to-head accuracy result was found. |
| Apple Watch Series 7, Garmin Fenix 6 Pro, Fitbit Sense, and Withings ScanWatch | Heart rate compared with ECG in a 2022 prospective study at University Hospital Würzburg | Results apply to those models and the study’s postoperative resting context. |
| Fitbit Charge devices | Step counts in studies summarized by a systematic review | The review described step-count accuracy as generally good, but protocols and reference standards differed. |
The study of the four named models enrolled 201 postoperative patients and analyzed 1,630 valid measurements. It reported correlation of at least 0.95, concordance of at least 0.94, and mean absolute percentage error below 5% across those measurements. The reported mean absolute error in observed deviations was under 5 beats per minute. These are encouraging results for the tested heart-rate measurements, not a universal accuracy guarantee for the brands or their other features.
Why a strong heart-rate result does not settle the comparison
ECG is an electrical measurement of the heart’s activity. Optical wrist sensors instead use light to estimate changes in blood volume near the skin. A wearable’s reading is therefore an estimate produced by a sensor and its processing, while ECG serves as a reference in the cited study. Agreement with ECG can be assessed for heart rate, but it cannot by itself validate steps, sleep, calories, or recovery scores.
Correlation is not the same as error
Correlation describes how closely two sets of measurements move together. A high correlation does not by itself show that readings are close in absolute terms: two measures could rise and fall together while one is consistently offset. Concordance and error measures add information about agreement. The Würzburg study reported all of these types of statistics, which makes its result more informative than correlation alone, but the numbers still describe the study’s measurements and participants.
Mean absolute percentage error summarizes the size of errors relative to the reference values. Mean absolute error expresses the average size of the differences in the unit being measured—in this case, beats per minute. Neither statistic tells a reader exactly how a particular watch will behave during a run, on a loose wrist, or for an individual with a different pattern of movement or physiology.
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Rest and exercise are different test conditions
The cited clinical study involved postoperative patients in a controlled resting context. That is not interchangeable with an intense workout, repeated wrist flexion, or movement that makes sensor contact less stable. A separate exercise-intensity comparison found that heart-rate and activity accuracy changed with both intensity and device. Results should therefore be interpreted by activity and metric, rather than transferred from rest to exercise.
What Hiyd says it will track
Hiyd describes its product as a small sensor that attaches to the back of a classic analogue watch, allowing the wearer to keep using that watch while adding tracking features. Its official pages list steps, activity or workouts, calories, sleep, and oxygen saturation; the product page also lists heart rate, recovery, resting heart rate (RHR), heart-rate variability (HRV), calories/macros, and daily readiness. Those are vendor claims about planned capabilities, not independently verified performance results.
Published hardware details and the size discrepancy
Hiyd’s reservation page specifies a 7 g sensor measuring 32 mm across and 3 mm thick. It describes green and infrared LEDs for heart rate and HRV, red and infrared LEDs for blood oxygen, a thermistor for skin temperature, and an accelerometer for movement. A separate product-page description calls the sensor 2.7 mm thick and says it attaches by microsuction. The stated thickness differs between the two pages; until the product ships, readers should treat these as published specifications rather than confirmed measurements of a retail unit.
The reservation page also states a four-day battery life, iOS and Android compatibility, and integration with Apple Health and Google Health. Hiyd’s product page says there is no monthly fee. These details describe compatibility, battery claims, and the company’s planned service model; none establishes sensor accuracy.
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Pre-launch status matters
Hiyd says its Kickstarter launch is planned for November 2026. The reservation page offers a refundable $10 reservation, not a product shipment. A planned launch and reservation do not provide the same evidence as an independently tested product in consumers’ hands. Prospective buyers cannot use those details to infer that the final device will match the stated specifications or achieve a particular tracking result.
How to judge each type of tracking
To compare wearables fairly, match the test to the feature. A result for one metric should not be used as a proxy for another. The relevant question is not simply which device has a sensor, but what it was compared against, in which conditions, and for which model.
Heart rate
Separate resting measurements from exercise readings. For a meaningful comparison, look for a specific model tested against ECG or another suitable reference during the activity you care about. The Würzburg study provides ECG-referenced resting-context evidence for the Apple Watch 7, Garmin Fenix 6 Pro, and Fitbit Sense, but not for Hiyd or every later model from those brands. Hiyd’s stated optical components and proposed case-back position are design information, not a test result.
Steps and activity
Step-count accuracy should be checked against a manual count or validated accelerometry—not inferred from heart-rate performance. The systematic review found generally good step-count accuracy for Fitbit Charge devices, while noting that study protocols and reference standards varied. That summary is not a model-by-model guarantee, and it does not establish Hiyd’s step accuracy. Activity labels and workout detection also need their own validation; they are not identical to counting steps.
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Sleep duration and sleep stages
Sleep duration and sleep-stage estimates are separate claims. Consumer wearables estimate sleep using movement and optical signals. Testing sleep stages requires comparison with polysomnography or an equivalent reference method; a tracker reporting stages does not demonstrate that those classifications are correct. No independent Hiyd sleep validation result is available in the evidence described here, so its sleep feature should be treated as an advertised capability rather than a proven measure of duration or stages.
HRV, recovery, and readiness
HRV is sensitive to sensor contact and motion artifacts. A stable position against the watch case may be Hiyd’s design rationale, but placement alone does not prove accurate readings. Recovery and readiness scores add another layer: they are interpretations built from underlying measurements, not direct readings of a single physiological quantity. Without published validation methods and results, a score from Hiyd cannot be meaningfully ranked against a different brand’s score.
Calories and oxygen saturation
Calories and other derived estimates depend on models that process sensor data; they should be treated as trend indicators, not clinical measurements. Hiyd lists calories and oxygen saturation among its planned tracking features, but the available claims do not establish how accurately either will be measured. A stated red and infrared sensor design for blood oxygen is not a substitute for validation against an appropriate reference method.
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Does Hiyd’s position under a watch help?
Hiyd’s technical explanation says optical heart-rate sensors use light to detect blood flow through the skin and require firm, consistent contact at a stable anatomical position for accurate readings. The company’s case-back attachment is presented as a way to place the sensor against the wrist while retaining an analogue watch. That is a plausible design objective, but a rationale is not evidence that the sensor maintains good contact across different watches, wrist shapes, movement, or wear conditions.
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The published descriptions indicate that Hiyd is meant to sit beneath a classic watch, but they do not establish that it will fit every mechanical watch or that it will retain reliable readings in every configuration. Nor do they show that wearing a mechanical watch over it preserves all health data. That practical question requires a final product and performance testing under actual wear conditions.
Which is more accurate: Hiyd, Apple Watch, Garmin, or Fitbit?
There is no defensible overall winner from the evidence available. The fairest answer depends on the metric and use case:
- Heart rate at rest: Apple Watch 7, Garmin Fenix 6 Pro, and Fitbit Sense have ECG-referenced evidence from the 2022 postoperative study. Hiyd has no comparable published result.
- Heart rate during exercise: Accuracy can vary by intensity and device. The cited resting-context study does not settle workout performance, and no comparable Hiyd result is established.
- Steps: A systematic review reported generally good results for Fitbit Charge devices, but differing methods limit broad comparisons. The evidence described here does not establish Hiyd’s step accuracy.
- Sleep, HRV, recovery, calories, or oxygen saturation: A heart-rate study cannot answer these questions. Look for validation specific to the metric and reference method; Hiyd’s listed features are currently vendor claims.
Apple Watch and Garmin models may also offer broader workout and navigation ecosystems. Hiyd’s published specifications emphasize phone syncing and do not list onboard GPS. This is a feature distinction, not evidence that one device is more accurate for health tracking.
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If Hiyd becomes available, a useful evaluation will need independent tests of the shipped device—not only sensor specifications or software demonstrations. When assessing any new tracker, check for the following:
- A named device version and a clearly described participant group.
- A reference method suited to the metric: for example, ECG for heart rate, a counted or validated reference for steps, and polysomnography or an equivalent method for sleep stages.
- Results separated by context, such as quiet rest and exercise intensity, rather than one combined score.
- Agreement or error statistics in addition to correlation, with enough detail to understand how the readings differed from the reference.
- Testing of the final attachment and fit, since sensor contact and motion can affect optical measurements.
Until such results are available, use Hiyd’s listed metrics as features it says it intends to offer, not as measurements whose accuracy has been demonstrated. A tracker’s trend may be useful to its wearer, but it should not replace a clinical measurement or be treated as a diagnosis.
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