1. Definition of Terms: HRV, RRI
1. RRI (RR Interval / R-R Interval)
The time interval between two adjacent R waves in an electrocardiogram (ECG), unit: ms (milliseconds), which is the interval duration between two consecutive heartbeats.
If the heartbeat is fast and slow, the RRI value will be long and short;
All HRV data comes from a series of continuous RRI sequences.
2. HRV (Heart Rate Variability)
The fluctuation rule of successive heartbeat intervals (RRI), not the instantaneous heart rate, but the slight fluctuation of heartbeat intervals, regulated by the autonomic nervous system (sympathetic + parasympathetic nerves).
A resting heart rate of 60 beats per minute while sitting quietly does not mean that each heartbeat interval is completely fixed. 5 heartbeat intervals may be (ms=milliseconds): 1020ms, 980ms, 1050ms, 990ms, and these differences are HRV.
3. Physical indicators reflected by HRV monitoring
(1) Autonomic nerve function (core)
High parasympathetic (vagus nerve) activity → High HRV value: good rest, low stress, good physical recovery;
Sympathetic excitation → Low HRV: fatigue, staying up late, high stress, anxiety, overtraining, sub-health.
(2) Physical recovery level
Commonly used in the sports circle: Decreased HRV = body not recovered, high-intensity training is not advisable; Increased HRV = physical recovery completed.
(3) Stress, sleep, fatigue, emotional load
Prolonged low level: chronic fatigue, long-term mental tension;
(4) Auxiliary reference for basic cardiovascular risk
Used in clinical medicine for preliminary screening of cardiac function and arrhythmia (only auxiliary, cannot replace medical equipment).
2. Can ordinary smart bracelets measure HRV? Two scenarios
1. Most affordable entry-level bracelets (tens to 100 yuan): Cannot accurately measure HRV
2. Mid-to-high-end bracelets/watches (above hundreds of yuan: Apple Watch, Huawei high-end, Garmin, Oura, etc.): Can estimate HRV
Why can't cheap bracelets measure HRV?
The mainstream method for bracelets to measure heart rate is PPG photoelectric green light sensor:
2-1. PPG Principle: Measures vascular volume pulse wave (blood flow), not ECG R wave
The definition of RRI is derived from the R wave of ECG (ECG), while photoelectric is pulse wave, which lags behind ECG by tens of ms with large single measurement error;
It is sufficient for measuring instantaneous heart rate, but the accuracy of continuous beat-by-beat RRI is insufficient, and small fluctuations are covered by errors, making it impossible to calculate real HRV.
2-2. Insufficient sampling accuracy
Entry-level bracelets have low PPG sampling rate (25Hz and below), unable to capture millisecond-level interval accurately; HRV requires 1ms-level RRI accuracy;
Hand shaking, vascular compression, and movement jitter bring a lot of noise, and cheap products have no hardware noise reduction.
2-3. No precise timing clock in hardware
HRV relies on millisecond-level timing, low-cost MCU has poor clock accuracy, leading to distorted beat-by-beat interval calculation.
Supplement: The "HRV" marked on ordinary bracelets is mostly an algorithm-estimated value, not medically standard HRV, only for reference.
3. Necessary hardware conditions for accurate HRV measurement (two sets of standards: [medical-grade accuracy] and [consumer-grade reliable])
(1) Essential hardware for medical-grade accurate HRV (gold standard: ECG)
- ECG electrocardiogram acquisition module (core)
- Captures myocardial electrical signals through electrodes to accurately locate ECG R waves and obtain standard RRI;
- High-sampling ADC analog-to-digital converter
- Sampling rate ≥250Hz (500Hz/1000Hz commonly used in clinical practice), time resolution ≤1ms;
- High-precision hardware clock (RTC)
- Beat-by-beat interval timing error <1ms;
- Hardware filter circuit
- Hardware filters out power frequency interference (50/60Hz mains electricity), myoelectric noise, and baseline drift;
- Dry/wet electrode ECG patches (chest patches, limb electrodes) to eliminate motion artifacts.
(2) Consumer-grade wearables (PPG for usable HRV, mid-to-high-end watch configuration);
Fit Monster smart bracelet adopts high-sampling-rate PPG sensor to achieve high-precision HRV collection.
- To get reliable HRV with only photoelectric PPG, the hardware must meet the following standards:
- High-sampling-rate PPG sensor: ≥100Hz (25Hz for entry-level), high-frequency sampling captures subtle changes in pulse wave peaks;
- Multi-light source + multi-channel photoelectric: green light + red light + infrared multi-band, multiple receiving PD photosensitive tubes, resistant to skin blood flow disturbance;
- Six-axis IMU gyroscope + accelerometer: hardware collects motion data in real time, and eliminates hand shaking noise at the hardware level;
- High-precision hardware timer, MCU hardware timing, RRI error controlled within ±5ms;
- Independent power supply voltage stabilization module: reduces photoelectric sampling drift caused by voltage fluctuations;
4. Supplementary knowledge points
ECG > PPG: ECG-based HRV measurement is the medical standard, while PPG is derived from pulse waves, only for health reference and cannot be used for clinical diagnosis;
The HRV measured by the bracelet for 5 minutes on an empty stomach while lying still in the morning is the most accurate. HRV data of all wearable devices becomes invalid when walking or hands are shaking.
Fit Monster smart bracelet adopts high-sampling-rate PPG sensor to achieve high-precision HRV collection.