Sports wearables have evolved from simple pedometers to health platforms providing dozens of physiological metrics. Understanding the algorithms behind core sports metrics — VO₂max estimation, heart rate zones, recovery scores — is the key bridge for translating device data into actual training improvements.
## How Wearables Estimate VO₂max
**VO₂max** (maximal oxygen uptake) is the gold standard for aerobic capacity measurement (ml/kg/min); laboratory measurement requires a graded exercise test to exhaustion on a treadmill or power bicycle. Wearables use a **Submax Protocol**: at a known exercise intensity (speed/power) and corresponding heart rate, using the linear heart rate-oxygen consumption relationship to extrapolate the maximum value. Garmin’s “First Beat” algorithm is the most widely validated consumer-grade approach, with correlation to laboratory measurement of approximately r=0.85-0.92 and average error ~5%.
**GPS speed accuracy**: VO₂max estimation depends on accurate speed/power data. Urban building interference and tree canopy coverage cause GPS signal instability, introducing speed errors that propagate into VO₂max estimation. Recommend completing “calibration runs” in open terrain to improve data quality.
## Heart Rate Zone Systems
Garmin, Polar, and similar devices typically use a 5-zone system: Z1 (<60% max HR, very low intensity) → Z2 (60-70%, aerobic base) → Z3 (70-80%, aerobic endurance) → Z4 (80-90%, near lactate threshold) → Z5 (>90%, high intensity/VO₂max). But zone definitions differ across sources, and max HR estimation (220-age) can err by ±20 bpm, making zone calculations inaccurate. Recommend performing a lactate threshold test (field test or laboratory) to personalize heart rate zones.
## Training Load and Recovery Metrics
**TRIMP (Training Impulse) and TSS (Training Stress Score)**: integrate exercise duration × intensity into a single number, tracking Acute Load (7-day) vs. Chronic Load (42-day) ratio (Acute:Chronic Ratio, ACR). ACR >1.5 is considered an injury risk warning signal (acute training volume far exceeds adaptive base).
**Sleep + HRV integration**: WHOOP, Garmin, and similar devices integrate HRV, resting heart rate, sleep duration, and sleep quality to calculate “recovery scores,” recommending high-intensity training on high-recovery days and reduced intensity or rest on low-recovery days. Research shows HRV-guided training produces marginally better endurance athlete progress than fixed periodization.
See [Digital Health Overview](https://sunqi.org/digital-health-overview-en/) and [Garmin official health data documentation](https://www.garmin.com/en-US/garmin-health/).
