What does my Apple Watch actually measure, and how accurate is it?

Your Apple Watch uses two primary technologies. Photoplethysmography (PPG): a green LED shines into your skin, a sensor measures how much light bounces back, and the variation in reflectance with each heartbeat generates a heart rate reading. This is how it measures resting heart rate and passive HRV estimates. Electrical sensing via the digital crown: pressing your finger on the crown completes an electrical circuit, generating a single-lead ECG trace (equivalent to lead I of a standard 12-lead ECG). Resting heart rate by PPG is genuinely accurate: validation studies found mean absolute percentage errors of 1–5% compared to ECG-derived measurements at rest. (Shcherbina et al., npj Digital Medicine, 2017)

During high-intensity exercise, accuracy degrades: movement artifact, wrist positioning, and skin tone variation all introduce error. The device you trust at rest is less reliable during sprint intervals than its confidence display suggests. The single-lead ECG is meaningful for one specific purpose, identifying atrial fibrillation, but cannot assess ST-segment changes that cardiologists use to detect ischemia, cannot evaluate bundle branch blocks, and cannot diagnose structural heart disease. It is a narrow but valuable window, not a clinical 12-lead ECG.

Cardiologist's calibrated position, Solid (1) for resting heart rate accuracy. Promising (2) for single-lead ECG in AFib detection. Not validated for ST-segment analysis, chamber hypertrophy, or structural evaluation.

What to do: Trust your resting heart rate trend data. Treat single-lead ECG traces as conversation starters with your cardiologist, not diagnostic conclusions.

For the full picture, read What Your Apple Watch Is Trying to Tell You.