HR Monitor Accuracy (Chest vs Optical)

Two technologies dominate consumer heart-rate sensing:

• Chest straps (ECG-based) detect the heart's electrical signal, the same principle as a clinical ECG. They are the more accurate option and the de-facto reference for consumer HR, with errors typically only a couple of bpm. They also track fast changes well, which matters for intervals and HRV.
• Wrist optical (PPG-based) shine LED light into the skin and infer pulse from blood-volume changes (photoplethysmography). They are convenient and often acceptably close at rest and steady efforts (reported errors at most a few bpm with limits of agreement under ~15 bpm in good conditions), but error grows during exercise.

PPG error is larger during exercise — studies report average absolute error meaningfully higher during activity than at rest — and concentrates in known failure modes:

• Motion artifact — bouncing/arm swing; worst in high-intensity and irregular movement.
• Wrist flexion / muscle activity — e.g. weightlifting, cycling grips, racquet sports disrupt the signal.
• Cold skin, poor fit, and tattoos reduce signal quality; skin tone can also affect PPG performance, an area of ongoing study.
• HR lag — optical (and HR generally) responds slowly, so short intervals are mistimed.

Practical guidance: for interval intensity, HRV, and zone fidelity (see time-in-zone) prefer a chest strap (or an optical forearm/upper-arm armband, which outperforms the wrist). For general all-day trends and resting metrics, wrist optical is fine. Always read HR data knowing the device and conditions (see wearable-accuracy-limits).

• Chest strap = ECG (electrical), most accurate, ~couple-bpm error; tracks fast changes.
• Wrist optical = PPG (light), convenient; good at rest, worse in exercise.
• PPG error rises with motion artifact, wrist flexion/muscle activity, cold, poor fit, tattoos.
• Skin tone may affect PPG performance (ongoing research).
• Use chest strap (or armband) for intervals/HRV/zones; wrist fine for daily/resting trends.

• heart-rate-logging — the data this accuracy governs.
• hrv — needs chest-strap fidelity for reliable readings.
• time-in-zone — wrong HR = wrong zone minutes.
• wearable-accuracy-limits — broader device-accuracy context.
• heart-rate-monitor — device options and selection.