Concept
Heart-rate sensor accuracy
How closely and consistently a device reflects the runner's actual pulse during a particular type of exercise.
Wrist and chest sensors
Wrist optical sensors estimate pulse from blood flow; chest straps detect electrical signals and often respond better to rapid changes.
Common error sources
Loose fit, cold skin, arm movement, sweat, poor contact, and cadence lock can distort a trace.
How to judge a reading
A believable trace should change plausibly and broadly agree with breathing, effort, and the session.
How to use it
Check fit and compare surprising readings with effort and cadence before changing training zones.
Common misconception
No consumer sensor is perfectly accurate in every session, and an unusual reading is not automatically sensor error.