Optical or chest strap heart rate monitor: which is more accurate?
Answer
They measure fundamentally different things. A chest strap picks up the electrical signal from your heart; a wrist wearable shines light through your skin and infers heart rate from blood flow. The electrical measurement is the more direct one, which shows up most clearly at the edges: during rapid changes in effort, and during sleep, where I compared a wrist optical sensor against a high resolution chest strap.