Unknown message slot CUSTOM_MSG_HEADER_SEARCH_FIND
Unknown message slot CUSTOM_MSG_HEADER_SEARCH_TIPS
When used properly* during athletic activity the vast majority of heart rate measurements are within a 5% error of a chest strap electrocardiogram.
We use a heart rate algorithm to measure heart rate. This algorithm allows us to combine motion information and sensor readings to better estimate your heart rate whilst you are doing activities or using continuous 24/7 heart rate monitoring.
Spark, Runner and Adventurer watches use technology from LifeQ to measure heart rate - LifeQ has many white papers explaining how the technology works, and its limitations.