A fitness tracker measures your heart rate using optical sensors or through electrical signals.
Optical sensors: Many fitness trackers use optical sensors, typically green LEDs and a photodiode, to detect your heart rate. These sensors shine light onto your skin and measure the amount of light that is reflected back. As blood flows through your arteries, the volume of blood in your wrist changes, and this affects the amount of light that is absorbed and reflected. By analyzing these changes, the fitness tracker can estimate your heart rate.
Electrical signals: Some fitness trackers, particularly those worn on the chest, use electrical signals to measure heart rate. These trackers use electrodes that come into contact with your skin. The electrodes detect and analyze the electrical activity of your heart, called electrocardiogram (ECG) signals. The fitness tracker processes these signals to determine your heart rate.
It is important to note that while fitness trackers offer a convenient way to estimate heart rate, they may not be as accurate as medical-grade devices like electrocardiograms. Factors such as movement, sweat, and skin color can influence their accuracy. However, for most individuals, fitness trackers provide a reasonable estimation of heart rate during workouts and daily activities.