Both wrist and chest strap heart rate monitors use algorithms to convert what they read into estimated heart rate. Their methods and accuracy differ.
Chest Strap Monitors
Chest strap monitors are the most accurate. One study showed a chest strap as 99.6% accurate when tested against an ECG. They have reduced lag times, more data points, and are less vulnerable to ‘drop-outs’.
Chest strap monitors need a little care to perform at their best. You need a good moist contact between pads and skin to pick up electrical impulses. Moistening the pad before putting it on improves accuracy.
Exposure to sweat can cause them to corrode, which means interference with the electrodes and inaccurate readings. Rinsing the strap after each use will extend its life and improve accuracy.
Disconnecting the sensor from the strap when not in use will extend battery life.
Smartwatches with inbuilt monitors work by shining light into the skin and measuring the amount of light that is scattered by blood flow.
The heart rate sensors at the bottom of the dial press up against the skin and track heart rate. Because they're reading your blood flow further from the source (your heart), they are less accurate.
Accuracy can be further reduced by light leaking in and affecting the sensor, the movement of your arm, or the flexing of your wrist. They need to be worn tightly and above the knuckle on your wrist for best accuracy.
Skin tone, hair, and moles can affect readings. Types of LEDs and algorithms also can affect their accuracy. Error rates can vary from +/- 1% to as high as +/-13.5%.
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