Smart watches have accelerometers that measure the direction of your movement and allow the software to figure out if you’re walking or running. They have GPS to work out how far you travel, which helps establish your stride length. They have altimeters that figure out if you’re climbing up or down.
Some smart watches also measure your pulse or detect irregular heartbeats. Your heart causes your blood vessels to dilate and contract every time it pushes blood. Using a process called photoplethysmography, green LEDs on the watch shine light through your skin and measure the reflection from the changing amounts of red blood cells within your wrist.
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Asked by: Tim Manning, via email
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