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How do smartwatches measure movements and body functions?

Smart watches have become real all-rounders in recent years. But how exactly do the wearables actually measure body data?

Smartwatches such as the Apple Watch not only show the time and measure the pulse. Modern smart watches can also determine blood oxygen levels, track and evaluate sleep, have GPS on board, can be paired with a smartphone, and much more. But how exactly does such a smartwatch work and how does it measure all these values from its position on the wrist? atechbook clarifies.

Smartwatch function of optical pulse measurement

By now, almost all smartwatches have both position and acceleration sensors. These sensors can record relatively accurately how you move and at what speed. Based on the walking pattern, they can distinguish whether you are walking, running, sprinting or climbing stairs, for example. At night, the sensors track your sleep and can even tell approximately how lightly or soundly you sleep based on your movements and pulse.

Many smart watches also have an integrated heart rate monitor. This is an optical sensor on the underside of the watch case. Green LEDs illuminate the veins and blood vessels on the wrist. When the heart beats accordingly, more blood flows, which absorbs the green light. Between beats, less blood flows and more light is reflected back to the sensor. By continuously measuring heart rate, for example, a smartwatch can also detect irregularities that indicate heart problems and warn of them.

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ECG and oxygen saturation

The measurement of oxygen saturation in the blood (SpO2) works in a very similar way. Oxygen-rich blood is bright red, while oxygen-poor blood is dark red. Instead of green light, infrared light is then used in this case.

A few smartwatches, including the Apple Watch, also have an ECG sensor. The necessary electrical heart sensor is installed in the rotating crown and the glass back of the smartwatch. The watch can thus detect sinus rhythm or atrial fibrillation. The waveforms are stored and can thus be shared with the doctor if necessary.

Each smartwatch is usually accompanied by a smartphone app that supports the functions. All the data from the smart watch arrives there and is processed accordingly. However, the wearable can perform an initial evaluation itself based on the measured values. Coupled with other functions such as GPS, very detailed analyses and evaluations are now possible.


  • Smartwatches