Smartwatch and fitness tracker: What they measure

In short: Smartwatches and fitness bands measure movement, heart rate and sleep with sensors on the wrist and connect to the smartphone.

In more detail: These wearable computers collect health and activity data and show notifications. They are practical, but their measurements are not medical diagnoses. Privacy and battery life matter too.

In Depth

Sensors

• Accelerometer and gyroscope: steps, movement, sleep phases.
• Optical pulse sensor (PPG): green LEDs shine into the skin, light reflection changes with the blood flow, from which the heart rate arises. It is sensitive to movement and skin tone.
• GPS for routes and speed.
• Barometer (altitude), compass.
• On some models: ECG, blood oxygen (SpO₂), skin temperature.

How accurate are the values?

Step counters and resting pulse measurement are usually usable, less so with intense intervals or wrist movement. Calorie burn and sleep phases are estimates. ECG functions of some watches can detect atrial fibrillation but are screening, not diagnosis. For complaints or abnormal values, clarification belongs with a doctor. Compare trends instead of single numbers.

Privacy

Health data belongs to the particularly sensitive data (Art. 9 GDPR). Check where it is stored (manufacturer’s cloud, on the phone), whether it is shared or used for advertising, and which permissions the app asks for. Location data of sports routes can reveal home and habits (avoid public profiles). With export and deletion functions data can be controlled.

Choosing and using

• Battery: fitness bands often last one to two weeks, smartwatches one to a few days.
• Compatibility: Apple Watch only with iPhone, Wear OS and many bands with Android and iOS.
• Water resistance, display, wearing comfort.
• Sensible as motivation and training overview; a watch replaces neither exercise nor doctor’s appointments. Reduce night mode and notifications so you are not constantly distracted.

See also: Battery and charging correctly, Data protection in everyday life, Connecting Bluetooth