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What’s inside a fitness tracker, anyway?

Just a couple years ago, fitness bracelets were basically glorified step counters worn on your wrist. Now, they’re doing everything from measuring your heart rate on a run to warning you to get out of the sun. And they’re everywhere. Every company from Fitbit to Jawbone and Microsoft makes a pretty advanced piece of wrist-worn technology now — all with the promise of better health.

But what’s in these things, anyway?

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Even though fitness bands are fairly simple compared to full-blown smartwatches, a growing number of sensors crammed inside has turned them into complex labs on your wrist. For example, Microsoft’s Band advertises 10 different sensors in the tiny package. With expectations higher than ever, bands are getting very technical and complicated to compete.

These are the sensors inside, making it happen.


The most common and basic tracker included is the accelerometer. It can be used for multiple things, but is typically put to work counting steps. By measuring orientation and acceleration force, they can determine whether the device is horizontal or vertical, and whether it’s moving or not.

Not all accelerometers are created equal. You’ll find both digital and analog ones, different sensitivities, and different numbers of axis. The very basic ones will only have two axis, while three-axis sensors can measure their position in three dimensions. At this point, most fitness trackers use fairly advanced accelerometers for increased accuracy.


GPS is decades-old technology, but its appearance in fitness bands is relatively new because the chips are becoming more efficient — nobody wants a huge band on their wrist to accommodate a giant battery.

GPS is still fairly power hungry compared to other sensors.

The global positioning system a comprises a network of 29 total satellites orbiting the Earth — at any location, a person should be in range of four satellites needed to pinpoint an exact location.

The GPS receiver receives a high-frequency, low-power radio signal from the satellites. The time it takes for a signal to reach your wrist can be translated into your distance from the satellite, which can be translated into precise coordinates with data from enough satellites. GPS chips continue to get better at handling battery usage, but GPS is still fairly power hungry compared to other sensors.

Unlike simple step counting, GPS allows runners, walkers and cyclists to easily map their exercise and analyze the terrain where they were excising.

Optical heart-rate monitors

Unlike the EKG a doctor might use to measure your heart rate, an optical heart-rate monitor measures your heart rate using light. An LED shines through the skin, and an optical sensor examines the light that bounces back. Since blood absorbs more light, fluctuations in light level can be translated into heart rate – a process called photoplethysmography.

Currently, using an optical heart rate monitor on the wrist just isn’t as accurate as using one on the fingertip or on the chest. The chest-worn models more closely mimic an EKG machine.

Basis B1
Basis B1 teardown (Image: iFixit)

There are also a lot of nuances to photoplethysmography, so there will be more variation from brand to brand. For instance, each band has to compensate for skin tone. Despite some lofty by manufacturers, the accuracy of results can vary significantly. These aren’t for professional athletes, they’re more used for overall heart-rate guidance – especially the wrist-worn types.

Galvanic skin response sensor

Galvanic skin response sensors measure electrical connectivity of the skin. When internal or external forces cause arousal — of any kind — the skin becomes a better conductor of electricity. Essentially, when you start to sweat, either from exercise or something else, the band will be able to monitor that.

An LED shines through the skin, and an optical sensor examines the light that bounces back.

Detecting when someone is sweating gives the software more information about what a user is doing, which allows for better health tracking. Being able to correlate the level of activity with a different source than just gravity from the accelerometer, allows these programs to take on a more trainer-like role — recommending specific exercises and levels of exertion.


Even a basic thermometer can provide valuable information by way of your skin temperature. Rising skin temperature can indicate to a fitness band that you’re exerting yourself, or if your heart rate isn’t rising accordingly, that you might be getting sick.

Ambient light sensors

Ambient light sensors are all around us. For instance, one tells your phone to dim its screen at night and brighten it in the sun. A fitness tracker uses it for the same purpose, and for detecting the time of day.

Samsung Gear Fit
Samsung Gear Fit teardown (Image: iFixit)

The way an ambient light sensor works, the light spectrum is narrowed so that only forms of light visible to the human eye are detected. That light is translated into a digital signal and fed to the processor inside the band.

UV sensors

But what about other forms of light? Instead of telling your fitness band how bright it is around you, UV sensors tell it when you may be absorbing harmful UV radiation – usually from the sun. Software compares this data to the values recognized by scientists to be harmful, and warms you to get out of the sun if you’re likely to burn.

Bioimpedance sensors

Jawbone’s new UP3 wrist band uses a single bioimpedence sensor to cover three bases: heart rate, respiration rate, and galvanic skin response. According to the company’s own  blog post explaining the technology, “The sensor measures very tiny impedance changes within your body. For heart rate, we are measuring the impedance changes created by the volume of blood that is flowing in the Ulnar and Radial arteries.”

The same sensor, worn around the wrist, will also be able to tell respiration and hydration by looking at metrics like oxygen in the blood. It does this by using four electrodes that drive a tiny bit of electrical energy to each other, and then measuring the results.


These sensors may provide a fitness band with reams of data about your heart-rate, body temperature and even elevation, but it’s not worth much without software to translate it into useful advice. From anticipating illness to spurring you on to more exercise tomorrow, it’s all of these sensors working together that truly provide a clear picture of your health today, and what you can do to improve it tomorrow.

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