How do Fitness Trackers work?

Many fitness trackers have several electronic sensors inside. These sensors are necessary to track the gadget’s environment and the wearer’s fitness. But how do fitness trackers work?

How do fitness trackers measure calories?

track calorie consumption

Fitbit Charge HR calculates calorie consumption

For a complete energy balance calculation you have to consider the calories consumed and compare it with the calories burned.

How do fitness trackers measure calories burned?

Measuring calories burned is a pretty difficult task. Most devices use acceleration sensors and additional heart rate sensors to calculate the used up calories. But because of the biological difference of every person and the individual movement’s the accuracy is most of the times not higher than 15%.

For further information you may take a look at an ACE-sponsored Research which shows how accurate the fitness trackers measured the amount of calories burned and how accurate the foot step tracking is. For the study they used some of the more popular devices like the Jawbone Up, Nike Fuelband and the Fitbit Ultra and compared those to scientific accurate measurement data.

How to measure the calories consumed?

Only Healbe FLOW Technology measures calorie intake through your skin—by reading the glucose in your cells.

Healbe
With a measured avereage error of 15% this is a pretty accurate method. And the user does not need to count calories anymore.

It is possible to measure the calorie intake automatically. Healbe developed a technology which tracks the amount of calories you intake indirectly.

However many fitness devices do not track the calorie intake at all. And there are some fitness devices where you have to enter your calorie intake manually. This means you have to weigh your food, scan bar codes or make a good guess. Typically your fitness application would then calculate a rough value for the calorie intake.

How do fitness trackers measure heart rate?

The heart rate sensing accuracy of current devices is pretty good, as the guys from wearbles state here. But what kind of heart rate sensors are typically used?

Basis Peak Heart Rate Sensor

Basis heart rate sensing technology

Optical heart rate tracking sensor

Smart Watches like the Basis Peak measures your heart rate with an optical heart rate sensor. A LED sends light directly into the skin and the heart rate sensor measures the amount of light which is reflected. The amount of reflected light depends on how fast your blood circulates so that it’s possible to calculate your current heart rate.

Bioimpedance sensors for heart rate tracking

Another technology that for example the Jawbone team is using is the bioimpedane sensor. Bioimpedance sensors are able to measure your resting heart rate. Changes in ones resting heart rate gives an indication of his or her cardiovascular health.

How do fitness trackers track sleep?

There are many devices that are able to detect deep sleep and REM phases. Basically there are two different sensors that are used for sleep tracking. The first one is of course the heart rate sensor which detects when you are at your resting heart rate. The second sensor for sleep tracking is the acceleration sensor. Whenever you move your arm and are tossing the device will detect that you are not in the deep sleep phase.

Image Sources

  • fitness-training-with-fitbit-charge-hr: Fitbit
  • basis-peak-back: Basis
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