Types of sensors in wearable fitness trackers

By Cashmere Lashkari, B.Sc.Reviewed by Afsaneh Khetrapal, BSc

A wearable fitness tracker makes use of special gizmos to keep track of various parameters of the individual wearing the tracker.

These sensors measure a number of parameters such as acceleration, frequency, duration, intensity and patterns in the movements that the individual makes. Then it is all put through an algorithm with some guess work thrown in to make sense of all the readings the data provides.

What types of sensors can be found in fitness trackers?

Many different types of sensors can be added to a fitness tracker - there is no limit to the variety of sensors available to designers. Each manufacturer will adapt the design of an existing sensor to build into their fitness tracker. Whilst some may be fully loaded with many kinds of sensors, others may have a few basic ones on the scaled down models.

3 axis accelerometer

To track movement in every direction. An accelerometer sensor takes inertial measurements of velocity and position. Usually on three axes, it can sense inclination, tilt, and orientation of the body as well. Naturally this is very important for any fitness tracker as most steps taken by the individual will be actually recorded by this sensor.

Gyroscope

To measure orientation and rotation. A gyroscope can be used for navigation and measurement of angular velocity. A 3 axis gyroscope can be paired up with a 3 axis accelerometer to provide a ‘6 degree of freedom’ motion tracking system. Most reliable fitness trackers use this combination to get a better grip on the 3D workout motions that an individual may perform.

Altimeter

Altitude measurement for mountain climbing. Pressure altimeters are actually an advanced version of the aneroid barometer. Where the barometer shows the measurement of pressure, the altimeter shows the height as there is an exact correlation between them. Most fitness trackers may not offer an altimeter unless the individual opts for one specifically designed for mountain climbing.

Temperature sensor

To keep track of temperature changes. While it is not exactly like a thermometer, the concept of the sensor is similar, i.e. ir provides a reading of the body temperature. The more the body heats up, the tougher the workout seems to the fitness tracker.

Bioimpedance sensor

This checks the resistance of the skin to a small electric current. The galvanic skin response is a method of measuring the electrical resistance of the skin and interpreting it as a certain activity of the body. It is also known as electrodermal response or psycho galvanic reflex. That does not mean that the fitness tracker will be giving shocks, but some of them may use this sensor to collect data for heart rate.

Optical sensor

This uses light on the skin to measure the pulse. The sensors can be used to measure the rate at which blood is pumped through the capillaries, thereby measuring the heart rate. They are preferred to galvanic skin response by most manufacturers of popular fitness trackers - these are the little lights that sit flush with the skin to measure the pulse.

The types of sensors mentioned here are indicative of typical fitness tracker sensors. This is not a complete list and other types of sensors may be used in wearable fitness trackers. Many manufacturers protect the information about the sensors that they use, along with the algorithms they program to gain an advantage over their competitors.

Sensors and errors

Each sensor has a slightly different threshold level and this makes it difficult to combine two fitness trackers to obtain an agreed upon activity level of an individual.

If an individual wears two different fitness trackers on the same wrist and does a workout, it is likely that the two trackers will yield varying measurements. This does not mean that one is at fault; it simply means that their sensors have a slightly different reading of the movements that were made during the workout.

For this reason it is important to remember that while fitness trackers worn by an individual may be a good reference point for the physical activities performed, they must be interpreted with care.

It is normal for the values of calories burnt, heart rate, number of steps and other parameters to differ between trackers. As long as the individual continues to use the same fitness tracker, there is some degree of continuity and the data will make sense over a period of time.

The user must remember that a fitness tracker is just another electrical gadget which can malfunction. It may be a good device to track basic parameters but the results should not be considered the absolute last word for physical fitness.

References

1. http://www.wareable.com/fitness-trackers/how-your-fitness-tracker-works-1449
2. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4683756/
3. http://www.explainthatstuff.com/how-altimeters-work.html

Further Reading

• All Exercise Content
• Where did 10,000 steps a day come from?
• Is it Safe to Exercise During Pregnancy?
• Why You Should Exercise in the Morning
• Wearable Fitness Trackers