How do Fitness trackers work?

How do Fitness trackers work?

Pictured above is an activity tracker, sometimes known as a fitness tracker, which is different from a smartwatch, even though very often it is both. Our concentration is on the fitness front. How do Fitness trackers work? For the purpose of this editorial, we will adopt the term fitness tracker.

You might have seen these shiny devices around and probably wondered what they do, how they work and most importantly, how they can improve your training and exercise experience. The focus of this article is on how these devices collect data and interpret it. What that data means and how that can improve your training and exercise experience.

Fitness trackers are constantly evolving and new designs are introduced to the market literally every day. All these designs have common features and seek to make your training and physical activity sessions a pleasure while getting you the results you desire. Kind of like personal trainers, but not quite there.

A fitness tracker can help an individual to monitor their physical activities. This helps in setting fitness or physical activity goals such as meeting the minimum daily number of steps, regular exercise activity goals, burning a specific number of calories in the day, and even drinking a specific number of glasses of water. Unlike personal trainers, however, there is a lot of guesswork involved with fitness trackers but it is still close enough. The data is recorded all the time the fitness tracker is worn and powered up, which enables the tracker to trace if the individual is walking forward, running fast, or even standing still [1]. This data is then presented through a complementary app or software for interpretation.

Depending on the design and functionality, most Fitness trackers are capable of:

  • Monitoring your Heart rate;
  • Monitoring your Sleep;
  • Counting steps;
  • Measuring distance;
  • Measuring Altitude;
  • Counting calories;
  • Some also have a silent alarm.

When you set up a wearable fitness tracker you are required to input essential physiological details such as your weight, height, gender, age, fitness goals and sometimes even favorite sports. These data will help the tracker make better sense of the data that it collects for interpretation and presentation. Fitness trackers have a built-in 3-axis accelerometer that helps in detecting motion and direction [2]. Additionally, some wearables also have a gyroscope that not only tracks movements but also measures orientation and rotation [2].

There are a number of sensors which may be added to a tracker, but most commonly motion sensors are used and they determine body movements and direction.

How do fitness trackers monitor heart rate and sleep?

How do fitness trackers monitor heart rate and sleep

You might have noticed that most, if not all fitness trackers have optical sensors underneath. Some of these optical sensors reflect light. This light shines on your skin and is able to measure your pulse through it in a similar way that fitness trainers do. This light illuminates your capillaries, then a sensor measures the rate at which your blood is being pumped and through that, it can calculate your heart rate. This is the number of times your heart beats per minute.

Through this same process, sometimes with the addition of a process called “actigraphy” a fitness tracker is also able to monitor your sleep. Your heart rate is lower when you are at rest and is typically at its lowest when you are asleep. Through that, an activity tracker is able to distinguish sleep between [very low heart rate] and activity. This data, in addition to actigraphy – which is when your tracker translates wrist movements into sleep patterns – is then collected and translated into what your fitness tracker presents to you as sleep patterns.

You can read this article about the different physical assessment tests and what they mean for your health and fitness. We discussed how knowing your heart rate is used to determine and improve your fitness.

How do fitness Trackers Count steps and Measure distance?

How do fitness Count steps and Measure distance?

As previously stated. A wearable tracker continuously senses the movements of the body on a 3 axis accelerometer. Through these sensors the fitness tracker is able to tell, well really it kind of makes an educated guess that you are walking forward, running fast, or standing still. Fitness sensors normally record acceleration, frequency, duration, intensity, and motion patterns [2].

Since many fitness trackers are worn at the wrist. It is easy for them to distinguish between a light jog and a sprint. But even those not worn at the wrist do a specific enough job. Think about the number of times and the different ways any sensor could be stimulated when you run. Most companies are keeping the specifics of what sensors they use and how they are stimulated close to their chest because they want to stay ahead of the curve.

The distance is mostly measured through built-in location sensors. Some high-end fitness trackers use a combination of GPS, Bluetooth, and networks to determine your location. Others use some, but not all of the technology described above. Some require that you take your phone with you if you want to track distance and use the GPS location sensor on your phone and translate that to the associated software or app to present to you the distance. Some fitness trackers just flat out lie but don’t assume that if a wearable tracker does not have a built-in GPS then it is incapable of measuring distance.

At the end of your workout or walk the fitness tracker is able to tell you, for example, that you have walked or ran 3 km at an average speed of 6km/h and you took 2019 steps to get there. Genius!

How do fitness trackers Measure Altitude?

How do fitness trackers Measure Altitude?

Fitness trackers that can measure altitude are most likely doing that through a built-in altimeter.

An altimeter is an instrument that measures the height above the ground. It is mostly used in navigation and now we have it in fitness. Don’t you just love it when industries integrate with each other? This can come in handy for working out the height of the mountains you’ve climbed or the number of flights of stairs you’ve managed to get up and down during the day [2].

How do fitness trackers count calories?

How can fitness trackers count calories?

If you practice in fitness as I do. You may already know of the different techniques used to determine the number of calories burnt during exercise. A calorie is a unit of heat equal to the amount of heat required to raise the temperature of one kilogram of water by one degree at one-atmosphere pressure. Which probably sounds complicated so I won’t bother you with the details.

The following formula is used to calculate calories burnt during exercise:

For men

Calories Burned = [(Age x 0.2017) + (Weight x 0.09036) + (Heart Rate x 0.6309) — 55.0969] x Time / 4.184

For women

Calories Burned = [(Age x 0.074) — (Weight x 0.05741) + (Heart Rate x 0.4472) — 20.4022] x Time / 4.184.

Computer engineers input these formulas [the way that they do] into the Fitness trackers or the associated software and app [again in the way that they do] and the fitness trackers interpret these formulas as algorithms which are then fed to the associated app or software. The final data is then presented to us in a language you and I can understand and interpret.

In both equations, there are values you input into the fitness tracker when you set it up. Such as:

  • Age
  • Weight

These are the values the fitness tracker already has. So the only thing left to do is to get the rest of the values to complete the equation. These values are:

  • Heart Rate
  • Time

Then those values are substituted into the equation and you are presented with the final values. The number of calories you burned during the duration (time) of your workout. This is one way. There may be other ways.

How are fitness trackers fitted with a silent alarm?

This is more of a smartwatch feature. But like I said, very often it is both. The fitness tracker simply vibrates at the time of the alarm as opposed to making a sound. In my experience, this is more pleasant and convenient.

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Bringing it all together

Fitness trackers collect health-related information like your heart rate together with location, and movements. This enables them to count steps, measure distance, altitude, and count calories. This data is then stored to the fitness tracker to be interpreted and processed further.

Since these values are stored as an algorithm. The data collected has to be interpreted and presented in human language. This is done through the software or app associated with the fitness tracker which does a great job in presenting these data to the user.

Most apps and software will offer this information to you in illustrated colorful graphs and charts. You can use this data to gauge your fitness goals and training in addition to knowing what you have achieved so far. For example, knowing this information can reinforce your motivation by showing you exactly how much of your goal has been met. It will also figure out how much of each exercise needs to be done for each workout session [1].

Now that you know the technical functions of fitness trackers you should be comfortable getting one. It can be very useful but I doubt it will ever replace personal trainers. What do you think? Share your thoughts in the comment section below.

We publish articles similar to this one every week. If you want to stay updated with our latest post and articles please subscribe and follow us on social media. Don’t forget to share to spread the word. Until next time. Keep Moving.


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My interest in Sport and Exercise Dev SA
[1] D. Nield, “How it works: ​We explain how your fitness tracker measures your daily steps,” 31 December 2017. [Online]. Available:
[2] C. Lashkari, “How do wearable fitness trackers measure steps?,” [Online]. Available:
[3] W. Contributors, “Activity tracker,” 14 March 2019. [Online]. Available:
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Thando W. Dlamini, BA

Sport Development and Exercise Science Practitioner

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