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How Do Fitness Trackers Work? They're More Complex Than You May Think

This article forms part of the IFLScience exciting editorial calendar for 2023.

Russell is a Science Writer with IFLScience and has a PhD in the History of Science, Medicine and Technology

Dr. Russell Moul

Russell is a Science Writer with IFLScience and has a PhD in the History of Science, Medicine and Technology.

Science Writer

fitness tracker on a hand with graphics and data coming out of it

Depending on the device, fitness trackers can provide all sorts of information about acceleration, movement, altitude, calories burnt, heart rate, and more. Image credit: ©IFLScience/ James Rodrigues.

From measuring the number of steps you take in a day to analyzing your heartbeat and sleep patterns, fitness trackers have become increasingly popular in recent years. But what are they and why are they so useful?

Physical activity has significant benefits for our bodies and our mental health. Aside from helping prevent and manage a range of non-communicable diseases, such as cardiovascular disease, diabetes, and some cancers, it can focus our minds and reduce the symptoms of depression and anxiety. 


According to the World Health Organization, adults should get as much as 150 to 300 minutes of moderate physical activity per week. This may feel like a daunting requirement, but for people who have less experience with fitness programs or exercise regimes, buying a fitness tracker could be an excellent step in the right direction.

While we might think of these devices as recent inventions, they actually have a far older heritage. In 1780, a Swiss-born watchmaker called Abraham-Louis Perrelet invented what is believed to be the first pedometer. Perrelet’s early device used a spring suspended lever arm to count steps and distance while walking. Later, in 1820, a different Abraham-Louis (with the last name Breguet), another eminent watchmaker, designed a pedometer and stopwatch to measure the distance and pace of Tsar Alexandre I of Russia’s army. 

These early step counters were state-of-the-art for their time, but it would be 145 years before we saw a fitness tracker that we would recognize today. In 1965, a team of Japanese researchers at the Kyushu University of Health and Welfare, led by Dr Yoshiro Hatano, created the manpo-kei, which translates to “10,000 steps meter”, as part of his efforts to combat obesity in Japan. Hatano’s research suggested that walking 10,000 steps a day was the amount needed to sufficiently decrease the chances of coronary heart disease. This number has remained a popular target for many modern fitness trackers, though some have suggested that 15,000 steps would be of more benefit, while others argue for fewer

Since the manpo-kei, fitness trackers have evolved. They can now be paired with a mobile device that gives you feedback and prompts behavior changes. They are useful for helping people monitor progress towards daily or longer-term goals (such as getting your 10,000 steps in) and can be compared with friends and wider communities of users – which improves consistency and personal accountability. 


Modern fitness trackers come with a wide range of functions to improve their utility and value. Common features include measuring step count, calories burnt, and heart rate. 

Movement and steps

Every fitness tracker, regardless of the make, will contain an accelerometer, an electro-mechanical device that measures changes in speed and direction. In most cases, they are three-axis accelerometers that track movement in three-dimensional space (along an X, Y, and Z axis). More advanced devices also include gyroscopes, which measure orientation and rotation as well, allowing them to track six degrees of motion. 

Accelerometers are responsible for recording your step count. The data they collect is stored on the tracker and then transferred to the software associated with the smartphone or laptop that it is synced with. The accumulation of this data allows the software to analyze your movements through a personalized algorithm (a veil of mystery surrounds these algorithms as different makes and models use different ones), which allows it to differentiate between movement recorded while walking, running, or standing still.  

Some fitness trackers also include altimeters, which measure altitude. Although it may not appear to be immediately useful for people who are not attempting to climb mountains, the altimeters are sensitive enough to identify changes in height caused by hills and even staircases. 

Burning calories and measuring heart rate

While setting up your fitness tracker, you will be asked to input personal information about your age, gender, height, and weight. This information will be used to estimate your personal basal metabolic rate (BMR), which is the number of calories your body burns as you do basic, life-sustaining activities. 

Once the tracker has estimated your BMR, it will use the data collected by the accelerometer to calculate the number of calories burned during the day. The accuracy of this reading will vary between devices and the information that you manually input. Some trackers even allow you to input daily updates about your diet so that they can provide greater details about your personal achievements.

You may have also noticed a small flashing LED on the back of your wearable fitness tracker, be it a smartwatch or other devices. This is an optical sensor that uses the light projected on your skin to measure your pulse which, in turn, can be used to record your heart rate through a method called photoplethysmography (PPG). Essentially, the blood absorbs green light, and the higher the blood volume level, such as when you exercise, the more light is absorbed from the LED and the less is sent back to the photodetector on the device. During a workout, the LED will flash hundreds of times per second to measure your heart rate.

Tracking your sleep

Another common feature of many wearable fitness trackers is the ability to measure your sleep. This is achieved by the accelerometer and gyroscope, which not only track your movement but also detect periods of inactivity. If you’re laying still for a prolonged period of time, the tracker will assume you’re asleep. This process is called actigraphy, and is a non-invasive way to measure rest/activity cycles. Some devices combine actigraphy and PPG to get a more accurate idea of your sleep patterns. 


While some fitness trackers can measure periods of inactivity, they are not so good at assessing the quality of sleep or detecting the different stages of sleep. The reason it is difficult to accurately assess the different stages of sleep is that they are controlled by the brain and do not necessarily translate into more or less physical movement that can be measured by the everyday fitness tracker. To be able to measure this more accurately, the tracker would need to incorporate ways to detect brainwaves or rapid eye movement (REM). 

How accurate are fitness trackers?

Ultimately, the accuracy of any fitness tracker will depend on the make and model, as well as the data it is able to collect. They are very good at tracking steps and movement, though it should be stated that the most accurate trackers are those that are located on the hip. In fact, mobile phones have been found to be more accurate than wrist-worn trackers as they don’t confuse random hand movements as a sign of walking. 

Measuring the number of calories burned in a day is also trickier to calculate accurately as our metabolism varies from person to person. A tracker will need to have significantly detailed and accurate information about you, your activities, and your diet to be able to give precise results. The ability to measure your heart rate can certainly help with this, while some devices also use skin perspiration to measure your progress and the calories you’ve burnt. 

As mentioned above, sleep is more difficult to measure accurately but your fitness tracker can at least give you a broad understanding of how much you move in your sleep and at what times. Unfortunately, the current technology is not yet able to provide more meaningful insights than that. 


Nevertheless, fitness trackers are a great way to start engaging in a healthier lifestyle. For people who are just setting out on the fitness road or are looking to simply monitor their progress towards specific goals, these portable devices are extremely useful. More seasoned athletes may need to look elsewhere for specialist performance information.

There are so many different types of fitness trackers available now that it is possible to find a device that is more suitable for your needs. And besides, anything that helps us set an exercise routine and to see progress, such as marching towards our 10,000 steps a day, can only be of benefit. 

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.


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