Tech Explained: Fitness Trackers

Wearable fitness trackers are continuously enhanced thanks to technological advances in material design and sensors. It's no longer enough to measure steps and active minutes. Find out how fitness trackers have evolved and how they're now been used in our latest Tech Explained.

Just a few years ago, fitness trackers were not much more than digital step counters. As fitness trackers have developed, manufacturers have packed more and more sensors into the devices. Today, they do everything from measuring heart rate to advising when you have had enough sun. So, how do they work, and what can we expect from the future of fitness trackers?

Trackers today still use accelerometers to measure orientation and acceleration force. This enables trackers to determine the position of a user and the amount of steps taken. An altimeter measures altitude, which the software can translate into the height of mountains or the number of stairs climbed. Most trackers also contain a GPS receiver. The GPS allows users to map their exercise and analyse the terrain where they were excising.

Many fitness trackers also contain a host of sensors to measure physical responses. One of these is an optical heart rate monitor. This uses an LED light to measure heart rate. The light shines through the skin onto the blood, which allows the sensor to determine how fast the blood is being pumped, and thus the users’ heart rate.

Sensors can also measure the amount of sweat given off by the user. This is measured by galvanic skin response – the electrical connectivity of the skin. When the body sweats the skin becomes a better conductor of electricity. Moreover the level of conductance can help determine the level of exertion. Software within the fitness tracker can then compare what the user is doing (e.g. walking or running) with their heart rate and sweat response to recommend specific exercises that will lead to more or less exertion. Data on sweat is also important in determining how many calories are being expended.

Ambient light sensors, such as the ones used to automatically control the brightness of phone screens, are used in fitness trackers to detect the time of day. These combined with UV sensors can determine how long the user has been in the sun for.

Some more recent trackers use sensors that measure very tiny changes in impedance (electrical resistance) within the body. The sensor uses electrodes that send electrical impulses to each other, and measures how quickly the impulses arrive. Changes in impedance in the blood are analysed by the trackers’ software to determine respiration rate, heart rate, and levels of hydration.

Some trackers can measure sleep patterns using a process called actigraphy. The tracker monitors wrist movements, which are analysed by an algorithm to determine how much time a person spends in deep sleep. This method is not as accurate as polysomnography, which measures brain activity, but it can be a useful guide.

While fitness trackers can provide users with a huge amount of data, they vary greatly in reliability. For some, a bumpy car ride can be enough to throw off the accuracy; optical heart monitors worn on the wrist can have difficulty in compensating for skin tone. In addition, fitness trackers are only ever as good as the algorithms used to analyse the data. These can vary greatly from company to company. The best manufacturers are constantly updating their algorithms. The most recent trackers use algorithms to automatically detect the type of exercise the user is engaged in, such as yoga or boxing, and set fitness goals accordingly.

The business of fitness trackers is also changing. According to analysis by CCS Insight, the wearables market will be worth 29 billion USD by 2022. However, CCS also reports that fitness trackers are losing ground to smartwatches, and estimate that around 140 million smartwatches will be sold in 2022. Many of these watches will be able to track the same data as today’s fitness trackers, as well as features such as pairing with connected gym equipment or streaming music.

At Springwise, we have been following closely this growth in the number of innovations involving smart trackers. These include a fitness tracking ring that can connect to Alexa and a wearable that automatically measures calorie intake based on cell glucose levels. Whether smart watches or trackers, it may be that the future of fitness is all in the wrist.