Millions of people around the world wear fitness trackers daily to record their physiological conditions. These devices contain a variety of different sensors that allow the user to measure heart rate, sleeping patterns, steps taken, and more. The physics behind these sensors can be relatively simple. However, the most interesting component of fitness trackers is not what is inside of them, but their external capabilities and applications to the real world. Though some inconsistencies and imperfections do exist, the future of the fitness tracker is rather bright. Amongst the endless possibilities for fitness trackers is the potential for mass data collection and surveying. The power of the fitness tracker does not have to be limited to individual use. Instead, it can extend its capabilities on a wider scale.
Introduction: What are fitness trackers?
We all have those athletic friends who send us pictures of themselves doing a new workout, following the new health trends, eating super foods, or drinking juice cleansers. Some of us may have even fallen jealous of these friends, wishing we had that much discipline. If their lifestyles weren’t enough to kick you into high-fitness gear, maybe their subtle invites to compare health statistics via their fitness tracker’s app could get your feet going. No matter what pushed your button, we can all admit that society as a whole is pushing each of us to eat healthier, exercise more, and sleep better. And the technological wave of fitness trackers – a $1.15-billion industry – is a major player in this game for a healthier world!
But what exactly are “fitness trackers”? Are they the same as a “wearable healthcare device”? Is it a little creepy that they track our every move? Technically, fitness trackers like FitBit, Garmin, JawBone, and many others are wearable healthcare devices. This is because they are autonomous, noninvasive, and able to perform certain medical functions for an extended period of time. Specifically, these fitness trackers can be categorized as activity monitoring devices since they track and record physiological data such as the number of steps taken, heart rate, and calorie expenditure, daily. Anybody can use them, and a variety of people do for different reasons. For example, an individual can use a fitness tracker to help self-monitor their activities. Many companies have an innovative app that consumers can use in conjunction with their fitness trackers in which they can set fitness goals, compare their achievements with competitive friends, record the number of glasses of water they drank during the day, and more. Others may use their trackers for research or preventative care, as heart rate monitors, or to record sleep activity. From an inanimate motivation booster to a personal medical assistant, these fitness trackers are an attractive product for anyone concerned with their health.
Just how do they work?
Have you ever wondered how these fitness trackers can track so many physiological measures at the same time? It’s a relatively simple science that has been around since 1883. All fitness trackers contain a variety of different sensors, and each is responsible for recording specific measures. Sensors have been around for more than one hundred years; they are in our phones, computers, TVs, and thermometers. But not many people know the details of how they work, since there are myriad types of sensors, and they all look different to the human eye. However, at its core, a sensor can be defined as a device that responds to an input from the environment and measures this physical quality. This input could be heat, temperature, pressure, light, motion, moisture, etc. Sensors then convert the measurement into a signal, and an electronic device (like your cell phone) will receive them and record the measurements.3
The basic sensor has three different terminals: Vcc, GND, and output (Figure 1). First, Vcc is the terminal responsible turning on the sensor. The GND terminal provides a fixed negative reference that is constant throughout the whole system. Any physical parameter, like light, enters the sensor, and leaves via the output terminal. Many sensors can have more than one output terminal.
So how many sensors are in fitness trackers? The answer: anywhere from 5-10 different types of sensors. There are accelerometers, Global Positioning Systems (GPS chips), optical heart rate monitors, galvanic skin response sensors, thermometers, ambient light sensors, UV sensors, and even more. Each of these sensors has a specific function for the fitness tracker. GPS receivers pinpoint your location on Earth by obtaining high-frequency, low-power radio signals from one of the 29 total satellites that are orbiting the earth. The time it takes the signal to reach the receiver in your fitness tracker is translated into your distance from the satellite, which is finally translated into precise coordinates from more than one orbiting satellite. Optical heart rate monitors measure your heart rate using light. The fitness tracker shines LED light through your skin, and an optical sensor examines the light that bounces back. Because blood absorbs more light than air, the tracker can interpret the fluctuations in light levels and translate it into a heart rate via a process called photo-plethysmography.
The most common sensor in fitness trackers is the accelerometer, which is responsible for counting your steps and measuring user orientation (whether the device is horizontal or vertical). Even simple pedometers have accelerometers! The physics behind this sensor is actually relatively simple. An accelerometer measures acceleration, which can be measured by the two equations in Fig 2. The first equation shows the definition of acceleration as the change of velocity (the rate of change of position) over the change in time, and the second equation is Newton’s law of motion.
Newton’s law of motion states that force is equal to mass multiplied by acceleration; therefore, it is possible to determine acceleration by measuring the force and dividing that by the mass. These sensors convert the motion (or acceleration) that they measure into an electrical signal, which is what your fitness tracker receives and interprets or stores. Basic accelerometers follow the format shown in Fig 3. Here, the piezoelectric material could be quartz or ceramic crystals, and is responsible for generating a piezoelectric effect that is then converted into an electrical output. The applied acceleration causes the mass to stress the piezoelectric material, which then generates an electrostatic charge output or signal.6
It is amazing to think about the sensors that work together to control a multi-faceted fitness tracker. Although a lot has stayed the same since the first thermostat sensor in 1883, there have been plenty of improvements as well. One of the biggest advances we have made is shrinking the sensors down in size, so that ten different ones can fit in an object the size of your wrist!
How beneficial are fitness trackers?
People spend a lot of money on fitness trackers, so it’s important to know how effective they can be. As previously mentioned, fitness trackers can measure so many different physiological conditions. These devices can replace costly doctor’s visits and painful lab-based tests. According to Marina Lu, Senior Analyst at ABI Research, non-invasive sweat sensors can actually make the same measurements that you get at a doctor’s office, but for less manual intervention and cheaper. On top of that, these sensors allow for continuous physical monitoring – not just the one stressed-out hour you may experience at the doctor’s. 
One recent study on children with cancer brought new light to the potential of fitness trackers. Many cancer patients have identified fatigue as one of the most stressful and prevalent treatment-related symptoms. This fatigue could be in the form of physical, mental, or emotional distress, characterized by a lack of energy. Children with cancer are no exception. Therefore, this study tried to determine a way to combat the chronic fatigue experienced during a period of chemotherapy. These children were given a fitness tracker and were told to follow the coaching provided by the fitness tracker’s app for 2 weeks. During the study, the children received chemotherapeutic steroid pulses. At the beginning of the study, the number of steps per day was measured (pre-pulse) along with fatigue levels. This procedure was then repeated after the chemotherapy treatment (post-pulse).
The results were impactful! Researchers found a negative relationship between number of steps per day and level of fatigue. This meant that a greater number of steps taken per day correlated with lower levels of fatigue. The bottom line: fitness trackers served as a motivation for children with cancer to get moving. As a result, these children were a lot happier and not as energy-drained. This highlights another powerful use for fitness trackers for everyone – these trackers can motivate us when we can’t motivate ourselves!
But they aren’t perfect… right?
With all of this positivity, one begins to question the validity of these claims. Are fitness trackers as perfect as they seem? Not necessarily. First of all, they aren’t always accurate. In one study, researchers wanted to determine the accuracy of fitness trackers in measuring heart rate (HR) and energy expenditure (EE). EE is defined as the amount of energy (or calories) that a person needs to breathe, move blood, digest food, physically move, etc. They had participants wear the devices while doing different types of gym activities: walking, running, cycling, and sitting. The results were clear; heart rate measurements were accurate while energy expenditure measurements were not. HR measurements were within an acceptable error range of 5%, but EE measurements had consistent error rates of about 20%. The highest error recorded for EE estimates was 43% different from the reference standard. This inaccuracy is important to take into consideration, especially if you want to use a fitness tracker as part of a health improvement program.
This isn’t the only evidence of inaccuracy regarding fitness trackers. One of the most exciting features of some fitness trackers is the ability to record sleep patterns, which is particularly helpful for people with insomnia. However, research shows that the accelerometer-based sleep-wake trackers have high sensitivity in detecting sleep but low specificity in detecting wake. There is higher discrepancy during nights with more disrupted sleep, indicating that trackers may not be as accurate for insomnia patients.
What does the future hold?
there are many areas of necessary improvement for fitness trackers, especially
relating to inaccuracies. However, the potential future of fitness trackers is
enough. There is a wealth of knowledge hiding inside the millions of fitness
trackers and cell phone apps around the world that researchers and medical
practitioners would love to get their hands on. Clinical studies require
monitoring individuals in a laboratory where space is limited, but fitness
trackers could expand the target groups to whoever owns one. Doctors
could compile the data and health statistics from fitness trackers and analyze
it to help explore the relationship between health indicators (physical
activity, heart rate, etc.) with other critical health outcomes. The
possibilities of improving health for everyone are endless, and it all starts
with the fitness tracker.
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