It sits quietly on your wrist: counting up your steps, tracking your sleep, monitoring your heart and calculating the difference between a light jog and a mad sprint. But how exactly does your fitness tracker come up with all the statistics that appear on the accompanying app?
Simply speaking, fitness trackers measure motion: most of today's wearables come with a 3-axis accelerometer to track movement in every direction, and some come with a gyroscope too to measure orientation and rotation.
The data collected is then converted into steps and activity and from their into calories and sleep quality, though there is some guesswork involved along the way. Then there's the altimeter that can measure your altitude, handy for working out the heigh of the mountains you've climbed or the number of flights of stairs you've managed to get up and down during the day. All of this information is collected and crunched to create an overall reading, and the more sensors your tracker has, the more accurate its data.
These sensors measure the acceleration, frequency, duration, intensity and patterns of your movement—taken together that's a good bunch of data and it can help a tracker work out if you're walking down the road or just waving at someone you know. Have a dive into the specs list of a particular tracker to see what sensors are included to collect data about you.
While the future of Jawbone is up in the air, its seasoned UP3 is still one of the most sensor-packed trackers, squeezing in temperature sensors and a bioimpedance sensor alongside the familiar accelerometer we've already mentioned. Bioimpedance sensors check the resistance of your skin to a tiny electric current, and the four electrodes on the inside of the UP3 fitness tracker are clearly visible.
Other wearables, such as the Fitbit Charge 2, use optical sensors to shine a light on your skin and measure your pulse through it: the light illuminates your capillaries, then a sensor measures the rate at which your blood is being pumped (and thus your heart rate). These optical sensors are less effective than bioimpedance as a gauge of your overall health but can be more useful if you want to check your heart rate as you exercise or work out.
It's a similar story with sleep tracking: using a process called actigraphy, your tracker translates wrist movements into sleep patterns as best it can, and as with steps there's some guesswork and estimating involved. It's a useful guide, but it's not as accurate as polysomnography - this is what the experts use to measure sleep in a lab, and it monitors brain activity rather than how much you're tossing and turning.
When it comes to calories, an app needs more than just a step count to make the calculation: that's why you'll often be asked for your age, gender, height and weight too. The algorithms used by each manufacturer aren't made publicly available, as each one likes to keep a lid on the 'secret sauce' they use to get the best and most accurate results, but the more sensors and data points used the more accurate the results are likely to be.
The below mentioned sensors helps to measure the body fitness :
Accelerometers
The most common and basic tracker included is the accelerometer. It can be used for multiple things, but is typically put to work counting steps. By measuring orientation and acceleration force, they can determine whether the device is horizontal or vertical, and whether it’s moving or not.
Not all accelerometers are created equal. You’ll find both digital and analog ones, different sensitivities, and different numbers of axis. The very basic ones will only have two axis, while three-axis sensors can measure their position in three dimensions. At this point, most fitness trackers use fairly advanced accelerometers for increased accuracy.
GPS
GPS is decades-old technology, but its appearance in fitness bands is relatively new because the chips are becoming more efficient — nobody wants a huge band on their wrist to accommodate a giant battery.The global positioning system a comprises a network of 29 total satellites orbiting the Earth — at any location, a person should be in range of four satellites needed to pinpoint an exact location.The GPS receiver receives a high-frequency, low-power radio signal from the satellites. The time it takes for a signal to reach your wrist can be translated into your distance from the satellite, which can be translated into precise coordinates with data from enough satellites. GPS chips continue to get better at handling battery usage, but GPS is still fairly power hungry compared to other sensors.Unlike simple step counting, GPS allows runners, walkers and cyclists to easily map their exercise and analyze the terrain where they were excising
Optical heart-rate monitors
Unlike the EKG a doctor might use to measure your heart rate, an optical heart-rate monitor measures your heart rate using light. An LED shines through the skin, and an optical sensor examines the light that bounces back. Since blood absorbs more light, fluctuations in light level can be translated into heart rate – a process called photoplethysmography.
Currently, using an optical heart rate monitor on the wrist just isn’t as accurate as using one on the fingertip or on the chest. The chest-worn models more closely mimic an EKG machine.
There are also a lot of nuances to photoplethysmography, so there will be more variation from brand to brand. For instance, each band has to compensate for skin tone. Despite some lofty by manufacturers, the accuracy of results can vary significantly. These aren’t for professional athletes, they’re more used for overall heart-rate guidance – especially the wrist-worn types.
Galvanic skin response sensor
Galvanic skin response sensors measure electrical connectivity of the skin. When internal or external forces cause arousal — of any kind — the skin becomes a better conductor of electricity. Essentially, when you start to sweat, either from exercise or something else, the band will be able to monitor that.
Detecting when someone is sweating gives the software more information about what a user is doing, which allows for better health tracking. Being able to correlate the level of activity with a different source than just gravity from the accelerometer, allows these programs to take on a more trainer-like role — recommending specific exercises and levels of exertion.
Thermometers
Even a basic thermometer can provide valuable information by way of your skin temperature. Rising skin temperature can indicate to a fitness band that you’re exerting yourself, or if your heart rate isn’t rising accordingly, that you might be getting sick.
Ambient light sensors
Ambient light sensors are all around us. For instance, one tells your phone to dim its screen at night and brighten it in the sun. A fitness tracker uses it for the same purpose, and for detecting the time of day.
The way an ambient light sensor works, the light spectrum is narrowed so that only forms of light visible to the human eye are detected. That light is translated into a digital signal and fed to the processor inside the band.
UV sensors
But what about other forms of light? Instead of telling your fitness band how bright it is around you, UV sensors tell it when you may be absorbing harmful UV radiation – usually from the sun. Software compares this data to the values recognized by scientists to be harmful, and warms you to get out of the sun if you’re likely to burn.
Bioimpedance sensors
Jawbone’s new UP3 wrist band uses a single bioimpedence sensor to cover three bases: heart rate, respiration rate, and galvanic skin response. According to the company’s own blog post explaining the technology, “The sensor measures very tiny impedance changes within your body. For heart rate, we are measuring the impedance changes created by the volume of blood that is flowing in the Ulnar and Radial arteries.”
The same sensor, worn around the wrist, will also be able to tell respiration and hydration by looking at metrics like oxygen in the blood. It does this by using four electrodes that drive a tiny bit of electrical energy to each other, and then measuring the results.
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