The Method Behind the Magic: How Trackers Work

Your activity tracker knows a lot about you and your daily habits, but how much do you know about your activity tracker? We asked Robert Havasy and Tim Hale, our tech gurus at the Center for Connected Health, to explain the magic behind these amazing gizmos.

What is the underlying technology of an activity tracker—in other words, what makes a tracker tick?

Walk, run, jog, sit, sleep or dance all night and your tracker will dutifully record every move you make.  How does it do it? All trackers rely on the same basic core technology—an accelerometer, which measures acceleration, which is the intensity and direction of movement.

If we only moved in one direction, tracking activity would be pretty simple, but that’s not the case. We live in three-dimensions: We move forward and backward, left and right, and up-and down. (In tech talk, these three directions are referred to as X, Y and Z axis respectively.) Unlike old school pedometers that were designed to just track forward motion–that is, they counted steps and distance in a linear path—any modern tracker worth its salt captures data from all three axes simultaneously. (Wellocracy Tip:  If you are shopping for a tracker, make sure that you buy a three-axis accelerometer.)

Think Like a Tracker: This graph shows how your tracker “sees” different patterns in your movements as you go about your daily activities.  (Courtesy of Rob Havasy’s smartphone.)

Think Like a Tracker: This graph shows how your tracker “sees” different patterns in your movements as you go about your daily activities. (Courtesy of Rob Havasy’s smartphone.)

How can trackers tell what you’re doing? For example, that I’m standing and not walking, or riding a bicycle and not running?

Trackers differentiate between specific activities (like walking, running or cycling) by detecting patterns in the force in the three axes that they measure. For example, walking or running will produce a pulse in the Z-axis, or up and down motion. Software can interpret the frequency of pulses to distinguish between running (high frequency) and walking (low frequency). Similarly, cycling will produce a bigger bang on the X-axis (forward motion) and a mere whimper on the Z-axis (up and down). It’s not magic, it’s just math!

Despite similarities in technology, each tracker has its own way of crunching the data. That’s why some trackers may be better at counting calories, detecting the intensity of your gym work-out, or monitoring the pivots and dunks of basketball.  It also explains why our Wellocracy testers who wore more than one tracker at a time often found discrepancies in the data collected from one brand to another.

How does a tracker measure calories burned?

The human body is constantly expending calories just to stay alive. Breathing, thinking, sleeping and digesting your food all consume calories. Your basal metabolic rate (BMR) is the sum total of all the calories burned by your body just doing these fundamental tasks; it accounts for the vast majority of calories burned by your body.

Your tracker estimates your basal calories using a formula based on the Harris-Benedict equation, which includes your height, weight, gender and age. Your BMR is the number of calories you need to consume on a daily basis to maintain your weight. If you want to lose weight, you need to burn calories in excess of your BMR.  The calorie total that is recorded on your tracker is a calculation of the calories that you burn in addition to your BMR. In other words, it’s based on the tracker’s best calculation of how active you are beyond simply burning calories to run your body and perform the tasks of daily life.  

Some trackers calculate calorie burns for specific activities. For example, a tracker may apply a different formula to calculate calorie expenditure for cycling than it does for swimming or running. Other trackers attempt to more accurately measure calorie burn by incorporating additional measurements. For example, Body Media uses skin temperature to calibrate intensity of activity, the assumption being the warmer the skin, the more intense the workout and the greater number of calories burned. Fitbit uses an altimeter (barometric measure sensor)  which helps it estimate altitude gain and enables it to give more credit (more calories burned) to walking up steps than walking on a flat surface.

So, how important is accuracy down to the nearest calorie? It’s actually probably not that important. The essential thing to remember is that if you’re trying to shed pounds, you need to increase your calorie burn by a designated amount, for example, 200 calories daily, which means you need to boost the number and intensity of your active minutes as well.  As long as you’re tracking with the same device, you will have the consistency that you need to help you keep an eye on your daily trends.

Can a tracker assess the quality of my sleep?

Many activity trackers also monitor sleep, as do several apps offered on smartphones.  At the moment, no home tracker or app can perform a sleep study that is equivalent to the polysomnogram, which is conducted at a sleep center. The polysomnogram gauges brain activity, eye movement heart rhythm and muscle movements to determine sleep quality. It can be hard on the patient, who while trying to get a good night’s sleep, is required to wear a breathing mask in addition to being tethered to a bunch of wires. Despite its obvious shortcomings—the very nature of the test itself can make sleeping somewhat challenging—it is still the gold standard for assessing sleep disorders.

Trackers and apps can just measure body movement. They are essentially bedtime activity trackers.  Most trackers make the assumption that the less you move at night, you are enjoying a deeper sleep and, therefore, the better your sleep quality. There is disagreement among experts as to whether this is valid. The one point that everyone agrees with is that tracking your sleep even with a less-than-perfect device can raise awareness about sleep, which is vital for health.

How “private” is my data?

Today’s trackers upload data to Internet-based servers that store your information, typically for about a month so you can track your progress and identify patterns in your behavior, which is helpful if you want to sustain positive change.

The company that is collecting your data is also responsible for keeping it safe.

Most companies have posted privacy policies which  are sometimes so long, and filled with so much legal jargon, that you may not even bother to read them. It’s worthwhile to give them a glance. Typically, companies assure their customers that they’re not selling names to third party vendors, and are making every effort to keep their customers’ information safe.  However, several companies sell aggregated, anonymous user data (separated by gender, age, weight, geography, etc.) to third parties like advertisers and marketers or donate it to health research groups.

Most companies do make an effort to de-identify data in a way that makes it unlikely that you will be identifiable. Having said that, hackers can find their way around anything. The bottom line is, if you want your life to be entirely private, don’t ever go online! In the case of using a tracker for health, the benefits may outweigh the risks.

There’s another way your data can become public. Many trackers allow you to share your data with family and friends via email, Twitter and Facebook. B This makes it possible for other, unintended people to see your data as well. To ensure that only the people you choose see your information, you need to be certain that your privacy settings on your accounts are adequate. Second, don’t send your data to people who you don’t completely trust. Third, be mindful that your data could record information that you don’t want revealed, like the time of day that you engage in specific activities. Following common sense, simple precautions can help keep your information free from prying eyes.