How the BodyMedia Fit Works

How can a sensor on an armband help you lose weight?
How can a sensor on an armband help you lose weight?
Courtesy BodyMedia

Come closer, I have a secret to share with you. It's the secret to losing weight. This secret has made people millions of dollars, and I'm going to give it away for free. Ready? You need to burn more calories than you consume. That doesn't mean starving yourself -- that can lead to more problems and your body will respond by slowing down and conserving calories. It just means you have to balance what you eat with your activity levels. Simple, right?

Many of us don't find it that easy. It can feel like a lot of work to keep track of everything you eat and do to make sure you stay on track with fitness goals. But while some of us see that as an obstacle, others view it as an opportunity. The people over at BodyMedia decided to take a crack at making it easier to see what progress -- or lack thereof -- we're making on a day-to-day basis.


The company developed a product called the BodyMedia FIT. It's not easy to categorize the FIT -- it's a device worn on an armband that uses an array of sensors to track how many calories you burn, the types of activities you do and even how much sleep you get each night. You synchronize the FIT with a computer to get a graphical representation of your activity, making it easier to see if you're sticking to the plan or getting a little lazy.

The company also got into the smartphone app game -- you can download a free app that lets you get a real-time look at your daily caloric burn as well as keep track of the foods you're eating. And it's not just about tracking and plotting data points -- in 2012, BodyMedia announced a partnership with IBM in which the two companies developed algorithms designed to give each user a personalized fitness feedback loop.

How can a little armband do so much?

FIT for Brains

This FIT device contains several sensors and a Bluetooth transistor.
This FIT device contains several sensors and a Bluetooth transistor.
Courtesy BodyMedia

At first glance, a BodyMedia FIT device just looks like a little square of plastic on an elastic armband. The company instructs you to wear the armband so that the FIT rests on the back of your arm against your triceps. From here, the FIT monitors your movements and activities. You wear the sensor all day and night, taking it off only when you shower or go swimming -- it's not waterproof.

Inside the FIT are the sensors that gather the data generated by your jogging, kickboxing or playing "Farmville." Those sensors include:


  • a thermometer
  • an accelerometer
  • a galvanic skin response sensor

Each of these sensors gathers data that feed into algorithms -- sets of mathematical instructions -- when you sync your device with a computer. That's how the system can tell whether you're feeling the burn or you're just burnt out.

So what does each sensor actually do? The thermometer is the simplest of the three -- it measures your body's temperature. It keeps track of when your muscles get warm and when they cool down again. As we expend energy by moving around, not all of that energy comes out in the form of physical work. We lose some energy in the form of heat. By measuring the amount of heat generated and lost by our muscles, the FIT can begin to create a picture of how active we are.

The accelerometer's job is to detect changes in velocity. Velocity has two factors: speed and direction. As you move around, the FIT's accelerometer detects changes in your movements. Walk at a leisurely pace and the accelerometer will detect that your body is active but not under a lot of stress. If you break into a prolonged sprint the accelerometer will sense your more aggressive movements.

The data from the accelerometer helps the BodyMedia system keep track of how many steps you take throughout the day. The motions you make while walking are different from those you make while jogging or running. The data the sensor gathers reflects these changes and the BodyMedia algorithms crunch the numbers to figure out how many steps you've taken that day.

The galvanic skin response sensor has the grossest job -- it tracks how much you sweat. Two stainless steel pads on the FIT sensor stay in contact with your skin. These two pads send a tiny electric charge that uses your skin to complete the circuit. As you sweat, your skin's ability to conduct electricity improves. The sensors measure the changes in the strength of the electric signal over time. If the signal remains strong for a long time it indicates that you're staying active and probably need a shower.

Put It All Together

When you pair a FIT device with a computer, the sensor sends all the collected data over for processing. BodyMedia's online Activity Manager takes the data and analyzes it using complex algorithms. The algorithms have a foundation in years of scientific study focusing on how various activity levels relate to caloric burn. One important thing to note, though, is that the Activity Manager requires a subscription to use; as of the time of this writing, the service costs $6.95 per month, although the company offers the first three months for free.

Based on your activity levels, including the intensity and duration of your activities, it generates information that's meaningful to you. This includes everything from how many steps you've taken that day to an estimation of the number of calories you've burned.


The software also breaks down your activity into types. If you spend all day walking around at a gentle pace the software will reflect that. But if you've been pushing yourself hard with intense workouts your profile will indicate that in your record.

With this information, you can get an idea if you're on the road to weight loss. But that's just part of the equation. What about caloric intake?

BodyMedia's Activity Manager includes a food diary feature that lets you keep track of everything you eat. The food database is extensive. You just have to enter information about what -- and how much -- you've had to eat for each meal. If the database doesn't have information about the type of food you're eating you can manually enter the nutritional information into the system and it will apply the data to your profile.

In this way, the software can plot how many calories you've burned against how many you're taking in. If there's a deficit of calories, you're on the way to losing weight! But if you're consuming more than you're burning, you'll need to make some changes if weight loss is your ultimate goal.

Some versions of the FIT device come equipped with Bluetooth technology. This wireless protocol lets devices communicate with each other through a connection called pairing. If you pair a FIT with a smartphone like the iPhone or an Android device, you can get an instant look at your daily progress.

Catching Some Zs

The FIT tracks your sleeping patterns.
The FIT tracks your sleeping patterns.
Courtesy BodyMedia

The only time you're supposed to take off your FIT is when you're going to get wet. You even wear it while you sleep. Is this because you're secretly sleep-exercising? Could it be you're a world-class athletic somnambulist? That's not likely, but it's clear that sleep plays an important part in weight loss.

A study published in the Annals of Internal Medicine found that dieters who cut back on sleep reduced their ability to lose fat by 55 percent [source: Science Daily]. Reduced sleep seems to have an effect on ghrelin production. Ghrelin is a hormone that plays a part in triggering the hunger response and reduces the amount of energy you expend. If you don't get enough sleep, your body will produce more ghrelin and you'll feel hungrier than normal.


The FIT tracks your sleep patterns the same way it tracks how far you've run or how many calories you burn as you pump iron. The sensors keep track of your movement, temperature and any changes in your skin's conductivity. Most people are pretty inactive once they go to sleep. When the FIT's data reflect this state, the software assumes that's when you've gone to sleep.

Throughout the night, you might move around in your sleep or even wake up a few times. The FIT captures all of these movements throughout the night and sends the data to your computer as you sync up. Based on this information, the software extrapolates how well you slept the night before. A steady signal could indicate you slept soundly for several hours, which is good news. But a data stream with a series of interruptions or spikes could show that you're not getting enough good sleep to help you in your goal.

What you do with this information is entirely up to you. You might need to look into changing your daily routine. You may have to adjust where you sleep. Keep in mind that you can be active and follow a sensible diet and still not lose the fat you want to get rid of if your sleeping patterns are bad.

Will FIT get you fit?

Keeping a diary of what you eat helps you stay on track to achieve your fitness goals.
Keeping a diary of what you eat helps you stay on track to achieve your fitness goals.
Courtesy BodyMedia

To use BodyMedia's FIT device, you'll have to purchase an armband and subscribe to the company's Activity Manager service. In return, you'll have access to all the data collected by the armband as you wear it throughout the day. You'll also be able to track personal bests and set goals for future achievements.

Through partnerships with other companies and organizations, BodyMedia may provide further services like meal planning and exercise routines. For example, in 2012 BodyMedia partnered with fitness expert Jillian Michaels. Michaels' program helps you plan out each meal and activity with the goal to help you achieve the results you want in a safe and healthy way.


As you enter information about the food you eat into the Activity Manager, you'll receive feedback about your diet. BodyMedia has included nutritional guidelines to alert you if you're consuming too many carbohydrates or aren't getting enough vitamins from the food you eat. A healthy diet is an important part of any fitness program. But if you slack off and don't keep your food diary up to date you'll lose this benefit -- the FIT isn't able to track every bite you consume. Yet.

On its own, the FIT is just a collection of sensors. It can provide you with a lot of data about what you're doing -- or not doing. But the armband isn't going to shed those pounds for you. You'll have to work to get fit. But the BodyMedia FIT might just give you the information you need to stay motivated while you strive to achieve your fitness goals.

Author's Note

Author's Note: At CES 2011 -- aka the 2011 Consumer Electronics Show -- I chatted with representatives from BodyMedia. The company made a big splash that year, even going so far as to track one man's body responses as he engaged in crazy activities like skydiving. I bought a BodyMedia FIT and tried it out over several months. The data collected from the FIT really opened my eyes to my lifestyle, and I made some changes to my diet and activities. The difference showed as I dropped a few pounds and began eating healthier foods -- a little information can be really powerful!

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More Great Links


  • BodyMedia. "More Details." (Feb. 12, 2012)
  • Hollis, Jack F. et al. "Weight Loss During the Intensive Intervention Phase of the Weight-Loss Maintenance Trial." American Journal of Preventive Medicine Vol. 2, No. 35. pp 118-126.
  • Kantrowitz, Barbara. "Three of the latest, greatest studies on what really helps when it comes to weight loss—and why keeping a food diary can be crucial." The Daily Beast. July 7, 2009. (Feb. 8, 2012)
  • Mann, Denise. "Sleep and Weight Gain." WebMD. Jan. 19, 2010. (Feb. 12, 2012)
  • MIT. "What is the skin conductance response?" (Feb. 12, 2012)
  • United States Patent & Trademark Office Patent Application # 20060122474.
  • University of Chicago Medical Center. "Sleep loss limits fat loss." Oct. 4, 2010. (Feb. 12, 2012)