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.