There are now small, portable trackers that attempt to quantify your activity, both formal and informal during the day. Theoretically, this lets you know how many kcal you have burned and if you have hit your goal.
They claim to track your steps, calories, distance and even sleep patterns. So how do they do it, and is it accurate?
These devices work one of two ways: either as accelerometers or GPS-enabled devices. In the case of accelerometers, they attempt to record the stop, start, and movement forces within the device and match to known patterns of movement. A good way to think of this is a jar with a ping-pong ball inside. Placed upon your hip, if you ran, the ping-pong ball would have a certain amount of vertical and horizontal motion in reaction to each step. Know what it does during running, and you can count steps and estimate speed and energy used.
Accelerometers work very well for movement patterns that are about propelling you from one spot to another, like walking or running. That is where the real relationship generally stops, as they do not know how to interpret motions like weight lifting or the elliptical. For running or walking, very accurate, for other activities, and the accuracy drops off dramatically.
The devices that use GPS signals tend to be more accurate for movement based activities in estimating actual kcal used because there is a general relationship between body size, distance traveled and energy burned.
The fitness bottom line is these devices are like Mr. Rogers, they get you in the neighborhood, yet do not accurately measure or record all activities. As a motivational tool, they tend to increase activity because you have a goal. However, do not rely on the numbers too much as they simply are not sophisticated enough to accurately gauge modes of exercise like weight training, Zumba or the elliptical.