It seems that everyone knows someone who is using an activity monitor these days. They come under a variety of brands including Fitbit, NikeFuel, Jawbone Up and others. Some report steps taken while others are more sophisticated and also report energy expenditure. Accuracy varies among devices and should only be used as a guide.
The most common way to measure steps is with accelerometry. The device measures change in speed to estimate the number of steps. One study took 16 healthy volunteers and measured activity recorded on various monitors while lying, sitting, standing, walking or shuffling. Error rates were between 4.5–17.2% when compared with manually counted videotaped steps. Error was speed dependent with slow speed having worse accuracy. NikeFuel band underestimated while Jawbone Up overestimated the number of steps at slow speed. Accurately counting steps on stairs was especially difficult for the monitors.
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“Although these devices are not precise, they provide a reasonable estimate of activity.”
Energy expenditure reporting is accomplished by converting activity data via an algorithm into energy consumption. This can be done with or without the aid of heart rate, temperature, heat flux or galvanic skin heat response. Monitors were tested in one study using 60 healthy volunteers. Resting energy expenditure was measured and the following week subjects were fitted with multiple monitors. They did activities including being sedentary, walking on a treadmill, jogging/running on a treadmill, and moderate to vigorous activities for 3–5 minutes during measurement. Energy expenditure over the entire duration was examined. Energy consumption estimates were be off by 9–23.5%. Fitbit error was approximately 10%, Jawbone approximately 12%, Nike Fuel 14% and Basis Band 23.5%. However, the standard error of the error measurements was between 6–17% in the study.
Another study examined shoe-based monitors. These sensed acceleration and foot pressure. Subjects walked in a calorimetric room that measures oxygen and carbon dioxide levels, which is a very accurate way to determine energy expenditure. The shoe-based monitors were accurate within 6% of energy expenditure while commercial monitors such as the Fitbit and DirectLife were off by approximately 14–18%.
Although these devices are not precise, they provide a reasonable estimate of activity. Effectiveness of the devices to motivate increased activity is an area that is being intensely studied. In the meantime, go ahead and use it if you have one.