Multi-Sensor Devices
Most of the single–sensor based systems that are appropriate for free–living activities involve surrogates for energy expenditure. Measuring steps, motion, heart rate, location on the planet, or even expired oxygen are only indirect measures of energy expenditure.

Take the case of accelerometry. Low motion might indicate rest, or it might indicate physical activity using a part of the body far from the accelerometer. Moderate motion might indicate physical activity or it might indicate riding in a moving vehicle on a rough road. By adding another variable, such as the heart rate, these different contexts can be disambiguated. Riding in a car will typically induce lower heart rates than will moderate physical activity; subjects at rest will typically have lower heart rates than those performing low–motion physical activity. By taking advantage of the science of data fusion, these systems typically achieve higher accuracies than single sensor systems while typically keeping overall costs moderate.