Systems and methods configured to process motion data associated with a user. The systems and methods are configured to receive motion data from a sensor, calculate motion attributes from the data, and classify the motion data using one or more mathematical models. Attributes may be calculated without classifying the motion data into an activity type (such as walking, running, swimming, or any specific or general activity). Attributes may be compared to activity models comprising motion data from several individuals, which may not include the user. Motion data within the models and attributes of the user may be independent of any activity type. Attributes may be compared to select an energy expenditure model from one or more energy expenditure models, which may be selected as a best-match to the one or more motion attributes. An energy expenditure associated with the motion of the user may then be calculated.