1. A method of limiting battery power usage, memory usage, processing cycle usage, or a combination thereof in a fitness tracking device configured to be worn by a user and configured to determine energy expenditure of a user, the method comprising: obtaining, by the fitness tracking device configured to be worn by a user, a plurality of physical characteristics of the user, wherein the plurality of physical characteristics comprise a first age and a sex of the user, the fitness tracking device including a memory configured to store an age lookup table comprising a plurality of age indices of a plurality of age ranges and a sex lookup table comprising plurality of sex indices of a plurality of sexes; mapping, by the fitness tracking device, each physical characteristic of the user to a corresponding index, wherein the first age of the user is mapped to a first age index of a first age range of the plurality of age ranges, and wherein the sex of the user is mapped to a first sex index of the plurality of sexes; selecting, from the memory of the fitness tracking device, a first combination of coefficients of a plurality of coefficients, wherein the first combination of coefficients is associated with each corresponding index, including the first age index and the first sex index of the user, wherein each of the plurality of coefficients are stored in the memory of the fitness tracking device, and wherein the combination of coefficients defines a first calorimetry model for the user; collecting, by a motion sensor of the fitness tracking device, sensed motion data of the user, wherein the fitness tracking device further comprises a low-power motion coprocessor, the low-power motion coprocessor communicatively coupled to the motion sensor; receiving, by the low-power motion coprocessor, the sensed motion data; analyzing, by the low-power motion coprocessor, the sensed motion data to determine a speed of the user; estimating an energy expenditure rate using the first calorimetry model and the speed of the user; and outputting an indication of energy expenditure for the user.