Abstract: In one aspect, the present disclosure relates to a method including obtaining, by a heart rate sensor of a fitness tracking device, a heart rate measurement of a user of the fitness tracking device; obtaining, by at least one motion sensor, motion data of the user; analyzing, by the fitness tracking device, the motion data of the user to estimate a step rate of the user; estimating, by the fitness tracking device, a load associated with a physical activity of the user by comparing the heart rate measurement with the step rate of the user; and estimating, by the fitness tracking device, an energy expenditure rate of the user using the load and at least one of the heart rate measurement and the step rate.

Abstract: A computerized method for determining maximal oxygen uptake for a user with incomplete data with data collected from a plurality of other users with complete data. The maximal oxygen uptake can be determined by computing similarity metrics between an incomplete data set of self-reported and measured data and complete user data sets, and using a weighted sum of the similarity metrics. The results of the maximal oxygen update calculation can be cross-validated with known user data sets.