A device comprising: an input configured to receive data from at least one body sensor; an aerobic processor configured to estimate aerobic energy expenditure of a user from the data using an aerobic metabolism model; an anaerobic threshold module configured to determine from the data whether the anaerobic threshold for the user has been exceeded; an output configured to output the estimated aerobic energy expenditure if the anaerobic threshold for the user has not been exceeded; an anaerobic energy expenditure estimator configured to estimate anaerobic energy expenditure of the user due to anaerobic respiration from the data; a processor configured to combine the estimated anaerobic energy expenditure and the estimated aerobic energy expenditure to produce a total energy expenditure when the anaerobic threshold for the user has been exceeded; an output configured to output the total energy expenditure if the anaerobic threshold for the user has been exceeded, wherein the aerobic processor comprises: an exercise intensity classifier configured to classify exercise of the user using data received by the input; and an estimator configured to estimate physical activity intensity and an estimated oxygen level in the respiratory gases according to the classification of the exercise wherein the output is configured to output the estimated physical activity intensity if the anaerobic threshold for the user has not been exceeded, wherein the anaerobic energy expenditure estimator comprises: a lactate estimator configured to estimate plasma lactate concentration; and a lactate to calorie convertor configured to convert the estimated plasma lactate concentration to a calorie value representing the anaerobic energy expenditure; and the device further comprises: an aerobic energy expenditure estimator configured to calculate a calorie value representing the aerobic energy expenditure using the estimated oxygen level in the respiratory gases; a processor configured to add the calorie value representing the anaerobic energy expenditure and the calorie value representing the aerobic energy expenditure configured to provide an estimate of the total energy expenditure when the anaerobic threshold has been passed, and wherein the plasma lactate concentration estimator is configured to estimate plasma lactate concentration using: LA = 10 0.082 lg V O 2 − 0.2 where VO 2 < 1.51 LA = 10 2.88 lg V O 2 − 0.7 where VO 2 ≥ 1.51 where LA is the plasma lactate concentration and wherein VO 2 is the estimated oxygen level in the respiratory gases. an input configured to receive data from at least one body sensor; an aerobic processor configured to estimate aerobic energy expenditure of a user from the data using an aerobic metabolism model; an anaerobic threshold module configured to determine from the data whether the anaerobic threshold for the user has been exceeded; an output configured to output the estimated aerobic energy expenditure if the anaerobic threshold for the user has not been exceeded; an anaerobic energy expenditure estimator configured to estimate anaerobic energy expenditure of the user due to anaerobic respiration from the data; a processor configured to combine the estimated anaerobic energy expenditure and the estimated aerobic energy expenditure to produce a total energy expenditure when the anaerobic threshold for the user has been exceeded; an output configured to output the total energy expenditure if the anaerobic threshold for the user has been exceeded, an exercise intensity classifier configured to classify exercise of the user using data received by the input; and an estimator configured to estimate physical activity intensity and an estimated oxygen level in the respiratory gases according to the classification of the exercise wherein the output is configured to output the estimated physical activity intensity if the anaerobic threshold for the user has not been exceeded, a lactate estimator configured to estimate plasma lactate concentration; and a lactate to calorie convertor configured to convert the estimated plasma lactate concentration to a calorie value representing the anaerobic energy expenditure; and the device further comprises: an aerobic energy expenditure estimator configured to calculate a calorie value representing the aerobic energy expenditure using the estimated oxygen level in the respiratory gases; a processor configured to add the calorie value representing the anaerobic energy expenditure and the calorie value representing the aerobic energy expenditure configured to provide an estimate of the total energy expenditure when the anaerobic threshold has been passed, and wherein the plasma lactate concentration estimator is configured to estimate plasma lactate concentration using: LA = 10 0.082 lg V O 2 − 0.2 where VO 2 < 1.51 LA = 10 2.88 lg V O 2 − 0.7 where VO 2 ≥ 1.51 where LA is the plasma lactate concentration and wherein VO 2 is the estimated oxygen level in the respiratory gases. an aerobic energy expenditure estimator configured to calculate a calorie value representing the aerobic energy expenditure using the estimated oxygen level in the respiratory gases; a processor configured to add the calorie value representing the anaerobic energy expenditure and the calorie value representing the aerobic energy expenditure configured to provide an estimate of the total energy expenditure when the anaerobic threshold has been passed, and