A method comprising: establishing, using one or more processors of a wireless wearable computer worn by a user, a wireless communication connection with a fitness machine; obtaining, using the one or more processors using the communication connection, machine data from the fitness machine while the user is engaged in a workout session on the fitness machine; obtaining, using one or more motion sensors of the wireless wearable device, motion data generated in response to motion of the user on the fitness machine; determining, using a digital pedometer of the wearable computer, pedometer output data based on the motion data; determining, using the one or more processors, a calibrated stride length of the user using the pedometer output data and the machine data; determining, using the one or more processors, a work rate caloric expenditure, the work rate caloric expenditure determined using a work rate calorie model with the calibrated stride length as input, wherein the work rate calorie model is a function of the user's speed; obtaining, using a heart rate sensor of the wireless device, heart rate data of the user; determining, using the one or more processors, a calibrated maximal oxygen consumption of the user based on the heart rate data and the work rate caloric expenditure; determining, using the one or more processors, a heart rate caloric expenditure by applying a heart rate calorie model to the heart rate data and the calibrated maximal oxygen consumption of the user, wherein the heart rate calorie model scales the calibrated maximal oxygen consumption by the user's heart rate; and sending, using the one or more processors, to the fitness machine or other device using the communication connection, at least one of the work rate caloric expenditure or the heart rate caloric expenditure. establishing, using one or more processors of a wireless wearable computer worn by a user, a wireless communication connection with a fitness machine; obtaining, using the one or more processors using the communication connection, machine data from the fitness machine while the user is engaged in a workout session on the fitness machine; obtaining, using one or more motion sensors of the wireless wearable device, motion data generated in response to motion of the user on the fitness machine; determining, using a digital pedometer of the wearable computer, pedometer output data based on the motion data; determining, using the one or more processors, a calibrated stride length of the user using the pedometer output data and the machine data; determining, using the one or more processors, a work rate caloric expenditure, the work rate caloric expenditure determined using a work rate calorie model with the calibrated stride length as input, wherein the work rate calorie model is a function of the user's speed; obtaining, using a heart rate sensor of the wireless device, heart rate data of the user; determining, using the one or more processors, a calibrated maximal oxygen consumption of the user based on the heart rate data and the work rate caloric expenditure; determining, using the one or more processors, a heart rate caloric expenditure by applying a heart rate calorie model to the heart rate data and the calibrated maximal oxygen consumption of the user, wherein the heart rate calorie model scales the calibrated maximal oxygen consumption by the user's heart rate; and sending, using the one or more processors, to the fitness machine or other device using the communication connection, at least one of the work rate caloric expenditure or the heart rate caloric expenditure.