Embodiments are disclosed for a wireless wearable computer with fitness machine connectivity for improved activity monitoring using caloric expenditure models. In an embodiment, a method comprises: establishing, by a processor of a wireless wearable computer worn by a user, a wireless communication connection with a fitness machine; obtaining, by the processor using the communication connection, machine data from the fitness machine while the user is engaged in a workout session on the fitness machine; obtaining, from a heart rate sensor of the wireless device, heart rate data of the user; determining, by the processor, a work rate caloric expenditure by applying a work rate calorie model to the machine data; determining, by the processor, a calibrated maximal oxygen consumption of the user based on the heart rate data and the work rate caloric expenditure; determining, by the processor, 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; and sending, by the processor to the fitness machine via the communication connection, at least one of the work rate caloric expenditure or the heart rate caloric expenditure.