A calorie is a unit of energy—it’s used to measure how much energy is contained in the foods you consume, and how much energy your body outputs in the form of physical work. These are objective measurements that have nothing to do with your body’s internal biology—you could measure the energy input and output of a robot or a car the same way. (In particular, calories in and out don’t tell you exactly how much weight you’ll gain or lose in the process—that’s dependent on your metabolism.)

The calories spent to work an exercise bike can be measured in terms of how much energy is needed to turn the pedals—it’s independent of whatever’s doing the turning.