The bike knows my height and weight and that’s about it. From that it can calculate how my body burns calories?
Every body is very different, so I don’t see how any calculation can be accurate.
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.
So it’s a measurement of the mechanical force needed to run the machine, but does one human body burn as many calories as another to exert the same force on a stationary bicycle?
It’s a measurement of energy needed. Bodies are going to take different amount of force to move the thing, but it’s also going to take a different amount of time, and impart less inertia, which starts averaging out to the same amount of calories. I bet they also raise the amount of calories shown to account for no process being 100% efficient.
Measuring how we consume our calories is also a bit tricky, the bike tells you the minimum energy you spent moving the wheel. It could be slightly more, but not by much because our bodies are pretty efficient at using stuff to burn into energy. The amount needed to move the bike is about as much as we’ll burn because there isn’t a lot of waste. Of course if you have a condition, or are out of shape this changes and the counter becomes more inaccurate, but in a positive way. (You burned more than the counter said). Some will record your body mass index and other info to try to shift you in the right average.
We still calculate how much calories are in food by burning them and seeing how much energy they emit (heat). It’s not really how we consume calories but very similar. All in all calories aren’t a great way to think about human strength, or exercise
Humans are about 25 % efficient. Not bad, but also not that good. So factor 4 to wherever actual work you did. The question is: Does the bike take that into account already?
It doesn’t need to be perfect, just reasonably close.
If you’re trying to track calories down to even double digits you’re going to have a ton of error on both the input and output.
Beyond what other people are saying, it should be pretty reliable for comparing between exercise sessions so you can see your improvement over time.
It’s a good guess. Maybe accurate enough for the average user. For an exact number you’d probably need to put on a mask attached to some machinery and breathe through that, to measure how much oxygen you take in and the CO2 you exhale in return… to get an exact measurement. Everything else is an approximation.
I would caution against using these numbers for any Calorie-in/out calculations. Even if they were 100% accurate, it still doesn’t take into account anything that happens outside of that machine.
Example: There’s something we call “non-exercise activity thermogenesis” (NEAT). This includes lots of things you do without thinking about it, like fidgeting, tapping your feet while sitting, or pacing around the room. In some people, NEAT can decrease significantly after exercise, which then negates much of the Calories burned.
