One of the great things about living in Taiwan is that so much stuff is made here. A lot of the fitness equipment that people enjoy is designed and/or manufactured in Taiwan so occasionally you’ll bump into someone that has experience with that design or production and they can give you some insight into how things work.
Below is quote from a treadmill designer and engineer who I asked about treadmills and how they calculate the calories you supposedly burn in “basic mode” where you get on the treadmill and just hit start.
“There are two different accepted calorie calculations, one for walking and one for running. We have to guess where the switchover is, and personally I calculate both values and then “fade” from one to the other either side of 6kph. The calculation gives a burn rate in ml per kg per minute; millilitres being oxygen and kg being bodyweight. Divide that value by 3.5 and you’ve got METs. This estimate is surprisingly accurate, especially for grade running; it’s about +/-10%. To estimate calories, you can assume 1 litre of oxygen represents 5kcal burned, but that’s only true for aerobic respiration. The metabolic reactions change throughout a long workout, so although the METs estimate is good, the final calorie value probably isn’t. And of course if the machine doesn’t know the user’s weight, then it’s even less accurate; in that case, I just use 70kg if the actual weight is unknown.
Distance is just measured as speed x time, which must (when you think about it) be completely accurate as long as speed is accurate. Speed is always slightly off though (a couple of %), especially with AC motors, but it will be a constant error, so you can compare distances with confidence.”
Here was my response:
“So 70kg is the guess of the average weight, meaning women under that weight are way off. So a 45kg women would be burning significantly less calories than a 70kg man.
I noticed we’re talking about body weight but METs don’t take into account body composition. So someone with higher than average lean body mass would also find their calorie count off, yes?
You’re point about it measuring aerobic respiration is well taken. I’ve long noticed that cardio machines don’t appear calibrated for anaerobic conditioning like Tabatas. This type of conditioning burns calories pretty well but the machines don’t reflect that fact. Naturally though, they can’t account for EPOC, etc. It’s just that clients are used to seeing a number and having that reflect everything when it actually doesn’t.
Finally, (and this is a more general comment) I’ve long noticed that “350 calories” burned for a beginner doesn’t seem to equal “350 calories” for someone fitter for the same length workout. Most machines don’t take into account efficiency and adaptation, which the body acquires as you do it more often. Most people don’t think about what they’re doing and use the exact same setting at the exact same speed for the same length of time and take the calorie count they get as a fact, even though they’ve been doing that same workout for a year. The 350 calories they might have burned the first few times seems to be much lower in reality even though the numbers are the same a year later.”