Exercise Equipment and Energy Spent

Factual Questions
1

I’ve noticed that a lot of exercise machines (specifically stationary bikes, stair steppers, cross trainers and treadmills) have display outputs that output the supposed energy that you are using during exercise.

Specifically this crosstrainer(dont’ remember the brand) displays both “Watts” and “Calories Per Minute”. Now, I assume Watts is the energy generated by the user of the machine, and Calories Per Minute is a guess at how much energy you are actually using(difference between the two being the inefficiency of the human body).

Well it seems that something (either my math, the machine’s math or my knowledge of the human body) has to be way off. The machine display roughly 15 Calories(kcal) per minute for 200 watts. If I did the math correctly 200 watts is 12000 joules per minute, which is just under 3 kcal a minute. This means that the machine assumes human metabolic efficiency to be only 20%. A very low number in my opinion.

So what is going on? Is the machine lying to make the user feel good about “calories burned”? Is it measuring energy input incorrectly? Since it asks for your age and weight, does it tack a guesstimate of your basal metabolic rate on top of energy spent on the machine?

To me 200 watts sounds a very reasonable number for how I feel (feels like sprinting in terms of effort) and so does 15 Kcal/minute (900 Kcal an hour is a good number for sprinting). But if we assume that the body manages to get at least over 50% efficiency on its metabolism then those numbers don’t match up at all. What is going on?

2

Pretty much. Ellipticals in particular are notorious for this.

3

I think that is a bad assumption. I once read an article in a magazine (Somewhat old, I know it was from 1974, and think it was Scientific American, or was in American Scientific?) titled ‘The Energetic Cost of Moving Around’. The gist of the article was an explanation of why larger land animals tended to migrate, and smaller ones did not. Muscular efficiency was one of the parameters, and although I can’t remember the specific details, around 25% was the maximum efficiency. I may still have it around somewhere, I’ll have to look around. It was interesting.