Exercise bike / ergometer - calories burned, or calories produced?

Question

Exercise bikes / ergometers normally display figures including cadence, power and calories.

Cadence and power are pretty explicit, but I'm not really sure what the calories figure refers to. Is it calories consumed by the body during the session, or calories produced at the crank?

I understand that the human body is only about 25% efficient, so theres a big difference between calories burned and calories produced.

If I pedal the bike for 30 minutes at 200 watts (i.e. 200 joules/sec) then I would expect to produce 200*30*60 joules of energy, a total of 360kJ or 86 Calories. If my body is only 25% efficient at converting food energy into mechanical power at the crank, then producing 86 Calories (at the crank) actually required my body to burn 344 Calories.

After 30 minutes at 200 watts, my exercise bikes tells me I've used about 200 Calories.

This doesn't seem to match either of my calculated figures, so either my bike is faulty, or I don't understand what it's showing.

Answer 1

The number that is displayed on your bike is an estimate and can be off as much as 40%: there is no standard for this and many machines are fairly inaccurate.

Some of the possibilities that your machine can show:

• Calories as a conversion of energy (joules) exerted.
• Average calories burnt for the style of machine based on a preset body type (e.g. 175 lbs).
• Average calories burnt for the style of machine based on your weight, height, and gender entries.
• Calories burnt based on your weight, height, and gender as a function of your heart-rate.

On top of that, as indicated in that first link, machines can display gross calories burnt vs net calories burnt:

Many models calculate the 'gross energy expenditure' and not the 'net energy expenditure.' Gross energy expenditure includes your resting metabolic rate, or the calories you would burn anyway," Olson says. "What you really need to know is how many more calories above your resting metabolic rate you are burning." So if the machine reads 400 calories burned, a 20-percent adjustment to find your net number reveals that the stair stepper workout is responsible for 320 of those 400 calories. "In other words, the workout caused you to burn 320 calories, and you would have burned the additional 80 sitting and reading a book anyway.

To find out what the number of calories on your bike means you need to dig into the user manual. Even then, you may not find out. The most accurate method will probably be to use a heart rate monitor and even then, they have limited accuracy.