Power helps give us a very accurate estimation of Calorie burn, however, to understand the reasoning behind it will require a quick science refresher.
Energy
In cycling, energy is absorbed from the foods we eat and stored as chemical energy within our bodies. This energy is then "converted" in order to produce mechanical work, driving us forward on our bikes. Since our bodies are not 100% efficient, a significant amount of energy is also lost as heat in the conversion from chemical energy to mechanical work. Mechanical work is a measure of power (watts)/second. Therefore, our power meters are easily able to measure mechanical work; we simply need to work backward through the equation to find the Calories burned. This is the brief overview. Now let's talk numbers.
Units of Energy
- 1 Calorie=1000 calorie: This one is confusing so bear with me. 1 Calorie (capitalized) refers to dietary calories we find in the food we eat. Sometimes referred to a kcal, these contain 1000 calories (little"c"), which are a smaller unit of energy used by scientists to measure smaller energy systems.
- 1 kJ=1000 joule: Joules are the SI unit of measuring energy (watts per second). In cycling, we typically use Calories to refer to food intake and kJ to define mechanical work completed during the workout
- To get an idea of what a kJ represents, consider this example: 400w x 5s = 2,000 J = 2kJ’s
- 1 cal=4.184 joules: This is the direct conversion from calories to joules (remember that they are both just energy)
- 1 Cal=4.184 kJ: Since Calories and kJ are both 1000 times larger than their smaller counterparts, their ratio is the same
So when I do 4.184 kJ of work, I burn 1 Calorie??
Welll... no. It's not quite that simple. This would be true if our bodies were 100% efficient at converting chemical energy into mechanical work, however that's not the case. Trained cyclists are anywhere from 19-24% efficient, therefore it takes more fuel (Calories) to produce the desired work output (kJ). So the unit analysis that takes efficiency into consideration looks like this:
- Assuming 24% efficiency: 1 Calorie x 4.184 kJ x 25% efficiency = 1.004 kJ
- Assuming 20% efficiency: 1 Calorie x 4.184 kJ x 20% efficiency = .8364 kJ
Nearly all human bodies operate within these bounds, so for practical reasons, we assume a 1:1 Cal to kJ conversion. Unfortunately, the only way to get a more accurate estimation would be through extensive (and unnecessary) lab testing.