Ask Dr. Mike: Which is best?
In this issue Dr Mike Posthumus answers a reader’s training related question.
“Dear Dr Mike
I have recently bought a new Wahoo Elemnt cycling computer and when setting up the variables to display, I can choose kilojoules and calories. How are these calculated and which is best to use and display?
Regards,
Martin”
Dear Martin
The majority of cycling computers have the ability to display both these metrics. Although both are measures of energy, generally within cycling terms the kilojoule reading is a direct measure of the accumulated energy you have placed through your pedals. Therefore, this metric is normally only present when you have a powermeter. Remember, by definition, 1 watt = 1 joule/second. Therefore if you ride at 100 watts for 60 minute you would have produced 360kJ. Now, calories, although also a measure of energy, generally refers to total body energy expenditure. On the majority of cycling computers this metric is a far more variable term as the metric is derived from heart rate and known normative data. We know that deriving calories from heart rate is not very accurate, but it is rather a crude indication.
The majority of readers, especially those who have followed strict “calorie-counting” diets, would already know that kJ’s may be converted to calories (more correctly, kCal). One calorie is equivalent to 4.2 kJ. Therefore if you ride for 100 watts for 60 minute, you would have produced a total of 360 kJ, or 86 calories.
Now if you have noticed from looking at your own data, the value for kJ is in actual fact normally very close to the predicted calorie value. Why is this you may ask? Well nature has very conveniently given us humans a cycling efficiency value (see text block) which allows energy put through our pedals in kJ, to be equal to the amount of calories our bodies have burnt. When we are cycling, our gross efficiency is approximately 25% (for ease of explanation). That means that when we are producing 1 kJ, our body is actually using 4 kJ. The 75% of “wasted” energy is all the energy lost to maintaining homeostasis in the rest of our body. Now as mentioned above, 1 Calorie = 4.2 kJ, therefore when you are producing 1 kJ of energy, your body is burning, roughly, 1 calorie of energy.
This concept of energy produced and energy expenditure is critically important when trying to ensure that you are eating enough on the bike or during a ride, and that you are eating enough after your ride to ensure full recovery. Measurements of total energy expenditure may also be of value when adjusting your eating plan or diet to accommodate the exercise you had performed.
Include in table:
What is cycling efficiency?
Cycling efficiency (or gross efficiency) is defined as the ratio of work generated to the total metabolic (whole body) energy cost. Elite athletes are extremely efficient (high values of efficiency). Values up to 28% have been shown in elite riders, but the norm is 22 to 27%. Recreational athletes will have efficiency values around below 20%.
