Many heart rate monitors and bike computers has a feature to measure the energy cost of the exercise. But can we believe these numbers? I have always been sceptical to these calculations since they are based on very few variables (percentage of maximum heart rate and total time).
I have always said to my riders that they can use these numbers for fun, but don’t count on them when they cook dinner. There is probably huge variability in the quality of calorie metres, some gives a rough estimate and some doesn’t.
Can heart rate monitors be used to calculate energy expenditure?
Yesterday I found a study published in Medical Science of Sports and Exercise that tried to figure out how accurate the energy expenditure calculator of the Polar s-410 heart rate monitor was. They used three different calculations of the energy expenditure: 1) Polar s-410 using predicted values of VO2 max and maximum heart rate. 2) Polar s-410 using actual values of VO2 max and maximum heart rate. 3) Indirect calorimetry (You might have heard about this one in school)
The results showed that the Polar s-410 watch did a quite good job for the men with no significant differences between the three calculation methods. The women’s numbers were overestimated when using predicted values of VO2 max and maximum heart rate. The estimation was better when they used the actual values but still overestimated with 12%.
Knowing your maximum heart rate is a good thing
It is very obvious that indirect measurement of energy expenditure is not necessary very accurate. By using the actual number for your maximum heart rate, you will get closer to the correct number of calories. This is just like when you calculate target zones intervals without knowing your maximum heart rate: You are in risk of getting different intervals than you were expecting.
The standard formula for calculating maximum heart rate is 220bpm minus age.
Because this formula has a standard deviation of 10, a 30-year old cyclist has a 95% chance to have a maximum heart rate in the area between 170 to 210bpm. I guess it is obvious that it makes a difference if your heart rate monitor knows your real maximum heart rate or it has to predict it.
Crouter SE, Albright C, Bassett DR Jr. Accuracy of polar S410 heart rate monitor to estimate energy cost of exercise. Med Sci Sports Exerc. 2004 Aug;36(8):1433-9.