Measuring Energy Expenditure on the Bike A friend of mine recently gave me a call, and asked how accurate the energy expenditure was in the treadmill at his gym. He told me how his girlfriend obsesses over the calorie count on the machines, so that she can be sure she burned off that extra latte she had the morning. The short answer to “How accurate are those calorie counts?” is… “not very” There are dozens of devices that claim to measure energy expenditure during exercise. Some of them are located directly in the exercise equipment, while others are devices the user straps to his body in some manner, and all of them make estimates of the energy expenditure by measuring different variables. The most accurate way to measure energy expenditure, would be to use a metabolic cart…the machine that measure VO2. As you exercise, you use Oxygen and create CO2, the amounts and ratios of O2 utilized and CO2 created can be used to calculate the energy expenditure. This works well in a lab, but it isn’t very practical for measuring energy expenditure in the real world. The devices that a users wear or strap on typically fall in to three categories: heart rate monitors and accelerometers, and GPS. Accelerometers such as the “body bug” or pedometers that measure your number of steps, basically measure how much the user moves and the use this information to guesstimate energy expenditure. As you can probably guess, there is lots of room for error in this method. A device that is going to be more familiar to athletes is the heart rate monitor. Heart rate monitors typically work by measuring the electrical current in the heart. The users wear a strap on his chest, and then the information is sent wirelessly to a watch or other device. These devices give accurate information regarding heart rate, but the energy expenditure is a calculation based on algorithms. The heart rate monitor also makes several assumptions about the individual’s level of fitness, cardiac output, and environmental factors. As you can probably guess, there is quite a bit of room for error here as well. My undergraduate exercise physiology students once did a project compared Polar heart rate monitor’s measurement of energy expenditure to energy expenditure as measured by a metabolic cart. The students discovered that the Polar heart rate monitors were frequently off by 30%! GPS units estimate energy expenditure in much the same way as the accelerometers and the heart rate monitors. They use algorithms based on the type of activity, distance traveled and elevation. Once again, there is plenty of room for error. The GPS doesn’t ”know” if the cyclist is drafting, if she is in an aero position, what type of wheels are being ridden, or other numerous factors. These devices all give all ballpark figure for the total energy expenditure, but do not have a high degree of accuracy, and can frequently be off by as much as 30%.
next time..gym equipment and bicycle powermeters