This is a long overdue follow up to: Measuring Energy Expenditure on the Bike So last time we talked about how indirect calorimetry is the gold standard, and why equipment such as heart rate monitors and GPS units are inaccurate. The next topic to examine is typical gym equipment such as stair climbers, and treadmills. These machines suffer from the same inadequacies as the heart rate monitors, in that they rely on equations and guesstimations to measure your energy expenditure. Just hop on any gym treadmill, and it will ask you your body weight so that it can calculate your energy expenditure as your exercise. The problem here is that there are many assumptions, and that the work you are doing is being calculated rather than truly measured. Something else you should know about the calories as given by these machines, is that they include your energy expenditure due to your basal metabolic rate. Most people burn between 50 and 80 calories an hour even if they are just sitting on the couch watching TV. So the calorie count given by these machines is inaccurate to start with, and then you add an additional 50+ calories that doesn't really count towards your energy expenditure from exercise, because you would be burning those calories even if you were sitting on your butt!
So the most accurate method of measuring energy expenditure is definitely indirect calorimetry (measurement of expired O2 and Co2), but it isn't practical because of these expense involved, and because you have to breath into a mouthpiece containing Oxygen and CO2 sensors. The next best way of measuring energy expenditure is going to be through the use of power meters. Power meters such as Power Tap and SRM, use strain gauges to measure force. The strain guages are little strips of metal at hub or crank, and the amount of deflection is measured. Power is force X distance/time. If you measure force with the strain gauges, you can measure distance with the rotation of the hub or cranks, and then time is measured with a simple clock. So power meters accurately measure power. From power and time you can easily calculate work, and work is measured in Joules. It's an easy calculation, 1 watt for 1 second = 1 Joule. Think of it like this: If watts were miles per hour, Joules would be total miles. So if you measure power, you can quite readily get Joules. At the end of the workout, you can look at your powermeter and and it will give the total joules. One joules is actually a tiny amount of work, so this measurement is typically expressed as Kilojoules, or thousands of Joules, also known as KJ.
Now we can calculate how many calories you burned during your workout. We know how many KJ you did, as it was calculated from watts and time. For demonstration purposes, let's just say it was 1000KJ. Calories and Kilojoules are both measurements of energy. There are 4.18 KJ in every Kilojoule , so you actually did only 239 Calories worth of work. However, the human body about 24% efficient at turning food energy into mechanical energy and pedal power, while the other 76% is lost as heat. So it actually took you about 4.16 times as many Calories to produce that 239 Calories of work. 239 times 4.16 = 996...basically the number of Kiojoules you did. This is why we typically tell riders that the number of KJ they do during their workout is the same as the number of calories burned. Riders frequently ask me if their body weight makes a difference, and the answer is no. A larger rider can typically put out more watts, and therefore do more kilojoules in a given amount of time. But is still takes a 100lb rider just as many calories to do 150 watts for an hour,as it takes a 200lb rider to do 150 watts for an hour. The only difference is that the larger rider will burn more calories as part of his basal metabolic rate, but he would burn those even if he were sitting at his desk typing on his keyboard, so that doesn't really count towards his energy expenditure from exercise.
So.... indirect calorimetry is still the most accurate way to measure energy expenditure on the bike, but power meters are definitely the next best thing, and are much more practical for every day use.
All of our bikes at Crank Indoor Cycling are equipped with powertap power meters. At the end of every ride, you can use your console to examine your data and a d find out your energy expenditure. This has obvious implications for weight management, but it is also one of many indicators of fitness. If you are able to do more KJ of work, and burn more calories in a similar workout, you know that your fitness level has increased.
Have any more questions about power measurement or measuring energy expenditure? Feel free to ask questions in the comment section or come to class and ask me afterward!