Fitness level vs Calorie burn
 

My brother and I were on a bike ride; we both weigh the same and ride similar bikes. We both were wearing a hear rate monitor. His average heart rate was 20 ticks over mine; according to his heart rate monitor he burned 20% more calories than I.

Correct me if I'm wrong, but my understanding is that a calorie is a unit of measure of energy. Regardless of our fitness level shouldn't it take the same amount of energy to get our similar loads down the road?

There are more variables to take into account. HRMs use heart rate in algorithms to estimate calories used. So if your brother's heart rate was higher it makes sense that the HRM said he burned more calories. The heart rate difference could just be a natural variance between you and your brother, your brother was having a bad day, you were having a good day, one of your HRM sensors was not reading and transmitting the same as the other due to position on the chest, a battery was getting low, the settings in the HRM were not the same (age, gender, weight). One might be tempted to make a judgement that 20 bpm is a big difference, but due to all the other factors and only a single ride for data I know better than to do that.

I've used the calorie counts from my Polar HRM to help with weight loss and maintenance. It works well for that but the numbers it produces are only good for comparison with my own history (and even then there is probably some variance for the reasons I mentioned in the first paragraph). But it is not sensitive or sophisticated enough to be used to compare my numbers to anyone else's beyond general curiosity.

Neglecting things like wind drag and drafting, it takes the same energy to move two bikes and riders down the same course.

Note, though, that's power to the pedals. If you're in better shape than your brother, you can generate that power more efficiently than he can. He probably wastes more energy as heat than you do.

Of course, there's also the problem of how accurate the HRM settings and algorithms are. Bottom line, a HRM may be useful for your training, but forget about comparing yourself to someone else based on their output.

I think there is confusion here over work and energy.

the work done, moving similar mass over equal distances during the same time period is the same.

the energy required to do that work can be different.


Say one bike had bad bearings, rusted chain and no lube and the other was perfectly lube and adjusted. The amount of energy input to get the work (mass, distance, time) done would be greater with bike with more internal resistance. so the guy on the junker would burn more calories to get the same amount of work done.

I can't vouch for the science, but when I am out of shape I sure feel like I work harder to get the same results than I do when i am in shape, so can see that making a differnces in calorie burn between two diferent cyclists.

humans aren't perfectly efficient and consistent machines. two people performing the exact same movements can be generating vastly different amount of power / engaging different muscles / burning different amounts of calories etc.

Forget about HRM estimates of calories burned. They are notoriously inaccurate. If you had power meters you could have calculated the calories used accurately, and would probably have found them similar.

Most of the energy you expend goes into overcoming wind resistance. If you were the same height and rode in the same riding positions i.e. drops or hoods and didn't draft off of each other unevenly then the total work you did should be the same.

To determine how many calories you burned you divide the total work by your efficiency. Efficiency of riders can vary between about 18-24% with most people falling in the 20-22% range. If one of you had a 20% efficiency and the other 22% you would expect a 10% difference in the total calories burned.

The 20% difference you noticed could be due to errors in HR/calorie algorithms, differences in riding positions and/or different amounts of time spent drafting. You would need powermeters if you want more accurate numbers.

You feel like you are working harder only because your cardiovascular system isn't efficient. When you are trained, you have a more extensive capillary network than the couch potato. Your ability to utilize glucose and fat is more efficient in the trained athlete. The couch potato with less muscle mass and a less efficient cardiovascular system is probably burning fewer calories than the more muscular fitter athlete. Oxygen consumption would be a more sensitive measurement than HR.

I'm in the middle on this one. Watts is the same. Physics is physics, it does what it does. However you're right about "energy." Sure your RPE, Rating of Perceived Exertion, will be higher when you're out of shape. It'll take more mental effort and energy to command your body to do the same thing. However, your body is still doing the same thing. The rusty chain is a flawed analogy. If you had a poorly maintained bike or didn't pump up the tires, yes, frictional and rolling resistance would be higher and you'd burn more calories doing the same thing. But for the purposes of this discussion, we're assuming identical bikes for the in-shape and out-of-shape repetitions.

Looking at it another way, you're the engine. Suppose you had a car with fouled spark plugs and clean ones. You'd burn more gas getting from A to B with the fouled plugs. But the horsepower put out by the car would be the same, assuming the same speed. It could not be otherwise. OTOH, your body has a chemical reaction that starts with glucose or glycogen and comes out with ATP. I don't see how that gets fouled up and any bits lost just because cellular structures might be less efficient at making lots of ATP on demand. I don't think the sparkplug analogy works. OTOH, people just starting a cycling exercise program do seem to lose weight more quickly at the start than they do later. It is said that the body is 20-25% efficient. Looks like a range to me.

However the theory might be, in the case under discussion a higher HR probably does not mean more energy burned. It just means a higher HR. Different people have different HRs. The reason that the HRM shows a higher calorie burn for your brother is that if the same person did one ride for an hour at one HR, and then did a second hour at a HR 20 beats higher, yes, the second ride would burn more calories. However with 2 different people riding side by side, comparing calculated calories by HR is silly. All HRMs give quite flawed calorie calculations because they don't know how your particular HR relates to watts throughout your HR range. So they estimate, and usually on the high side so you'll feel good about owning and using that HRM.

Yes, a calorie is "the amount of heat energy required to raise one gram of water from 14.5 to 15.5 degrees Celsius at atmospheric pressure." [Peter Attia]
You're assuming the amount of energy you expend to do the same work should be identical, because of your equal weight and bikes. But the energy you each are expending is a product of your internal metabolism, the conditioning of your muscles, the efficiency of your blood circulation, etc. This is why, as you get into better shape, you will expend less energy, i.e. use fewer calories, to ride the same distance.
Think of each of your hearts as a little gas burning motor. If motor "a" has to go at 1,200 rpm to produce the same result as motor "b" does at 1,000 rpm., motor "a" will definitely use more gas.

Well, remember heart rate monitors have no way to measure caloric burn. It is just an estimation and probably not a particularly good one. Still people like getting the caloric burn so it makes the customers satisfied. More importantly, I bet the feature sells heart rate monitors which is more to the point. I recall when I was fishing, suspecting that fishing lures were not designed to catch fish as much as they were designed to get fishermen to buy them.

You both rode the same route on very similar bikes at the same speed. So those variables are dealt with. Now if you are both about the same level of conditioning and the same weight, you should both burn a very similar number of calories. Heart rates can vary quite a bit. I have compared heart rates with friends whilst riding (there were three of us). One tended to have a fast heart rate, I was in the middle and the last guy was very low (these were max heart rates). Max heart rates seem to be pretty constant for the individual. They also seem to vary considerably between individuals. Two people can be burning calories at similar levels with two different heart rates. If one person has a larger heart and a higher level of hemoglobin in their blood, they will have a relatively low heart rate. A person with a smaller heart (for their size) and a lower hemoglobin concentration will need more heart beats to deliver the same amount of oxygen to their muscles. It is quite possible that the two of you burnt close to the same number of calories in the ride.