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Calories burned during exercise--it's the INTENSITY, not the HEART RATE that counts.

A couple of other blogs and many of my comments concern the topic of caloric expenditure during exercise and the use of HRMs. Rather than write a magnum opus on the subject  that might take too long to read for some people, I occasionally elaborate on smaller parts of the big picture, usually after seeing a high number of topics and comments on the subject. 

HRMs are great tools, but the introduction of the calorie estimating feature by Polar years ago and their widespread use has led to what appears to me to be widespread misunderstanding  about calories burned during exercise and heart rate. 

I have written on this subject before--see for an overview of the relationship between heart rate and oxygen uptake--the background info might be helpful. 

As I explain in that blog, heart rate is not a direct indicator of caloric burn during exercise. Heart rate can be used to estimate caloric burn during certain exercise conditions because of the relationship between heart rate (HR) and oxygen uptake (VO2) that occurs during steady-state aerobic exercise. It is actually VO2 that is the most direct way to measure caloric expenditure. The way we calculate calories burned per hour during steady-state aerobic exercise is relatively simple:

Intensity x body weight = Kcals burned per hour.

Intensity is expressed in METs (1 MET= 3.5 ml/O2/kg) and body weight is expressed in kilograms.

Simpler, steady-state aerobic activities have a relatively fixed energy cost, or VO2, regardless of who is doing them. For example, the intensity/oxygen cost of running at 6 mph (10:00 mile) is  about 10 METs. Whether you are doing it, I am doing it--it's still 10 METs (exceptions are those with some medical conditions such as COPD and obesity). 

The absolute intensity of running 6 mph is relatively the same--the difference in caloric burn will depend on weight.

A 60 kg (132 lb) runner will burn ~600 Kcal/hour 

An 80 kg (176 lb) runner will burn 800 Kcal/hour

and so on......

The intensity (10 METs) is mostly the same regardless of how fit the individual is--the difference is the % of maximum the energy cost represents. 

A less fit person who has a VO2max might find running at 6 mph represents 70% of their VO2 max and a highly fit person might find that the same speed represents only 45% of their VO2 max. However, for both runners, the intensity is 10 METs.

A beginning exerciser may find that running 6 mph is 80% of their VO2 max when starting a program, yet after a year of training, that speed now represents only 60% of max. The intensity it still ~ 10 METs.

The problem with many people who use HRMs is that they will see the heart rate decrease for any given workload  and they assume that because heart rate is lower, the caloric expenditure must be lower as well. This is confirmed for them when they see the calorie total on the HRM going down as well. 

This is not the case. What has happened is that while relative intensity has decreased, the fixed intensity has not.  With training, the individual has increased their VO2 max. The intensity has not changed--it just feels easier because the person's maximum has increased. 

When improvements in VO2 max occur, in order to maintain any accuracy at all in your HRM, it is necessary to manually increase the VO2 max number in your HRM setup. (How to do this is a topic for another day). If your HRM does not allow you to manually increase your VO2 max, then you may just have to live with the fact that it is less accurate than other models or brands.

A person's absolute heart rate during steady-state aerobic exercise doesn't mean very much. I see comments where people try to equate their effort/calorie burn with someone else's based on heart rate alone--that can't be done. A heart rate, for example, of 150 bpm is relatively meaningless by itself. It only means something if we know that, for person A, 150 bpm represents"Z"% of their HRmax, that their VO2 max is "X" amount and they weigh "Y" kilograms. 

You might be thinking "gee, Azdak--that sounds like an awful lot of stuff you gotta know before you can even put one of these babies on". To some extent, that would be correct. Polar and Suunto try to even out some of the variables with their technology--the average person really has to rely on that. I have no idea about other brands--I suspect it's pretty much a crapshoot in many cases. The main thing is to have realistic expectations. HRMs are never going to be exact, even under the best of circumstances. Realistically, I think it is unreasonable to expect even the good models like the Polar F6 and Suunto t3 or t4c to be any better than 80%-85% accurate. And that is fine. 

Keep in mind that the relationships I have described are primarily true for steady-state aerobic exercise with relatively simple movements--walking, running, stationary bike, etc. (Cross trainers/ellipticals are a different matter--there is no common movement patterns, so there is no way in general to determine the fixed cost of intensity). 

More complex activities (mostly aerobic classes) are a different story. Because the movements are more complex, mechanical efficiency DOES play a more important role, and, again, oxygen uptake cannot be "fixed" because intensity is always changing. In a class that is primarily aerobic in nature, an HRM is probably the only way to get even close to a reasonably accurate estimate of caloric expenditure. Since mechanical coordination does play such a large role,  a decrease in HR that occurs over time CAN reflect a decrease in absolute oxygen uptake-although there is also a training effect at work here as well. It's just more difficult to measure. Again, look at your HRM calorie numbers as an estimate, and err on the side of caution when replacing those calories.  

I hope to write another blog in the future exploring the whole issue of efficiency, "muscle confusion", etc. Suffice to say for now that, again for steady-state aerobic exercise--the amount of decrease in caloric burn for a given workload that is due to "increased efficiency" is not that much. The idea that you have to change your cardio every couple of weeks to avoid a decreased rate of caloric burn is utter nonsense. But that's for another day.

Suffice to say that, under the conditions I have described above, you should not assume that a decrease in heart rate for a given exercise workload means you are burning fewer calories. If your weight has remained the same, it's most likely just your HRM that is "burning" fewer calories, not you. 

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