Calorie Burn Question

I'm curious on this too. I've done 3 days of The 30DS and burned pretty much the same amount of calories. It also is about 10 per minute going by my HRM (has the same options that yours does).

OK--couldn't do this at work--too many people looking over my shoulder.

First of all, and I always have to start with this: heart rate monitors do NOT measure calories burned--not directly anyhow. They estimate calories burned based on the relationship between heart rate and oxygen uptake (or VO2). During steady-state cardio exercise, there is a relatively consistent relationship between heart rate and VO2, so the HRM can look at heart rate, figure out what percentage of your maximum heart rate you are working at, translate that % of heart rate max to a % of VO2 max and calculate calories.

Got it so far? The key is that the HRM has to be able to estimate your oxygen uptake from your heart rate, because that's how you calculate calories. Heart rate by itself doesn't mean anything--it's just a relative indicator.

That term--relative--is key to this discussion. Heart rate is a *relative* number. By that, I mean that the actual number --120, 130, 160, etc--doesn't mean very much. What percentage that is of your maximum heart rate--50%, 70%, 80%, etc--IS significant.

One of the fundamental physical changes that take place with exercise training is that the heart rate for any given submaximal workload with decrease. That's because your maximum fitness level has INcreased and now that workload represents a smaller percentage of your maximum. For example, you start an exercise program and running 5.0 mph represents 75% of your maximum. Your heart rate is 150 bpm. After several weeks of training, your maximum fitness level increases by, say, 15%-20%, Now, if you run 5.0 mph, your heart rate might only be 126 bpm. Why? Because now that workload only represents 63% of your maximum instead of 75%.

Here is where the error comes in. The energy cost (calories burned) of a workload is relatively fixed. In the above example, running 5.0 mph is a workload of 7.3 METs (a MET is a measurement of oxygen uptake). When you first starting running and going 5.0 mph was harder, the workload was 7.3 METs. Now that you are more fit, even though the workload is a lower % of your maximum and feels easier and your heart rate is lower, the workload (and thus the calories burned per minute per kg of body wt) is STILL 7.3 METs. What has changed is that your MAXIMUM has increased, therefore it feels easier.

The energy cost of an activity is FIXED, but the heart rate response is RELATIVE. Since the HRM doesn't know that your maximum fitness level has increased, it assumes that, since your heart rate is lower, you are now working at a LOWER workload, and thus estimates that you are burning fewer calories.

Since people have been conditioned by HRM manufacturers and the popular press and ignorant trainers to believe that heart rate and HRMs directly measure calories, this myth has arisen that, the longer you do any activity, the fewer calories you burn doing it. And people are running around doing odd things to try and "shock" their bodies into burning more calories.

So what to do with your HRM? Depends. Ideally, you should be able to update your VO2max in your HRM, so that the device calculates the percentage of a higher number. In the case mentioned above, ideally, the HRM could be programmed to know that your VO2 max increased from 9.7 METs to 11.7 METs, so that, even though running 5.0 mph is now 63% of your maximum, it is now 63% of 11.7, rather than 75% of 9.7, therefore the calories burned should stay relatively the same.

Some of the more advanced Polar models -- the F6, F11, RS300x, RS800x--allow you to manually enter your VO2 max (of course, you still have to be able to figure out what that number is, which is not easy for many people, but at least it's an option). Most Polar models above the F4 have the "Polar fitness test" which uses resting heart rate to estimate VO2 max. You would just retake the Polar fitness test and update your settings. That would probably be acceptable for someone just starting a program -- that person would be more likely to see significant decreases in resting heart rate with training. If you are a more experienced exerciser, I don't know how much more resting HR is going to drop. Suunto models can also track these changes and maybe some of the top end, newer Garmin models as well. If you don't have one of these monitors, I have no idea how or even if you can update these settings--which is why I don't recommend any of them (Timex, Mio, Omron, etc).

So, there you go--hopefully that's not too complicated.

It's one of the reasons why I always advise that people exercise caution when using HRM calorie numbers in their eating plans. Based on what I read here, I think people think these things are a lot more accurate than they are. I would advise that people never eat more than 50% of the exercise calories as measured by an HRM. Your risks of overeating are significant higher than any possible risks of adverse effects from undereating

I'm so confused now! I asked my husband to get me a Polar HRM for Christmas...is it really worth it if even those aren't accurate?

bump- very interesting discussion