New HRM? How to Make the Calorie Estimate More Accurate

(published in my blog--http://www.myfitnesspal.com/blog/Azdak/view/new-hrm-how-to-make-the-calorie-estimate-more-accurate-183102)

Also applicable to your old HRM as well.

HRMs are not omniscient, intelligent devices that "know" you and your body. They don't measure calories. They are passive tools that reflexively respond to programmed instructions and the setup data that you enter.

They are not always "the most accurate way to count calories"--sometimes they aren't any more accurate than just making up a number out of thin air.

If you are interested in more detail, I have written about this subject in one of my blogs: http://www.myfitnesspal.com/blog/Azdak/view/the-real-facts-about-hrms-and-calories-what-you-need-to-know-before-purchasing-an-hrm-or-using-one-21472

For this topic, I am going to focus 2 things you can do that will help you make the calorie estimates from your HRMs a little more accurate.

They are: Maximum Heart Rate (HRmax) and Maximal Oxygen Uptake (VO2max). The VO2 max information may only be relevant to Polar HRM users, but the HRmax info is appropriate for any HRM.

Maximum Heart Rate (HRmax)
To estimate calories, an HRM needs to know how hard you are working. By itself, your ACTUAL exercise heart rate is not that useful. The important part is THE PERCENTAGE OF YOUR MAXIMUM at which you are working.

In other words, it doesn't make much difference whether your exercise heart rate is 115, 130, 150, 170 beats/min, etc. It DOES matter if that heart rate represents 60%, 70%, 80%, etc of your maximum.

So, one of the most important pieces of set up information for your HRM is your HRmax. For liability reasons, most HRMs and HRM user manuals will instruct you to use a common formula such as [200-age] to estimate HRmax. The problem with those formulas is that probably 30% of population has a TRUE HRmax that can be 15-30 beats ABOVE the age-predicted number.

Example: a 40 yr old sets up an HRM. The HRM uses the 220-age formula and defaults to a HRmax of 180. This person actually has a HRmax of 210. During exercise, that person might reach a heart rate of 170, which would be 80% of HRmax, a good effort, but one that can be sustained for 45 min. The HRM assumes, however, that the person is working at 94% of HRmax (170 divided by 180)--in that case it will significantly overestimate calories burned.

Having an HRM set to too low a HRmax is probably the biggest single mistake people make when setting up HRMs.

How do you determine HRmax? One way is to do a graded exercise test or other field test that pushes you to a maximum level of exertion. Googling "maximum heart rate test" will provide some examples. I DO NOT RECOMMEND THIS METHOD , ESPECIALLY FOR BEGINNERS. It can be risky, it is definitely uncomfortable, and chances are you won't be able to push yourself hard enough to get an accurate reading anyhow, so it will still be a guesstimate.

While it may not be possible for the average exerciser to determine their true HRmax, you can get a lot closer by just observing your heart rate at different aerobic intensities and comparing it to your level of perceived exertion. Start with the age-predicted number and compare from there. For example, if your [220-age] estimated HRmax is 175, but you are cruising along at 160, finding it fairly easy to talk, and able to hold that pace for 30-60 minutes, that is a good indication that your actual HRmax is higher than 175--bump it up to 185 or 190.

In general, a 60% effort means that an increase in breathing should be noticeable, but it should be possible to carry out a conversation without undue effort.

70% effort means you will struggle a bit to speak in complete sentences and talking to someone will require more mental effort and concentration.

75%-80% effort means you will be able to gasp in short phrases only and will struggle to concentrate on the both conversation and your exercise effort at the same time. Often one will suffer.

85%-90% effort--you will have extreme difficultly conversing at all.

Using this method will take longer and it will still be an estimate. There's no way of getting around that. However, after a few sessions, you should be able to make your HRmax, and thus your calorie estimates MORE accurate.

Maximal Oxygen Uptake (VO2max)
Giving the difficulty in achieving true VO2 max, this is often referred to as "VO2peak" now, but it is essentially the same thing. VO2max is the maximum amount of oxygen your body can deliver and process during physical (esp aerobic) activity.

The process of oxygen uptake is what truly generates energy expenditure, not heart rate. However, VO2 is impractical to measure directly during exercise. During steady-state aerobic exercise, there is a relationship between VO2 and percentage of HRmax. (Example: 70% or HRmax is equivalent to 57% VO2max). It is that relationship that allows us to use HRMs to estimate calories expended during steady-state aerobic exercise.

So, if one knows HRmax and one knows VO2max, those are the "scales" that can be used to estimate calories burned from exercise heart rate.

Again, VO2max is not something that is easy to determine. Most people do not have access to a metabolic cart. You may or may not be able to adjust this setting in your HRM. But, if you can, this is a way to improve the accuracy of your calorie estimates. Here are three common methods:

1. Polar Fitness Test -- Some Polar models have a "fitness test" feature. It essentially looks at resting heart rate and uses that rate to project VO2max. Polar claims an 85% accuracy rate for its test, but I am skeptical of that claim. First of all the validation studies they used were done over 20 years ago. Second, research has shown that the use of resting HR to predict VO2 max has a substantial standard of error. Still, it can be better than nothing, so if you have the feature, it is worth trying.

2. Manual entry of VO2max--if a Polar model has the "fitness test" feature, it is usually possible to manually enter VO2max as well (complete the fitness test--when the HRM asks "keep results?" you press "NO", then it allows you to manually enter a number). Even if you don't have access to a research lab, there are a number of submax exercise tests on common commercial cardio pieces that you can get a rough idea. (However, these tests will be inaccurate if you have a normally high HRmax, so beware). There are also field tests, such as a 1-mile run, or the Cooper 12 min Run Test that can give you an estimated VO2max number that you can plug in.
Regardless of the method you use, you will want to periodically re test yourself and update the setup information.

3. VO2max estimates based on activity habits: Here is a sample of a VO2max estimator that uses physical activity habits--it seems to be accurate enough to be useful: https://docs.google.com/viewer?url=http://www.cardiofitnessanalyzer.com/Download/Test%20methodology/Assessing%20fitness%20without%20exercise%20test.pdf
The result you get is expressed as METs -- just multiply the MET number by 3.5 to get VO2max and plug that into your HRM.

Again, only a select few Polar models like the FT40, FT60, RS300x, and the top-end HRMs have the ability to manually input VO2max. For lower Polar models, or lower-end non-Polar models I don't really know how they estimate your fitness level or, if they don't estimate fitness level, what criteria they use to estimate calories.

One way to do a quick and dirty "accuracy check" on your HRM is to check your calories burned estimate while doing simple activities whose energy cost has been well-validated.

Example: Walking 3.0 mph on a treadmill: Multiply your weight in Kg x 3 to get calories per hour burned. Divide calories per hour by 60 and then multiply that number by the number of minutes exercised to get your workout calories.

Walking 4.0 mph on a treadmill: Do the same thing, but multiply weight in Kg x 4

Running 6.0 mph (best outside on a level surface w/ little or no wind): Do the same thing and multiply body wt in kg x 10.

These numbers should be within 10%-15% of what you get on your HRM. If not, you either need to try and tweak your settings, or you may just have an HRM that isn't very accurate. In general, over a wider range of exercise activities, HRMs should be expected to be no more than 80% accurate. If that seems low, well, I'm sorry but that's just the way it is.