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The Real Facts about HRMs and Calories -- What you need to know before purchasing an HRM (or using one)

Every week, there are at least a dozen questions about heart rate monitors--which model to buy, what the numbers mean, how the calories compare to readings from machines, etc. I have answered these questions numerous times, but wanted to go into more detail for those who are interested.

HRMs can be great training tools. I have used various types of heart rate monitoring for my training for over 20 years. I currently have a Polar F11 (edit: I now use an FT60--the F11 went to Polar heaven about a year ago), although I don't use most of the features. About 10-15 years ago, Polar introduced their "OwnCal" feature--using heart rate to estimate calories burned during exercise.

This has proven to be extremely popular and has really driven the sale of HRMs. Now virtually every manufacturer includes some type of calorie-counting feature with their HRMs. Effective marketing has created the following beliefs about HRMs and calories:

1.HRMs directly "measure" caloric expenditure. 

2. HRMs are the most accurate way to measure calorie expenditure during exercise.  

3. All HRMs have the same level of accuracy when counting calories. .

4. HRMs can be used to accurately count calories expended during strength training and during rest and 24-hour activity periods. 

5. HRMs are always more accurate than the readout from exercise machines.  

6. If your heart rate response becomes lower when doing a certain activity, it means you are burning fewer calories.  

None of these are true.  HRMs only indirectly estimate calories expended during certain types of exercise. Unless they are set up properly and the profile information updated regularly, they can have significant inaccuracies. And not all HRMs are the same. 

Let's look at all of these:

HRM Theory

First of all--how do HRMs count calories? First thing is that HRMs do NOT measure caloric expenditure--neither directly nor indirectly. HRMs measure heart rate and that's it. They estimate caloric expenditure during steady-state cardiovascular exercise using the relationship between heart rate and oxygen uptake (or VO2).  

The most commonly accepted method for measuring the calories burned for a particular activity is to measure oxygen uptake (VO2). 

During *steady-state*, aerobic exercise, oxygen is utilized at a relatively consistent rate depending on the intensity of the exercise. There is an observable and reproducible relationship between heart rate and oxygen uptake.When workload intensity increases, heart rate increases and vice versa. 

If we have some individual data--resting heart rate, maximum heart rate, VO2 max, weight--it is possible to make reasonably accurate estimates of caloric expenditure based on the percentage of HRmax or percentage of HRreserve at which someone is working.

For example, we know that 70% of HRmax is equivalent to approx 57% of VO2max. If we know that person's VO2 max is 12 METs and they are working at 70% of HRmax, we can estimate that they are working at (57% x 12 METS) or 6.8 METs. If that person weighs 80 kg, we can estimate that they are burning ~545 Cals/hour (MET value x Body wt in kg). 

The major HRM manufacturers (Polar and Suunto) use much more sophisticated sensing technology and algorithms, but they are still based on the same basic principle--the association of HR to oxygen uptake.  

So, it must be emphasized again: HRMs calorie counts are only accurate when there is a consistent and measurable relationship between heart rate and oxygen uptake.  That means exercises and exercise movements that are aerobic in nature and that are performed at intensities between 40% of VO2 max and the lactate threshold

There are a number of conditions under which heart rate can increase, but without an increase in oxygen uptake:

-Stress, Illness

-Dehydration

-Environment (high heat and humidity)

-Heavy strength training (HR increases because of increased pressure)

-Changes in posture

-Cardiovascular drift during extended aerobic exercise

In other conditions--arm work, overhead work, "anaerobic" or sprint exercise--an HR increase will reflect in increase in VO2, but it is not a consistently measurable and reproducible relationship, therefore the HRM calorie count is not as accurate. 

Note: Certain higher-end HRMs--the Polar RS800 and Suunto T6c use a very sophisticated method of analyzing the R-R interval of heart beat signals. When using this method, it is claimed that they can more accurately estimate caloric expenditure at rest, during anaerobic exercise and even during post-exercise oxygen consumption. Since these models are in the $350-$400 range, I am not including them in this review--if you are interested in more detail, check out www.firstbeattechnologies.com and www.suunto.com

HRM Setup 

Even if you have purchased a quality HRM, the readings will only be as good as your setup information. An accurate estimation of caloric expenditure requires the following input:

Resting heart rate (HR rest)

Maximum heart rate (HR max)

VO2 max

Weight

Age

Gender

Not only must this data be accurate when you set up the HRM, it must be updated as well if your fitness level increases or if your weight decreases.

Determining  HR max and VO2 max are the most difficult tasks. You can use a prediction formula (e.g. 220-age) or the built-in "fitness test" of a Polar, but these have a not insubstantial standard of error. For HR max, I would recommend that you Google the web for various predicted HR max fornulae and determine a range of possibilities. Then compare your exercise heart rate to your feelings of perceived exertion. If your HRM shows your HR during exercise is 90+% of maximum, but you feel like you are cruising at an easy effort, your HR max is probably higher than the predicted number. 

VO2 max is even trickier. You can try some field tests (1-mile run, Cooper 12-min run) or a submax fitness test protocol programmed into a treadmill  (if there is one) if you have the ability or access. 

Keep in mind that as you continue to exercise, it is likely that you will lose weight and that your cardiovascular fitness level will improve (that's the whole point, right?). When that happens, you need to adjust the VO2 number up and the weight number down in your setup.

Choosing an HRM 

The first thing you must do is determine what features are important to you.

Accurate heart rate measuring

Accurate (as possible) caloric estimation 

Exercise planning or "coaching"

Stopwatch features

 Data storage/transfer/analysis

If you are primarily interested in heart rate monitoring and basic stopwatch functions, then many different brands and models will probably fill your needs.

However, if you want the most accurate caloric estimation, there are significant differences between brands and models.

Just because an HRM provides a calorie count doesn't mean that number is any more accurate than one from a machine or even a number you just made up. Manufacturers know this feature is popular--some will stick any old number on the display to make you think you are "measuring" calories. 

For greatest calorie count accuracy, an HRM must have the following features:

1. Chest strap sensor for continuous monitoring

2. Ability to manually input HR max, VO2 max, gender, age weight and HR rest.

3. Sophisticated analysis technology and software which has been validated on large numbers of test subjects.

For those features, your choices are going to be limited and you cannot go super-cheap. If you cannot enter VO2 max, then the HRM is using a more general format to determine your fitness level, which means greater inaccuracy.

Note: The only HRMs I know that meet the above criteria are the Sunnto T-series HRMs, the older Polar F6 and F11 models, and the newer Polar FT40 and FT60 models. FT4 and FT7 do not. Suunto does not use VO2 max, but they have a detailed series of "activity levels" that accomplish the same thing. They may not be the only ones, but they are the only ones I can say with certainty. I have looked at the owner manuals of other brands (Timex, Sportsline, Mio) and they do not allow manual VO2 input. Don't know about Reebok or Nike products. 

 Note #2 Update: Someone in the comments suggests that the Garmin Forerunner 405 also uses Firstbeat Technology software, which means they might also use R-R analysis--worth checking out. 

So I bought a cheapie, now what? 

Well, it's not a complete waste--but you do have to be careful about how you include those calories in your overall eating and exercise plan.

Even if the HRM uses a less accurate method of determining your caloric expenditure, the relative changes in the numbers should give you some insight. Calories are a measure of total aerobic work performed. If you do 30 minutes on a cross trainer and burn "300" calories for example with an avg HR of 130, then 3 weeks later burn  "330" calories on the same machine in the same amount of time at the same average HR, that represents a significant increase in your aerobic work capacity and probably a similar percentage increase in the caloric expenditure. You can use the relative numbers to gauge your progress for different machines or activities. 

I hope this is helpful. 

 UPDATE SEPTEMBER 2011: Looking at updated manuals for the most commonly used Polar models (FT4, FT7, FT40, FT60), I think I have a better idea of the product features. 

The FT4 and FT7 models do NOT allow manual entry of VO2 max and do not include the Polar "fitness test" feature--at least I did not see it mentioned in the manuals. I don't know how they assign a "fitness level" or how the HRM takes into account changes in your fitness level over time.

 The FT40 and FT60 DO allow manual entry of VO2max, plus they have fitness tests included. One of the tests uses your setup info, resting HR and a description of your physical activity status to estimate VO2 max. I have seen this formula described elsewhere and it is probably adequate if you don't know your VO2max. If you ramp up your exercise program or notice improvement, these features should be able to capture that improvement and reflect that in your calorie estimates.  

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