HRM Calories--A tale of 2 workouts

Azdak

I have posted many comments and a number of blogs about HRM calorie counts. Amidst the constant chorus of "I LOVE IT!!!", I am usually the dissenting voice -- or at least the "parent" that has to tell the kids to settle down.

I am not anti-HRM, I just urge people to exercise caution (how many calories does THAT burn? I dunno, maybe you should wear an HRM....) because they are not as "accurate" as most people think,

Here is an interesting example. I have unfortunately suffered a seemingly unending string of injuries over the past 6 months, which has resulted in a notable decline in my fitness level. About 5 wks ago I reinjured my back, which put me completely on the shelf -- no cardio, no weights, no nothing- for 3-4 weeks. I just started back a few days ago.

As we know, HRMs estimate calories based on your heart rate response to exercise and your setup information. If your setup information is inaccurate, so are your calories. One key factor in determining HRM calories is overall aerobic fitness level (aka VO2 max).

Machines also measure calories expended, but they do so in a different way. Machines measure the ACTUAL aerobic workload you are performing and then they calculate calories burned based on that workload and your body weight. Measuring actual aerobic workload is more accurate than estimating calories from heart rate. However, for most machines, the equations used for estimating calories from workload are not very accurate. With the exception of treadmill walking, some calibrated stationary bikes and certain brands/models/model revisions of cross trainers, most equipment readouts are off --some by a little, some by a lot.

However, even if the machine readouts are not spot-on accurate, they are still MORE CONSISTENT than HRM readings. In other words, day to day differences in the calories displayed on a machine are a more accurate reflection of changes in fitness than day to day fluctuations in HRM calorie readings.

Here is my example:

I have an older Stairmaster 4600 Freeclimber in my basement. along with my treadmill. Stairmaster (at least the older models) put a lot of effort into providing user performance data in their machines. In the past, when my fitness level was a lot higher, the difference between the calorie readout on my Stairmaster vs my HRM was about 20% -25% (Stairmaster was higher). Part of that could be explained by the fact that --even though the model has the "bullhorn" handgrips and I try not to rest my weight on the grips--I cannot work the pedals without holding on at all, so I am getting some handgrip assistance.

However, I have also noticed that the discrepancy is much higher during the first 1/2 of the workout. For example, the calorie difference between machine and HRM for the first half of a 45 min workout is 30%-35% and the second half, the difference drops to about 10% or sometimes less. This occurs even when I keep the workload constant for the entire 45 min workout. The reason for the difference is "cardiovascular drift". This means that during longer workouts, heart rate will drift upward, even with no increase in workload. Since workload hasn't increased, calorie burn does not increase, even though heart rate increases. The HRM doesn't know this -- it just counts beats and spits out a number.

Now back to the two workouts. As I said, my cardiovascular capacity is way down, but I haven't bothered to change my HRM settings. I did a 45 min workout on Friday--total machine calories was 615, with an average heart rate of 126. My HRM calories were 641--more than 4% higher.

So, regardless of whether or not the actual numbers are accurate, you can see a huge discrepancy. For years, the HRM number was consistently 20-25% less than the machine number--now it's higher. Why? because my lack of conditioning sent the HR zooming, esp during the last 20 min of the workout.

Saturday, I did a short workout and did not wear the HRM. Today, I repeated the 45-min workout. Short-term losses in aerobic fitness are often reversed fairly quickly, and this was evident.

Today, I did 636 calories per the machine, with an average heart rate of 118. My HRM showed 570 calories. So my machine number went from being 4% lower than the HRM to 16% higher in 3 days. According to the HRM, that's a drop of 70 calories for a 45 minute workout--but in fact I worked harder and burned MORE calories than before.

It's important to realize that, especially when you are starting out a program, you are going to get variable readings as your body sorts itself out and gets used to a new routine. You cannot look at HRM calorie numbers as being precise. As I have said before, they are at best about 80% accurate -- usually on the high side.

It is also important to remember that, if you are doing the same workout and your HRM calorie numbers start to decline over time, unless your weight has decreased, you are likely burning the same number of calories--or even more--than you were previously. The decline in HRM calories is a problem with the HRM setup, not with you.

I realize this may be confusing to those who think that "HR equals calories" under all circumstances, and I'm not sure I explained it very well. But I hope this was informative.

cmriverside

I always love your HRM posts, thanks.


Sorry to hear about the injuries. Feel better :flowerforyou:

kenlad64

I too look forward to reading your posts and appreciate your insight, thanks again for sharing.

MzMiller1215

Wow! I am happy you decided to post this because I've been so confused about my HRM readings that greatly differ from the treadmill. Your explanation makes sense though.

sheri02r

It is also important to remember that, if you are doing the same workout and your HRM calorie numbers start to decline over time, unless your weight has decreased, you are likely burning the same number of calories--or even more--than you were previously. The decline in HRM calories is a problem with the HRM setup, not with you.

I always thought that as your level of fitness and endurance increased that you would burn less calories at the same workout than when compared to your less fit self. Am I wrong?

Azdak

It is also important to remember that, if you are doing the same workout and your HRM calorie numbers start to decline over time, unless your weight has decreased, you are likely burning the same number of calories--or even more--than you were previously. The decline in HRM calories is a problem with the HRM setup, not with you.

I always thought that as your level of fitness and endurance increased that you would burn less calories at the same workout than when compared to your less fit self. Am I wrong?

In short, yes. That's one of the reasons I posted this--to give a concrete example.

HRMs seem to have distorted many peoples' understanding of exercise physiology (or they never knew it, and are drawing conclusions based on HRM observations).

The difference is the difference between ABSOLUTE workload (e.g. how much oxygen you consume) and RELATIVE workload (the percentage of your maximum the workload represents).

Exercising at a certain intensity level has a relatively fixed energy cost. Over time, as fitness level improves, that cost represents a lower percentage of your maximum--that's why HR goes down, because HR is a relative indicator. The absolute energy cost is still the same--it feels easier because your max has increased. Unless you update the HRM with your new maximum, it assumes you just aren't working as hard and shows a lower calorie number.

Over a long period of time, mechanical efficiency can lower calorie burn for the same workload, but only by a modest amount, and that is easily made up by the fact that the increased efficiency allows you to work harder.

sheri02r

It is also important to remember that, if you are doing the same workout and your HRM calorie numbers start to decline over time, unless your weight has decreased, you are likely burning the same number of calories--or even more--than you were previously. The decline in HRM calories is a problem with the HRM setup, not with you.

I always thought that as your level of fitness and endurance increased that you would burn less calories at the same workout than when compared to your less fit self. Am I wrong?

In short, yes. That's one of the reasons I posted this--to give a concrete example.

HRMs seem to have distorted many peoples' understanding of exercise physiology (or they never knew it, and are drawing conclusions based on HRM observations).

The difference is the difference between ABSOLUTE workload (e.g. how much oxygen you consume) and RELATIVE workload (the percentage of your maximum the workload represents).

Exercising at a certain intensity level has a relatively fixed energy cost. Over time, as fitness level improves, that cost represents a lower percentage of your maximum--that's why HR goes down, because HR is a relative indicator. The absolute energy cost is still the same--it feels easier because your max has increased. Unless you update the HRM with your new maximum, it assumes you just aren't working as hard and shows a lower calorie number.

Over a long period of time, mechanical efficiency can lower calorie burn for the same workload, but only by a modest amount, and that is easily made up by the fact that the increased efficiency allows you to work harder.

That makes a lot of sense.Thank you for your reply!