# Training & Nutrition - How does a HRM calculate power?

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View Full Version : How does a HRM calculate power?

elicheez
03-29-05, 08:11 PM
Hi-

My dad got a heart rate monitor that estimates power output based on HR and weight. It's surprisingly accurate compared to a trainer with power monitor.

I have a HRM. It doesn't calculate power, but I'm curious. Does anybody know the calculation that estimates power from HR (or where to find it)? I googled a bit, without luck.

thanks.

trirmk
03-29-05, 09:58 PM
That can't be accurate. The determinants of power have nothing to do with HR, and using weight alone won't help at all either. It's only the HR monitor and nothing else that he has to estimate power? What monitor is this exactly? I'm curious to look it up.

JavaMan
03-29-05, 11:56 PM
My Polar 210 heart rate monitor displays calories burned. If it displayed calories burned per second, that would be power output. The problem is that only some of that power would make it to the rear wheel, so how useful is that?

trirmk
03-30-05, 12:13 AM
My Polar 210 heart rate monitor displays calories burned. If it displayed calories burned per second, that would be power output. The problem is that only some of that power would make it to the rear wheel, so how useful is that?

:roflmao: :lol: (that is a joke, right?)

JavaMan
03-30-05, 12:23 AM
Not sure what's so funny, trirmk. Let me know and I'll laugh with you.

Power is in units of energy per time. Calories per second is power.

The human body is only about 25% efficient, so most of the power you develop is waste heat, which does not help turn the wheel.

GreyGoat
03-30-05, 08:50 AM

elicheez
03-30-05, 06:58 PM
It's a Polar. It gives calories burned during the workout, so that is energy and you could divide by time to get power.

I'm guessing that the company measured people's HR and power and put some kind of correlation together. I assume that higher HR and higher body weight lead to higher power, but I'm curious to know the details.

I haven't tried it, but my dad reports that it's accurate to about 10% of the bike readout, over a variety of workouts.

elicheez
03-30-05, 07:06 PM
The problem is that only some of that power would make it to the rear wheel, so how useful is that?

If you know how much power you're putting out, there are other places you can calculate how much power is needed to drive the bike
http://www.kreuzotter.de/english/espeed.htm

Dutchy
03-30-05, 07:13 PM
Calories burnt is based on a persons HR over a given time that's all. Take two riders with exactly the same body weight with identical bikes. Rider A can ride up a hill in 5 minutes with a low HR. Rider B can also do it in 5 minutes but his HR is higher. Rider B would have burnt more calories but their power output would be identical. If calories were a measure of power then Rider B would have produced more power, which is impossible considering their same weight and ride time.

Estimating Calories has nothing to do with Power output.

CHEERS.

Mark

operator
03-30-05, 07:39 PM
Nothing is going to give you as accurate power measurment as an SRM. The polar power output feedback was among the worst of the three, powertap and SRM being the other two.

supcom
03-30-05, 08:00 PM
That can't be accurate. The determinants of power have nothing to do with HR, and using weight alone won't help at all either. It's only the HR monitor and nothing else that he has to estimate power? What monitor is this exactly? I'm curious to look it up.

Heart rate certainly is related to power output. For a given individual, the more power output, the higher the heart rate will be. Accuracy is a relative term. For some applications, the cheap \$300 power meter sold by Polar would be considered crude. For others, it would be overkill.

It would be a fairly straightforward task to take a sample population and test them for heart rate at various output power levels. You can analyze the data to generate an algorithm that provides a best-fit power output based on heart rate and weight. Chances are your results will be fairly close for the 'average' person. You could probably do even better if you include VO2max values as a third variable in the algorithm.

Of course, the algorithm will end up being much less accurate for elite athletes and couch potatoes, but that's probably not the target market. I doubt that the HRM manufacturer claims any great accuracy any more than they do for calorie estimates based on heart rate and weight or the calorie expenditure function found in cycle computer.

SSP
03-30-05, 09:31 PM
Calories burnt is based on a persons HR over a given time that's all. Take two riders with exactly the same body weight with identical bikes. Rider A can ride up a hill in 5 minutes with a low HR. Rider B can also do it in 5 minutes but his HR is higher. Rider B would have burnt more calories but their power output would be identical. If calories were a measure of power then Rider B would have produced more power, which is impossible considering their same weight and ride time.

Estimating Calories has nothing to do with Power output.

CHEERS.

Mark

I don't think so. In your example, riders A and B will produce the same power output, and their calories burned will be nearly identical as well (despite the differences in heart rate).

Heart rate is just a measure of how efficiently your heart is able to pump blood. Some people have naturally higher heart rates, and others have naturally lower heart rates, but that doesn't mean that they are burning calories at differing rates. Many people with naturally low heart rates have higher metabolisms (i.e., burn more calories at rest) than people with higher heart rates.

ewitz
03-31-05, 08:06 AM
Polar does make a heart rate monitor with a power output option. It is mounted on the chainstay a calculates wattage based on chain speed and tension.

See:

http://www.polarusa.com/consumer/powerkit/default.asp