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 03-29-05, 08:11 PM #1 elicheez Senior Member Thread Starter   Join Date: Aug 2004 Bikes: Posts: 102 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) How does a HRM calculate power? 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.
 03-29-05, 09:58 PM #2 trirmk Senior Member     Join Date: Mar 2003 Location: PA Bikes: Quattro Assi & Trek 7000 Posts: 122 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.
 03-29-05, 11:56 PM #3 JavaMan Senior Member     Join Date: Sep 2004 Location: Southern California Bikes: 2013 KHS Flite 747 Posts: 1,068 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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?
03-30-05, 12:13 AM   #4
trirmk
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 Originally Posted by JavaMan 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?
(that is a joke, right?)

 03-30-05, 12:23 AM #5 JavaMan Senior Member     Join Date: Sep 2004 Location: Southern California Bikes: 2013 KHS Flite 747 Posts: 1,068 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.
 03-30-05, 08:50 AM #6 GreyGoat Senior Member     Join Date: Feb 2005 Location: NY midstate Bikes: 85 Ross Mt Rainier(for winter road use), 86 Centurion Ironman Master, 92 trek 2300,2005 Iron Horse HollowPoint Expert Posts: 395 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) you can up your power output by patting your head and rubbing your tummy while riding using that method....
 03-30-05, 06:58 PM #7 elicheez Senior Member Thread Starter   Join Date: Aug 2004 Bikes: Posts: 102 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.
03-30-05, 07:06 PM   #8
elicheez
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 Originally Posted by JavaMan 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

 03-30-05, 07:13 PM #9 Dutchy We drive on the left.     Join Date: Sep 2001 Location: Adelaide, South Australia Bikes: Posts: 1,096 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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
 03-30-05, 07:39 PM #10 operator cab horn   Join Date: Jun 2004 Location: Toronto Bikes: 1987 Bianchi Campione Posts: 28,321 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 9 Post(s) 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.
03-30-05, 08:00 PM   #11
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 Originally Posted by trirmk 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.

03-30-05, 09:31 PM   #12
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 Originally Posted by Dutchy 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.

 03-31-05, 08:06 AM #13 ewitz Banned.   Join Date: Dec 2003 Location: toronto Bikes: specialized allez pro, giant tcr composite Posts: 456 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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