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The case for Cycleop’s PowerCal

27 June 2011 14 Comments
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July 2012 UPDATE: I have now made a review and comparison of PowerCal. See my new post with detailed review and powertap comparison here.

One of he big, and most surprising product announcements from Cycleops two weeks ago was the all-new PowerCal heart rate monitor.

This new device is an ANT+ heart rate monitor strap with a twist… Based on your heart rate, it derives the power you are producing. This is a very surprising announcement from a company that is almost a standard and helped open up and democratize the power meters to all cycling enthusiasts.

There are other products out there that derive power from other metrics. Some do it real time like the iBike, others after the fact, taking into consideration things like weight and grade, like Strava and SportTracks, but to my knowledge this is the first one that does it from the heart rate.

It is a very smart product. It takes your heart rate, derives the power using advanced algorithms, and then broadcasts it over ANT+ so it can be read on your cycling computer. Very smart !!!

Steve Chapin, CycleOps’ marketing director said “Lab tests have shown a very consistent correlation between heart rate and power indoors, the correlation is almost one-to-one,” said Chapin. “The problem is that there’s all sorts of variables outside, hills, heat, etcetera. What we’ve done over the last few years is to analyze literally thousands of ride files and we’ve been able to identify and establish trends in instances where there’s a deviation in the correlation between heart rate and power and we’ve taken it one step further by building algorithms that identify those trends.”

The data analysis, subsequent algorithms and programming allowed CycleOps to build the PowerCal, which will sell for $199. “It’s not going to replace a PowerTap,” said Chapin, but it allows riders to have conversations about training and power who don’t have power meters.

The PowerCal must be initially calibrated for a rider using a power meter and a specific protocol in which heart rate and power information are captured then uploaded to the PowerCal unit. The unit is currently in trial at the University of Colorado to validate its accuracy. Chapin estimated the PowerCal’s accuracy to be between 5- and 10-percent based on CycleOps testing.

I remain skeptical.

Heart rate and Power generated are very different and react very differently. One of the big differences is that Power is immediate whereas Hear Rate is not.

When you accelerate and increase the power you immediately see the increase in power, but the hear rate will take a while to catch-up to the physiological change that just occurred. Every cyclist has experienced this.

The reverse is also true. When you stop powering on the pedals, power goes immediately away, whereas the heart rate will decay slowly, depending on your fitness, freshness, etc… How does Cycleops account for this?

The first I heard about this, I immediately remembered my VO2max test. There I could see a very obvious correlation between heart rate and power… That was a test under very rigorous conditions, pedaling smoothly and constantly, indoor on a trainer, very controlled environment.

The results are remarkable:


However… how does it work in the real world?

Out of curiosity, I‘ve took some of my powertap files and plotted Heart Rate vs Power, and the results do not show any type of correlation. This is caused mainly by the immediateness of power versus the inertial nature of heart rate.

The plot is presented below:


Looking at this graph, I can’t see how you’d be able to derive power from heart rate.

Even when including only cadence between 80 and 100, the diagram still does not hold much promise:


Looking at data like this, although you start to identify a trend, it’s hardly a strong correlation.

Possibly indoors, when you are always pedaling at the same cadence, it’d would work… The next graph plots watts and hear rate only when pedaling at cadence between 77 and 93 rpm.


Still no strong correlation is visible…



In conclusion… Let’s just say… I’m very curious to see side-by-side comparison, see a powertap graph against a powercal graph… Even with the calibration done with a power-heart-rate test… I simply don’t see how you can derive power from heart rate outside the lab under very strict conditions.

6/30 UPDATE: Some comments were send inquiring if results would be different if the workout would be done indoors on a trainer, instead of outside, on the road.

It so happens that I have a Computrainer, that I have for indoor training (more infrequently than I would want, but that’s a different issue). I used one of the workout files generated by the computer connected to the trainer and analyzed the heart rate vs watts:


There was in fact, a slightly better correlation between heart rate and power in an indoor environment, but still the correlation factor (0.331) is too low to formulate any conclusions.

In general, when analyzing correlation,  a general way to interpret the calculated r value is as follows:

0.0 to 0.2 – Very weak to negligible correlation
0.2 to 0.4 – Weak correlation
0.4 to 0.7 – Moderate correlation
0.7 to 0.9 – Strong correlation
0.9 to 1.0 – Very strong correlation

In this case, we have a weak correlation factor, which means that the error margin is too large to generate any meaningful conclusions.

So, why is the first graph presented in this article so neat and perfect, whereas, any correlation from a workout, either outdoors or indoors, does not present any meaningful correlation?

In my opinion, this has to do with the VO2max test protocol, where you are supposed to pedal at constant cadence without any accelerations. Since the heart rate is a trailing indicator of the effort, every time there is an acceleration (or deceleration), the correlation between heart rate and power will fail. In other, it does not work for any workout where you won’t maintain a constant cadence and constant effort (or slowly increasing, which is the case of the VO2max test).

So, conclusion… I’d say that it may work in the lab (under very strict testing conditions), but does it work in real life? In my opinion, either indoor or outdoor, it will have an extremely high error of margin since the effort is not constant.

As I said, I’m very curious to see side-by-side comparison, see a powertap/computrainer graph against a powercal graph… I’m sure Cycleops will thoroughly test before launching the product, so maybe they’ll include some black magic that makes everything work.

What do you think? I’d love to hear your opinions.


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  • Andrea said:

    I agree. I think that Power and Heart Rate, while related, are only going to be directly correlated indoors. The other thing they can’t possibly address is that as your fitness level changes, the correlation between power and heart rate will change as well. And relatively small changes can have big results. If your FTP is 200 Watts, and you up it by 5% to 210 Watts through effective training at the same HR, that would make a real difference in your riding. How could PowerCal possibly capture that? I guess you would need to recalibrate pretty frequently… In any case, I’m a skeptic as well.

  • MaverickNH said:

    If accurate within 5% indoors, I would be happy. When I can’t ride my bike with PowerTap on a CompuTrainer, I’m on a spin bike at the gym. That is where it will be handy for me.

  • Rodrigo L said:

    I’ve updated the article to reflect the analysis of one of my indoor workouts in my Computrainer.

    @Andrea , @MaverickNH: even indoors, a strong correlation does not seem to exist due to intervals that you will inevitably will be doing during a normal indoor workout. So any conclusion might be difficult to make.

    As I said… very curious when the product comes out to test it and compare.

  • Peter Zampardi said:

    If you do an actual test designed to look at the correlation (there is one specified for the Computrainer) where you increase your power for something like 3 minute intervals and record heart rate, the correlation is actually quite good (I think Max Testa uses this sort of data for training professional cyclists). So, the AVERAGE power – which is probably more appropriate for training purposes, should be correlated very well, while the INSTANTANEOUS power, which seems to be where you find this device deficient is not as good. Overall, it’s a question of what you are trying to get out of using the tool. If you want something low-cost that provides information for training, this probably looks okay. If you are worried about absolute accuracy, then you need the more expensive tool.

  • MK said:

    Would you please please label all the axes. I know one can figure it out, but it is a common courtesy in the scientific world, and adds to your credibility. You know your data very well inside out, but this does not hold true for your readers, some of whom may not be as familiar with the topic. A big part of my career is explaining things to other people, and it is the little things that sometimes make a difference. I liked your data, interesting discussion.

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  • John Macgowan said:

    Hi Rodrigo,
    My club just added FreeMotion Indoor bikes with power and I’m researching the correlation between heart rate and power. I agree that it seems pretty far fetched that CycleOps can predict watts using HR – although it would be a pretty neat trick if they could. One thing my class has found; during steady state intervals at ~ FTP their HR would climb, while maintaining a constant wattage.

    • Rodrigo Loureiro said:

      Yes, that is one the critical issues with trying to correlate power with heart rate, is that typically within the same ride, the correlation varies from the start of the ride to the end of the ride, where at the start, while feeling fresh you can maintain power with constant heart rate and then the power starts to go lower if you try to maintain the heart rate, or to maintain power, you need to increase hear rate.
      Also, I’ve plotted several rides, and the hear rate average and power average vary wildly from ride to ride.
      Not event to mention what happens when you heart rate is elevated at the top of the climb, then you cost downhill, while you hear rate remains elevated (at least for a couple minutes).
      Heart rate is just an unreliable indicator of power.
      On the other hand, I could see how you could use speed, grade and rider weight to calculate power, but that is not what cycleops is proposing to do here.

  • » Tech Tuesday – Measuring Power Using Heart Rate? » Indoor Cycle Instructor Podcast | ICI/PRO said:

    [...] Here is a very comprehensive review / article that explores PowerCal from a road cyclist’s perspective. [...]

  • Riding with CycleOps PowerCal said:

    [...] power output during a bike ride through wearing a heart rate strap and nothing else. Back then, I wrote about it, and I was very [...]

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    Team R – Awesome Bike Ride Adventures » Blog Archive » The case for Cycleop’s PowerCal

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