A few weeks ago I was reading through my RSS feeds and came across a blog post that interested me. If you’re not a powermeter freak like I am, feel free to skip over this. It’ll bore you to death!
Here’s the jist of it:
Groover (the rider and the blog writer) did 2 rides. The first ride (below) she described as being a tough session with the boys (although the boys were doing a recovery ride). The average wattage was 118 watts and you can see that her heartrate (bars in red) was largely in the recovery zone.
The second ride (below) was done on the same loop but perceived as being easier, but if you look at her heartrate distribution you’ll notice that it was in much higher zones. The average wattage was 112 watts – almost the same as the first ride, but surprisingly slightly lower considering her HR was higher.
Basically, two rides were done and had nearly the same average power, but one was perceived as being much more difficult than the other. Let’s forget about how this was percieved for now as there are many variables that could affect this.
This is a great example of why looking at average power of a ride is not a sufficient indicator to reflect true physiological demands of the ride. There are many variable factors that affect your rides: wind, hills, accelerations, steady tempo, descents, etc. To be able to more accurately reflect the actual demands of a ride, Hunter Allen and Andrew Coggan developed an adjusted method to quantify the power for a ride for analysis. They called this Normalized Power.
Normalized Power combines two factors. First, the fact that physiological responses to rapid changes in intensity follow a time course that is predictable. Secondly, the fact that many physiological responses (e.g. lactate production, glycogen utilization, etc) are not linear when related to exercise intensity. For example, a ride that has massive accelerations and then lulls (perhaps a criterium) will be more demanding on your energy systems than a ride that is much more steady – even though they may average out to being the same wattage in the end. Normalized Power is basically an estimate of the power that you could have maintainted for the same physiological “cost” if your power output had been constant. Because of the factors that are taken into consideration, Normalized Power gives you a better indication of the true demands of a ride than Average Power does. This can all be found in the book “Training And Racing With A Power Meter” (I almost copied this description right out of the book). It’s well worth the read if you have a power meter and want to learn how to use it properly.
Another good example of where average power could be misleading is in criteriums. There is lots of coasting through corners and then hard accelerations in crits that end up giving you a low average power in the end. However, Normalized Power will give you a better reflection of it’s true intensity.
I don’t see why the same can’t be said if you train by using Heartrate. If you come back from a ride and judge how intense the session was by using average heartrate, you won’t be getting a good indication of the true demands that were placed on the body.
The above power summaries from Groover’s two training sessions give a great example of how the Average Power of rides can be nearly equal, but the demands on the body greatly differ. The Poweragent software that comes with Powertap in this case does not calculate Normalized Power, however, TrainingPeaks WKO+ software does. I’m willing to bet that the second ride had a higher Normalized Power than the first ride if we were to analyze it on WKO+ (i.e. the ride was more difficult, even though the average power was slightly less).
I see many people who use a powermeter only look at the average power at the end of a ride to judge how difficult it was or as a basis to gauge their progression. Hopefully the concept of Normalized Power broadens your analysis to look for more than Average Power after your ride. Thank you Groover for letting me use your power graphs as an example to illistrate this.
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Thanks for explaining normalised power vs average power. It makes perfectly sense. I will definitely read Allen and Coggan’s book now, too.
I looked at a Garmin 705 this week and I want one although then you have to buy a wheel to use with it…….what is a good Power Meter to use????? The Garmin looks great to race yourself and map your rides…..
Power training is something I have never done. I own a Polar 725 with Power (bought it overseas and cheap) I had it on my road bike for a while but never really used it….What is a good Power Meter?
Hi Buttsy, I am at the Freedom Machine in Prahran Melbourne, and am one of those “Boys with all the toys” kinda guys. I was the same, had tried a few wattage units without any satisfaction. I did get a Powertap in December and found it to be the most useful tool I own. It is amazing how it changes your training mind. You start doing intervals properly, Seatet Hill climbs etc, and you can quantify your improvement so accurately. I even can quantify the power I can gain or lose depending on my set up (on the Drops/on the Hoods/shorter stem/longer stem etc)! The Powertap does come with a computer head and a fantastic download program, so you dont need a Garmin. But of course I have one! I have used Garmin now for a couple of years and found the User friendly nature of it such a pleasure to use. The only downfall with this combination is Garmin have not quite worked out a great program including wattage, however the best combination I have found is Sportstrack if you own a Garmin and Powertap. Sure this total package is not cheap, however I can definitely suggest that it is better spending a few Grand on this combo, than race wheels. I have tried Polar (terrible), IBike (great if you are a triathlete (not suitable for a roadie) and the only unit I suggest that is probably an improvement is SRM, however your bike will need to be mortgaged!
Your reference to HR monitoring is quite valid, only instead of the concept of “average” vs “normalized”, the common approach is to use “time in zones”. In the world of HR training, Sally Edwards’ Heart Zones emphasize the different benefits of training in each zone, and thus any good HR monitor will give you “time in zone”. Just getting into Power Meters myself, I’m not sure if the software out there for tracking power will also give you “Time in POWER zones” – is this something people look at?
I can’t believe anyone would look to averages as a useful summary statistic, I say this of course after having 10 yrs of post graduate training in statistics :-)
I think Normalized power is a great start although I am still skeptical, as I would have to read the book to see how “physiological cost” is calculated. Just like max HR (in my thirties and I still have a “freakishly” high 210 max) my guess is the energetic costs per power zone vary greatly from person to person and will be very hard to predict (just as max HR is quite hard to predict accurately). For example, I am willing to bet your muscle fiber composition alone (not even touching to differences in metabolism – e.g. catabolism and anabolism) would create very different cost profiles, thereby negating to some degree the concept of normalized power. That said, it would be worlds better than a simple average of the raw readings.
Speaking of analyzing workout data, I am terribly disappointed that none of the programs I have played with to date (e.g. Garmin and trainingpeaks online) have let me download the data as a .csv (or equivalent) so I can start doing my own analyses! Does anyone have a different experience?
And as a side note, I found a statistical error in the iBike software a while back, when I contacted the makers they basically told me I had no idea what I was talking about and basically got no where with them. I was a bit annoyed with the experience as I was giving them free consulting, something I usually charge $200/hr for and I had even went so far as to confirmed the error with 2 people (both with PhD’s in statistics) prior to contacting the maker.
So you may not be surprised that I am a tad skeptical with what any of these programs are even doing with our numbers!!!
This is not a good example; If you take a close look at these rides you will see the the first is 58.62km and the second is 49.34km! Honestly at this low wattage and looking at the data I think you will find that the NP is VERY similar. Also, RPE is only useful if one shows the training diary. What was the load 5 days prior to the 05/22/09? What was the load 5 days before the 05/29/09?
Great post on NP!
Hi – you can get a full csv output from the polar power meter..
You want to have a look at Golden Cheetah then (http://goldencheetah.org) … its fully open source, so there is no mystery as to what the program is doing with your raw data.
It also incorporates a range of different normalising algorithms.
If you have good ideas about number crunching, or doubts about the statistical methods in use with the program … you can freely modify the code and submit your patches upstream for peer review. All being well, your code patches may make it into the mainline program at the next release cycle.
Worth a look – I think you would enjoy getting involved with it,
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