NP = Avg Power
Hi guys, got a question. Have a friend who now has power. She does not have a yellow computer. She uses a garmin. She did her test the other day and for the 42 minutes she got an avg power of 170. Would that be considered her normalized power number?
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It depends on the ride really. They might be close (even identical) or they could be materially different.
http://www.peaksware.com/articles/cycling/normalized-power,-intensity-factor,-training-stress-score.aspx
To account for this variability, TrainingPeaks uses a special algorithm to calculate an adjusted or normalized power for each ride or segment of a ride (longer than 30 seconds) that you analyze. This algorithm is somewhat complicated, but importantly it incorporates two key pieces of information: 1) the physiological responses to rapid changes in exercise intensity are not instantaneous, but follow a predictable time course, and 2) many critical physiological responses (e.g., glycogen utilization, lactate production, stress hormone levels) are curvilinearly, rather than linearly, related to exercise intensity, By taking these factors into account, normalized power provides a better measure of the true physiological demands of a given training session - in essence, it is an estimate of the power that you could have maintained for the same physiological "cost" if your power output had been perfectly constant (e.g., as on a stationary cycle ergometer), rather than variable. Keeping track of normalized power is therefore a more accurate way of quantifying the actual intensity of training sessions, or even races. For example, it is common for average power to be lower during criteriums than during equally-difficult road races, simply because of the time spent soft-pedaling or coasting through sharp turns during a criterium. Assuming that they are about the same duration, however, the normalized power for both types of events will generally be very similar, reflecting their equivalent intensity. In fact, normalized power during a hard ~1 hour long criterium or road race will often be similar to what a rider can average when pedaling continuously during flat 40k time trial - the normalized power from mass start races can therefore often be used to provide an initial estimate of a rider's threshold power (see below).
As everyone here has pointed out, the AP and NP are not identical, and in fact are DESIGNED to be different. But really, the NP is of value to us for a slightly different reason than for road bike racers.
We want our NP and AP to be as close as possible during a race. NP/AP = VI (=variability index). If you ride very steady, the NP and AP approach one another, and a good IM bike split will have an IF of under 1.05 on most normal courses. So, on an IM or HIM course, she is not losing much by looking at AP instead of NP while riding AS LONG AS she is riding pretty steady.
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On another note, on a realistic IM-style ride, there is a coincidence of errors you should know about:
If you set your Garmin to "include zero", it will give you the true actual power for the ride; e.g., if you coast for a few seconds, it counts that time as zero watts.
You can also set your Garmin to "exclude zero" as it averages. Thus, your coasting breaks don't lower the average, and you get an artificially high "average power".
However, it's an observation that I and at least 3 other people I've talked to have seen that the "average power excluding zero" power ends up being very close to the NP over a ride of any significant distance as long as you're riding reasonably steady. I should emphasize that there is no good mathematical reason for this other than that both measurements (AP excluding zeros and NP) are going to be a few percent higher than the true AP. So, if she wants to know what her NP is, and she is not spiking power badly, this is one way to be very close.
On my last FTP test each of the 20' intervals had Average Power (shown in both Garmin Connect and WKO) within 1 watt of NP.
HOWEVER, when measuring the entire 42' interval, ie including the 2' recovery, the NP was 4 watts higher than the Average Power (btw, the average power per WKO was identical to the time-weighted average of the numbers straight from Garmin Connect).
My understanding is that, consistent with some above comments, that when you include the recovery interval all of a sudden your VI is no longer 1.0, so NP and AP are diverging...
I'm not sure if there is free software with WKO capabilities to measure NP, but otherwise my experience above would suggest that adding ~2% to the average power should get you pretty close.
I wrote a Wiki entry on this "free alternative to WKO", which is found at this path:
Wiki / (Bike) EN Power Training Resources / WKO+ Alternatives for the Mac
The article is concerned about Mac, but everything it says can be transferred to a Windows machine as far as the alternatives.
And yes, it is correct that just because the Garmin doesn't "have" NP, IF, etc, that doesn't mean you can't get it by using any of these programs. The Garmin doesn't calculate those things on the fly, but all of those things are treatments of the data that the Garmin does collect and record.
William