By PCG Elite/Master Coach Karen Mackin
Choosing the correct overall intensity for the bike split of
your race is probably the most important thing you can do to have a successful
triathlon. If you have a power meter, choosing the correct intensity is as easy
as looking up a number in a table (see table 1) and either displaying Intensity
Factor (IF) or Normalized Power (NP) on your bike computer during the race.
Since IF = NP divided by Functional Threshold Power (FTP), you can calculate a
target NP by simply multiplying your target IF by FTP. Once you’ve decided on
your target intensity, wouldn’t you like to ride as fast as you can at that
intensity? If so, read on:
Riding the fastest split possible for a given intensity
factor is all about managing variations on that intensity. Some small
variations in intensity during the bike split may be necessary (i.e. to take
advantage of terrain and or wind conditions), however, these changes should be
relatively small. Your power meter is the ultimate tool to learn how to manage
these variations. In this article I will first provide you with a little
background, and illustrate why lower variability allows you to go faster, then
I will provide you practical advice on determining how well you are managing
variations, and finally, give some examples of how to keep these variations to
a minimum.
We all know that Normalized Power represents an estimate of
the power you would have maintained “if” your ride had been perfectly constant.
However, it is important to know that AVERAGE power, not normalized power,
determines how fast you cover the course. Variability Index (VI) is a measure
of how variable the ride is, and is computed as the ratio of your normalized
power over average power (VI=NP/AP).
Consider the scenario of an ironman triathlete with a Functional
Threshold Power of 280 watts, who wants to have a good run split. He chooses an overall Intensity Factor (IF)
of .70 to target. Since IF = NP/FTP,
that means he would target 196 watts as his normalized power. Now, let’s
compare what happens when he is able to manage variations during his ride with
a VI of 1.05, as opposed to if he rides with a higher VI of 1.15.
|
Case 1
A Lower VI of 1.05
Since VI = NP/AP, then AP = NP/VI
AP = 196/1.05 = 187
|
Case 2
A higher VI
of 1.15
AP = NP/VI
AP =
196/1.15 = 170
|
You can see that the average power is higher when the
variability is lower. Essentially, this means he is able to go faster given the
same target normalized power, or intensity factor, than he would have if he
rode with a lot of variability. As I stated earlier, a certain amount of
variability may be necessary to take advantage of terrain. This is due to the fact
that it is not optimal to put out exactly the same power going up a hill as it
is going down because the additional wind resistance on the downhill will only
give you marginal increase in speed compared to going uphill. So, your VI
should reflect the terrain of the course. For flat courses, target VI’s should
be between 1.00 and 1.03. For courses with hills, good VI goals would be
between 1.04 and 1.07.
So, how well do you manage your variability? In
TrainingPeaks you can look at your variability index after you download your
workout. I recommend that all of my athletes do a test ride on a course that is
similar to their event (or ideally on the course) and see how they fare. Now
that most bike computers/watches will enable you to display both your average
power and your normalized power you can see how you are doing during your ride.
If your goal is to keep your VI to 1.05 or less, then you just need to make
sure that your AP and NP are within 5% of each other and it is a simple matter
to compute what that wattage difference is.
Two common reasons for excessive variability are 1)
non-optimal gearing for hilly courses, and 2) power spikes. In order to
minimize your variability, it is very important that you are not forced into
riding at too high a wattage on a hill because your cadence otherwise would
become too low. Ideally, even on the steepest of climbs, you should be able to
maintain at least 70 rpm while climbing. If you are unable to do this, you’d
best head out to your local bike shop and get a new gear setup. AND, once you
have those gears, don’t be afraid to use them! The second reason, power spikes,
occur with excessive surging coming out of corners, or at the bottom and top of
hills where the gradient changes, trying to pass someone, or getting out of the
saddle to power over a short climb. These variations should be minimized! Focus
on every one of those little things that can cause micro bursts in your power,
become very aware when you do them, review your power files after your rides
and make a concerted effort to avoid those spikes.
With a little awareness and practice, you will be able to
get to the target VI that is appropriate for your race. And when you do, you
will maximize your speed for your chosen goal intensity.
