Dear Coach: What Is VI?
Knowing how to read and interpret the VI (Variability Index) data in your power files can help you improve your bike splits in training and racing.
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VI, or “Variability Index,” represents the steadiness of your effort level for a bike ride, with 1.0 representing a perfectly steady effort and increasing values representing greater variability. In triathlon racing and training, we are aiming for a VI between 1.0 and 1.05. A VI of 1.2 to 1.35 is typical for cycling road races and riding styles; 1.2 to 1.5 is typical for mountain bike and cyclocross races and riding styles.
*It’s important to note that the VI measurement is calculated based on power data. If you don’t ride with power, you won’t be able to measure VI.
Visually, VI translates to the “spikiness” of your power data in graph form. In the image below, the overall ride segment shown has a VI of 1.15; the highlighted section has a VI of 1.02.
Mathematically, VI is the ratio between Normalized Power (NP) and Average Power (AP): NP / AP = VI. Average Power is the mathematical average of your power data. Normalized Power is a revised average of your power data that drops the zeros, re-scales the data to exaggerate the “spikiness,” averages the re-scaled data, and then brings that average down to its original scale.
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Why Should You Care About VI?
Conceptually, your ride VI indicates whether your legs feel like you just completed an endurance ride or an interval ride. How does it do that? Let’s compare two actual rides from recent training. The first was on rolling roads with relatively few turns or cross-streets and yielded a VI of 1.02 (NP was 2% higher than AP). The steady effort is reflected in the low variability of the power output, as shown in the graph below.
The second ride involved some climbing and descending on a narrow, often winding multi-use path and yielded a VI of 1.28 (NP was 28% higher than AP).
Unlike the first ride, this ride involved a great deal of both high wattage spikes and soft pedaling and the greater variability of power output is easily seen in the graph below.
In reality, these rides differed in terms of absolute power data – they neither had the same Average Power nor Normalized Power. But let’s say they did: whether we’re talking about AP or NP, you’re arriving at that average either by way of data points that all cluster around the same value (the first, low VI ride) or by way of data points that are both way above and way below the average (the second, high VI ride). And a string of alternatingly very high and very low power sounds a lot like intervals, doesn’t it?
Whether your legs interpret your ride as a steady, endurance effort or as a series of intervals really comes into play when you are off that bike and onto your run. There is no question that you are going to run better after a steady-state ride than after a series of hard intervals. And that is why you should care about VI.
How To Improve Your VI
The ultimate question, then, is how to achieve an optimal VI for your ride (1.0-1.05, as a reminder). First, let’s set aside the idea that you can only achieve a low VI on a ride with low elevation gain. As it turns out, VI isn’t dramatically affected by the trend-level changes in power, such as when your power increases uphill and decreases downhill. The bigger impact on VI is how much your instantaneous power varies around that trend-level power.
So what kind of riding does effect VI? Predominantly three things: route variables including stop signs/lights, turns, and winding terrain; non-continuous pedaling patterns; and low frequency of gear changes. Each of these ride characteristics results in a combination of spikes in power and drops in power, which is exactly what contributes to a high VI.
Route variables aren’t under your control, but your choice of bike route is. Even if it means going out of your way, it’s helpful to incorporate relatively uninterrupted and generally straight sections of road into your bike training so that you can both evaluate your baseline VI and work to improve it.
When you’re within those race-like sections of road, pay attention to your pedaling patterns. Do you pedal continuously with consistent cadence, or are you more of a pedal-coast-pedal-coast kind of rider? If you’re more the latter, it’s time to focus on changing that habit.
Once you are pedaling consistently, spend some time paying extra attention to your power (I prefer 3s average power to instantaneous power) and cadence. Specifically, you’re looking to keep those two metrics within 5W / 5rpm of your target. Are you able to do this on flat terrain, or do those numbers jump around? If the numbers are a bit erratic, your next goal is to fine-tune your pedaling consistency to maintain stable power and cadence on flat terrain.
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The final piece of the puzzle is being able to keep your power and cadence within 5W / 5rpm of your target even when the terrain is changing. In practice, this means you are changing gears any time your power or cadence is more than 5W / 5rpm above or below your target. (You will know your skills are improving when you see power or cadence moving away from your targets versus power and cadence.)
Start by honing this skill on a relatively flat road (+/- 3% grade) since your gearing should be sufficient to hold power and cadence on that terrain. Once you’ve mastered that, move on to rolling hills and climbs. This terrain is a bit trickier, as you are likely to run out of gears at some point and both power and cadence will stray from your targets. Your goal is to maintain your targets as long as possible, and then to keep both power and cadence smooth even as they are deviating from those targets.
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And yes, all of this plays out as a constant string of gears changes, which requires close attention. The attention toward gear changes and even paying close attention to your data may be a big shift from your current riding style, but with practice you’ll get used to it – and be a stronger triathlete.
Alison Freeman is a co-founder of NYX Endurance, a female-owned coaching group based in Boulder, Colorado, and San Diego, California. She is also a USAT Level II-certified and Ironman University-certified coach as well as a multiple iron-distance finisher.