What is VI (cycling) and why does it matter?

What is VI?  Variability Index is a key workout metric for those who ride with power.  A simple formula, VI is calculated by dividing your normalized power by average power.

Normalized Power vs. Average Power? Average power is a reflection of what you actually did during a ride, including coasting. Normalized Power approximates what you could have done with that same level of effort had you ridden at a perfectly steady pace, taking out the coasting.  Personally, I like NP to track performance across different disciplines and terrain.

You can see my VI from Saturday's ride.  1.10 is NOT steady riding.  It's surging, me hanging with a group that came by me about 10 miles in and holding on for dear life, burning matches.  A VI of 1.10 left me shattered!.  The intensity was only 75%, between Ironman and 70.3, but how I got, there is what did the damage.   Too many peaks vs a steady 75%.  Reviewing a power file you have to consider the VI or you are not looking at the entire story.   

From Training Peaks Blog 
How those watts are put out is also a key factor. An IRONMAN® is all about being steady and keeping the effort relatively moderate. Big spikes in power cause a much faster burn rate of glycogen, creating fatigue faster. It is better to keep your power as steady as possible. Using the Variability Index (VI), we can see how smooth or erratic the power output was. For every rider, pro or age grouper, a VI of 1.05 or lower is the goal. While it’s harder to do on a hilly course, it is still necessary to keep the effort steady. The bike course in Kona has around 3,000 ft. of elevation gain, so it’s not flat by any means. Still, every file from the pro athletes does have a VI of 1.05 or lower.   

Looking at my Kona 2022, I rode 1.05, IM StG 1.06 (much hillier), and Oceanside 1.03.  

The last 2 weekends, my VI was high, and neither Saturday did I have a quality run OTB. It would have been a struggle.   It's a good reminder and something to practice.  

This is 1.03 - steady pink lines

This is 1.10 looks more like a heart attack.  You can see the power spikes, and the high variability.