By Coach Nick Morrison
If you ride with a power meter, one of the most beneficial metrics to analyze after you ride is your variability index (VI). VI tracks the fluctuation in your power output. Low fluctuation means you held a relatively constant power output during the whole ride and the VI score will be in the 1.00-1.05 range. Lots of fluctuation means you were constantly changing your power output – at times going really hard and at other times going really easy. These are two very different approaches to riding – sprint/recover/sprint/recover vs. pace evenly. As shown in the example variability chart below, these may still end up with the same average power. So, which approach is more appropriate for you?
If you are training for time trial style (non-drafting) triathlons or for endurance cycling events like century rides, low variability is the way to go. These events are all about pacing. In triathlon, you need to look at your bike portion not only to look at your bike split, but also to make sure you set yourself up for a good run. Riding with low variability will set yourself up for a great run while riding with high variability will make you struggle through the run (even if you get through the bike alright).
Some courses and conditions will challenge you more than others. On nice calm days with a flat straight course, it’s pretty easy to ride with low variability. You’ll barely even need to change gears. If it’s a windy day or if you’re riding a winding or hilly course, the conditions will be more challenging to keep a low variability. When you’re riding in those challenging conditions, keep an eye on your power output and try to keep it constant. Shift frequently. Avoid the temptations to attack hills (even short ones) or accelerate aggressively out of corners – both cause your power to spike. Challenging courses and conditions are just that – challenging. But if you’re smart about how you approach the ride, you can still do well.
There are times where high variability is the norm. The biggest factor here is if it is a draft-legal event. Drafting completely changes race strategy because riders can tuck in behind the riders in front of them or the full peloton and save a ton of energy while maintaining speed with the other riders. Then when it comes time to try and create separation from the other riders, you must be able to produce a tremendous amount of power to sprint ahead and lose others on your tail.
Take a look at these two power files: Rigoberto Uran’s 2017 Stage 9 Tour de France win and Lionel Sanders’ 2016 Ironman World Championships. Both had impressive power outputs – Uran’s normalized power was 291 watts and Sanders’ NP was 306 watts. The big, big difference is that Uran’s VI was 1.24 while Sanders’ VI was 1.02. That clearly and dramatically shows the difference between a Tour de France ride and an Ironman ride. Uran spent much of his day drafting, but then when the time came to create separation from the pack, he opened up! Sanders on the other hand needed to set himself up for a good run and didn’t have anyone to draft behind nor anyone drafting behind him that he needed to create separation from.
Keep this info in mind next time you’re out for a ride. What is the appropriate pacing strategy for your workout or event?