Training Peaks: using VI to keep an eye on your pacing and control

Have you ever noticed how different rides leave your legs feeling? One day you finish and the legs feel peppy, fresh even. On another you finish and feel the ride in the legs, even experiencing some residual soreness the next day. Interestingly, when you look at Training Peaks both these rides have the same average power. What’s up with that?

The difference in these rides is how you paced them. Was your effort stable, like we would want on an endurance day? Or did you jam at some points and then go easy, like you might on a local club ride? You already know which ride would be harder on the legs.

The Variability Index (VI) in Training Peaks is a good way of looking at this aspect of your riding. VI = normalized power/average power. The lower the ratio the better the stability of your power output on a ride. For rides like endurance we want to see a VI of 1.05 or less. Or, we can check VI in our tempo efforts on a course. Are we able to keep the wattage stable as the gradient changes?

Here is a tempo effort on Zwift, starting in London and going out onto Leith Hill in the Surrey Hills. Note the stability in heart rate (red) and power (purple), even as the gradient and cadence change.

Here is the lap selection stats, which you will see on the right-hand side in Training Peaks.

Aside from checking on the impact a ride might have on the legs, the VI is also a good way to check on the control we have of our wattage.

At first getting your VI down might be challenging. One trick that has worked in the past is having riders put their head units in their pocket and not watch the wattage all the time. In this way you feel the tension in your legs for your ride and try and keep it constant. This is a great trick that has worked in helping riders that couldn't keep their wattage stable get their VI <1.05.

The big take away with VI is in understanding how the different rides affect your legs and fatigue level. Despite the same average wattage between rides the training impact can be very different. Keep this in mind when doing your endurance rides for instance. They should be manageable days that leave you not feeling tired. If you are doing average power in your endurance zone but with a high VI then they are not endurance days and will end up negatively impacting your ability to work hard when it matters.