Training with a power meter is hands-down the most effective way to train and race on a bike. If you’re new to using power meters or are considering getting one, this will help you understand how to make the most of it.
Power is the most accurate way to measure a cyclist’s work effort and in turn to use for training zones and race pace targets. Speed fluctuates far too much with hills and wind to be helpful on the bike. Heart rate is better than speed, but still leaves a lot to be desired since it is a lagging metric and because it is influenced by too many other variables than just current effort. Power though instantly tells an athlete how hard they are working. It is a factor of the resistance level (how hard you are pushing on the pedals) and cadence. Increase one while maintaining the other and your power output will go up.
I further like to think of the resistance level in two forms – internal and external factors. The internal factors are the ones you control and the external are the ones out of your control. The main internal factor is gear selection. Shift down to an easier gear and you reduce resistance or shift up to a harder gear and you increase resistance. The main external resistance factors are wind and grade (uphill/downhill/flat). Handling these external factors is what really gets at the heart of why power is so beneficial.
Imagine you are riding on flat ground at a given power output and given speed. Now the road changes to a slight uphill. If you were pacing by speed, you would have to increase your power output to maintain the same speed up the hill – you shouldn’t do that. Instead, with a power meter you can see that you can maintain your cadence and shift down to an easier gear to balance out the uphill grade. The result is that you maintain a constant power output and accept that your speed drops on the uphill.
That example also illustrates the importance of shifting frequently. With every change in grade and wind, you should be shifting. In a non-drafting triathlon, your goal should be to maintain a constant power output at your race pace target. How well you do this is measured by something called the variability index (VI). VI measures how much your power fluctuates during a ride. VI of 1.00-1.05 shows that you kept your power output relatively constant while higher VI numbers show that there was a lot of fluctuations in power. Notice that I did say in a non-drafting race.
Different race types require significantly different strategy. You can see this by comparing power files from Lionel Sanders during Ironman Arizona (non-draft) and Ben Kanute from the Rio Olympics (draft legal). Sanders kept a VI of 1.01 – that’s about as steady an output as you can get. Meanwhile Kanute’s VI was 1.20 and the article also shows his time in zones which shows that he was constantly going from sprinting to coasting/drafting and back. Both were fantastic races, and both are fantastic athletes, but as you can see from the power files, the races themselves are very different.
The last thing that I want to point out is that exact power numbers are only worth comparing you to you – not to other athletes. What I mean by that is that you can track your power output during training and see that you increased your average power during a 20-minute test from 240 watts to 260 watts and know that you have improved. You cannot however compare one cyclist’s 20-minute test of 240 watts to a second cyclist’s 20-minute test of 260 watts and assume that the second cyclist is faster. If the first cyclist is lighter, it’s very possible that the first cyclist is actually faster. If you want to compare two cyclists, the best way is just to compare their race or time trial times over the same course.
Hopefully you have a better understanding of what power is on the bike and how you can use it to benefit your training. With this understanding, the next step is to work with your coach to determine your functional threshold power (FTP) and to set training zones to use for pacing during workouts and races.