I came across this interesting article about the differences in measuring FTP (Functional Threshold Power) indoors and outdoors and thought it would be good to add some other ideas to complement those in the article.
In the Power Meter Handbook, Joe Friel talks about how Andrew Coggan, PhD came up with the idea of FTP as a measurement of the power an athlete can sustain at their lactate threshold intensity for about an hour (Friel 2012: 53). And how do we do this? Do a test.
The test is a simple one; find out the power you can hold you 60 minutes. While simple, it obviously has drawbacks – it would really challenge you to do a test this long, both mentally and physically, to go out on the road and ride as hard as you can for 60 minutes (Friel 2012: 54). Not only this but there is traffic on the road which might get in the way during your test!
Being a Trainerroad user, I am already familiar with testing my FTP at home on my turbo trainer using one of their tests. I always feel like my tests are not accurate though, and I think this is for a number of reasons:
- Trainerroad suggest resting for 2 days before testing (Friel also suggests resting for a few days before testing (2012: 60). I rarely want to rest for 2 full days every 4-6 weeks before testing, especially as the triathlon season draws closer.
- Testing on a turbo trainer is difficult – there is nothing to distract you from the effort (except if you have a window nearby 🙂 ). I’m pleased that Joe Friel says that most riders find this test more difficult indoors! (Friel 2012: 55)
- When I do have a couple of easy days before a difficult Trainerroad workout (either after a holiday, illness or business trip), I always find the workout easier than I would without this break. That might be obvious to most of us but once my routine gets back to normal these workouts are more difficult. Does this mean my FTP is set too high (I have trouble completing it now) or too low (I did it easily last week!)??!!
I have never tested my FTP outside, but it makes sense to me that for the reasons above and considering how I can move in and out of my saddle on roads that are not flat and consistent like testing indoors that my FTP will vary somewhat when testing outdoors vs. indoors. I have, however, used race files to estimate my FTP and to estimate target power for a given event (by using the 5% Friel describes (2012: 45), stating that when the duration of a race/test doubles, your power reduces by 5%). Using this formula I can estimate my FTP for any given distance quite easily.
This blog post explains more on how to estimate your FTP if you don’t want to test every 4-6 weeks or are just starting out.
So now we come to the nitty gritty! How do we test our FTP outdoors?
As we see in the article, it is suggested that you ride an 8-minute or 20-minute test (as you would when testing on Trainerroad indoors) based on your experience and the type of races you will participate in, then deduct 10% or 5% depending on the length of the test.
Friel proposes a slightly longer test (30 minutes) but with NO percentage reduction – UNLESS you are with other people! (Then it becomes more like a race scenario and you should deduct 5%)
This is the most interesting difference between Trainerroad and Friel’s methodology and the main reason I consider I test low when testing indoors, and not necessarily due to fatigue as I suggest in points 1 and 3 above but more likely due to mental toughness. I am sure that I could work harder if I was racing or outside with other people than I could on my own, at home, sweating away on my trainer, and this is why I would consider he thinks it is best not to make the reductions suggested.
Based on the same 5% theory, with no reduction for a 30-minute test, perhaps the reductions suggested by Trainerroad are a little high and you could consider reducing a bit closer to 3% for a 20 minute test…. the question is, how much difference does it actually make to your wattage!
I’ll leave you to ponder 🙂
So however you decide to assess your FTP now, here is a post about useful tips to remember when doing FTP assessments.